One advantage of the new .NET Core (now .NET) apps is that they are cross platform. Once you create an app, the same code will run on Windows, Mac or Linux with no change. But there are some exceptions to this: WPF or WinForms apps are only for Windows. You cannot create a WPF app and run it on Linux, for example.

If you want to create a cross platform app, you will have to use another technology, like Blazor or MAUI. Blazor UI is based on Razor components and it’s not compatible with XAML, so it may be very difficult to convert. MAUI, on the other side, uses XAML and can be used to port your app to Mac, iOS or Android. But MAUI apps don’t run on Linux. If you want a Linux app, this is not the way. You can use Uno Platform (see my article here), it can run on the Web (as a WebAssembly), Linux, Mac or Windows, or you also have the option of using Avalonia UI.

In this article we will show how to convert an existing WPF app to Avalonia UI. We will use the project described in my MVVM Community toolkit 8.0 article. This project has some interesting features to explore:

  • It uses a .NET Standard library for the repository
  • It has a Datagrid to display the data
  • It uses the MVVM pattern and the MVVM Community toolkit NuGet package

Avalonia UI is an open source cross platform framework for .NET to develop cross platform apps using XAML. To use it, you need to install the Avalonia project templates with:

dotnet new install Avalonia.Templates

Once you do it, you can create a new basic project with:

dotnet new avalonia.app -o BasicApp
cd BasicApp
dotnet run

When you run it, you will see a basic app:

The generated code has these differences:

  • The extension for the view files is axaml instead of xaml (and the code behind extension is axaml.cs)
  • The default namespace for the view files is https://github.com/avaloniaui instead of http://schemas.microsoft.com/winfx/2006/xaml/presentation
  • The project includes the Avalonia, Avalonia.Desktop, Avalonia.Diagnostics and XamlNameReferenceGenerator NuGet packages
  • The project targets net6.0 (or net7.0), instead of net6.0-windows
  • There is no need to include the UseWPF clause in the project file
  • The Program.cs file and Main method are explicit
  • There is some initialization code in App.axaml.cs

Apart from that, designing the UI and the C# code aren’t much different from the standard WPF app.

For this app, we will use Visual Studio 2022 and the Avalonia Extension. This extension will provide all templates and a designer for the views. If you don’t want to use Visual Studio, you can use VS Code, but you won’t have the visual designer. In Visual Studio, go to Extensions/Manage Extensions and install the Avalonia for Visual Studio 2022 extension:

Let’s start converting our app. We have two approaches, here: convert our app in place, making changes in the files as needed, or create a new basic Avalonia project and add the features incrementally. I prefer to use the second approach, in this case all the basic infrastructure is already set and we can make sure that things are running while we are adding the features. In the in place conversion, it’s an all-or-nothing, and at the end we may not have any clue of what we’ve missed, in case it doesn’t run.

The first step is to clone our app from https://github.com/bsonnino/MVVMToolkit8.git. Then, we will create our app in Visual Studio:

That will create a new basic app. If you open MainWindow.axaml, you will see the code and the visual designer:

Let’s start converting our app. The first step is to add the two NuGet packages, CommunityToolkit.Mvvm and Microsoft.Extensions.DependencyInjection.

Then, copy the CustomerLib folder with all files to the folder of the Avalonia solution. We will use this project as is, as it’s a .NET Standard project and it can be used by Avalonia unchanged. In the solution explorer, add an existing project and select the CustomerLib.csproj file. That will add the lib to our solution. In the main project, add a project reference and add the CustomerLib project:

Then, copy the ViewModel folder to the project folder, it will appear in the solution explorer. Open MainViewModel.cs, you will see an error in ColletionViewSource:

That’s because the CollectionViewSource class doesn’t exist in Avalonia and we need to replace it with this code:

private readonly ICustomerRepository _customerRepository;
private Func<Customer, bool> _filter = c => true;
public IEnumerable<Customer> Customers => _customerRepository.Customers.Where(_filter);

[RelayCommand]
private void Search(string textToSearch)
{
    if (!string.IsNullOrWhiteSpace(textToSearch))
        _filter = c => ((Customer)c).Country.ToLower().Contains(textToSearch.ToLower());
    else
        _filter = c => true;
    OnPropertyChanged(nameof(Customers));
}

Instead of using the WPF CollectionViewSource class, we are creating our filter and using it before displaying the data. Just to check, we can copy the Test project to the solution folder, add it to the current solution and run the tests to check. For that, we must do the following changes:

  • Change the Target Framework in the csproj file to .net6.0
  • Change the reference for the main project to MvvmAvalonia

Once we do that, we can compile the project, but we get the errors for the CollectionViewSource. For that, we must change the tests to:

[TestMethod]
public void SearchCommand_WithText_ShouldSetFilter()
{
    var customers = new List<Customer>
    {
        new Customer { Country = "a"},
        new Customer { Country = "text"},
        new Customer { Country = "b"},
        new Customer { Country = "texta"},
        new Customer { Country = "a"},
        new Customer { Country = "b"},
    };
    var repository = A.Fake<ICustomerRepository>();
    A.CallTo(() => repository.Customers).Returns(customers);
    var vm = new MainViewModel(repository);
    vm.SearchCommand.Execute("text");
    vm.Customers.Count().Should().Be(2);
}

[TestMethod]
public void SearchCommand_WithoutText_ShouldSetFilter()
{
    var customers = new List<Customer>
    {
        new Customer { Country = "a"},
        new Customer { Country = "text"},
        new Customer { Country = "b"},
        new Customer { Country = "texta"},
        new Customer { Country = "a"},
        new Customer { Country = "b"},
    };
    var repository = A.Fake<ICustomerRepository>();
    A.CallTo(() => repository.Customers).Returns(customers);
    var vm = new MainViewModel(repository);
    vm.SearchCommand.Execute("");
    vm.Customers.Count().Should().Be(6);
}

Now, when we run the tests, they all pass and we can continue. We will start adding the UI to the main window:

<Grid>
	<Grid.RowDefinitions>
		<RowDefinition Height="40" />
		<RowDefinition Height="*" />
		<RowDefinition Height="2*" />
		<RowDefinition Height="50" />
	</Grid.RowDefinitions>
	<StackPanel Orientation="Horizontal">
		<TextBlock Text="Country" VerticalAlignment="Center" Margin="5"/>
		<TextBox x:Name="searchText" VerticalAlignment="Center" Margin="5,3" Width="250" Height="25" VerticalContentAlignment="Center"/>
		<Button x:Name="PesqBtn" Content="Find" Width="75" Height="25" Margin="10,5" VerticalAlignment="Center"
                Command="{Binding SearchCommand}" CommandParameter="{Binding ElementName=searchText,Path=Text}"/>
	</StackPanel>
	<DataGrid AutoGenerateColumns="False" x:Name="master" CanUserAddRows="False" CanUserDeleteRows="True" Grid.Row="1"
              ItemsSource="{Binding Customers}" SelectedItem="{Binding SelectedCustomer, Mode=TwoWay}">
		<DataGrid.Columns>
			<DataGridTextColumn x:Name="customerIDColumn" Binding="{Binding Path=CustomerId}" Header="Customer ID" Width="60" />
			<DataGridTextColumn x:Name="companyNameColumn" Binding="{Binding Path=CompanyName}" Header="Company Name" Width="160" />
			<DataGridTextColumn x:Name="contactNameColumn" Binding="{Binding Path=ContactName}" Header="Contact Name" Width="160" />
			<DataGridTextColumn x:Name="contactTitleColumn" Binding="{Binding Path=ContactTitle}" Header="Contact Title" Width="60" />
			<DataGridTextColumn x:Name="addressColumn" Binding="{Binding Path=Address}" Header="Address" Width="130" />
			<DataGridTextColumn x:Name="cityColumn" Binding="{Binding Path=City}" Header="City" Width="60" />
			<DataGridTextColumn x:Name="regionColumn" Binding="{Binding Path=Region}" Header="Region" Width="40" />
			<DataGridTextColumn x:Name="postalCodeColumn" Binding="{Binding Path=PostalCode}" Header="Postal Code" Width="50" />
			<DataGridTextColumn x:Name="countryColumn" Binding="{Binding Path=Country}" Header="Country" Width="80" />
			<DataGridTextColumn x:Name="faxColumn" Binding="{Binding Path=Fax}" Header="Fax" Width="100" />
			<DataGridTextColumn x:Name="phoneColumn" Binding="{Binding Path=Phone}" Header="Phone" Width="100" />
		</DataGrid.Columns>
	</DataGrid>
	<customerApp:Detail Grid.Row="2" DataContext="{Binding SelectedCustomer}" Margin="5" x:Name="detail"/>
	<StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Margin="5" Grid.Row="3">
		<Button Width="75" Height="25" Margin="5" Content="Add" Command="{Binding AddCommand}" />
		<Button Width="75" Height="25" Margin="5" Content="Remove" Command="{Binding RemoveCommand}" />
		<Button Width="75" Height="25" Margin="5" Content="Save" Command="{Binding SaveCommand}" />
	</StackPanel>
</Grid>

There is an error with the DataGrid. That’s because we need to add the package Avalonia.Controls.Datagrid. Once we add that, we can see some other errors:

  • The ItemsSource property has been changed to Items
  • The columns don’t have the Name field and should be removed
  • The CanUserAddRows and CanUserDeleteRows do not exist and should be removed
  • We should add the themes for the DataGrid in App.axaml:
<Application.Styles>
    <StyleInclude Source="avares://Avalonia.Themes.Default/DefaultTheme.xaml"/>
    <StyleInclude Source="avares://Avalonia.Themes.Default/Accents/BaseLight.xaml"/>
    <StyleInclude Source="avares://Avalonia.Controls.DataGrid/Themes/Default.xaml"/>
</Application.Styles>

We can also see that this code is missing the Detail control. Add to the project a new item of type UserControl (Avalonia) and add the content from the original project:

<Grid>
    <Grid Name="grid1" >
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Label Content="Customer Id:" Grid.Column="0" Grid.Row="0"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="0"   Margin="3" Name="customerIdTextBox" Text="{Binding Path=CustomerId, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Company Name:" Grid.Column="0" Grid.Row="1"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="1"   Margin="3" Name="companyNameTextBox" Text="{Binding Path=CompanyName, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Contact Name:" Grid.Column="0" Grid.Row="2"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="2"   Margin="3" Name="contactNameTextBox" Text="{Binding Path=ContactName, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Contact Title:" Grid.Column="0" Grid.Row="3"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="3"   Margin="3" Name="contactTitleTextBox" Text="{Binding Path=ContactTitle, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Address:" Grid.Column="0" Grid.Row="4" HorizontalAlignment="Left" Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="4" Margin="3" Name="addressTextBox" Text="{Binding Path=Address, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center" />
        <Label Content="City:" Grid.Column="0" Grid.Row="5"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="5"   Margin="3" Name="cityTextBox" Text="{Binding Path=City, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Postal Code:" Grid.Column="0" Grid.Row="6"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="6"   Margin="3" Name="postalCodeTextBox" Text="{Binding Path=PostalCode, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Region:" Grid.Column="0" Grid.Row="7"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="7"   Margin="3" Name="regionTextBox" Text="{Binding Path=Region, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Country:" Grid.Column="0" Grid.Row="8"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="8"   Margin="3" Name="countryTextBox" Text="{Binding Path=Country, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Phone:" Grid.Column="0" Grid.Row="9"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="9"   Margin="3" Name="phoneTextBox" Text="{Binding Path=Phone, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
        <Label Content="Fax:" Grid.Column="0" Grid.Row="10"  Margin="3" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="10"   Margin="3" Name="faxTextBox" Text="{Binding Path=Fax, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}" VerticalAlignment="Center"  />
    </Grid>
</Grid>

We must remove the , ValidatesOnExceptions=true, NotifyOnValidationError=true from the code, as it’s not available in Avalonia. Then, we should add the correct using clause in the main xaml:

xmlns:customerApp="using:MvvmAvalonia"

Once we do that and we run, we can see the UI (but not the data):

For the data, we must add the configuration for the services, in App.axaml.cs:

public partial class App : Application
{
    public override void Initialize()
    {
        AvaloniaXamlLoader.Load(this);
        Services = ConfigureServices();
    }

    public override void OnFrameworkInitializationCompleted()
    {
        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
        {
            desktop.MainWindow = new MainWindow();
        }

        base.OnFrameworkInitializationCompleted();
    }

    public new static App Current => (App)Application.Current;

    public IServiceProvider Services { get; private set; }

    private static IServiceProvider ConfigureServices()
    {
        var services = new ServiceCollection();

        services.AddSingleton<ICustomerRepository, CustomerRepository>();
        services.AddSingleton<MainViewModel>();

        return services.BuildServiceProvider();
    }

    public MainViewModel MainVM => Services.GetService<MainViewModel>();
}

Then, we must set the DataContext on MainWindow.axaml.cs:

public MainWindow()
{
    InitializeComponent();
    DataContext = App.Current.MainVM;
}

Now, when we run the code, we can see it runs fine:

We’ve ported our WPF project to Avalonia, now it’s ready to be run on Linux. We’ll use WSLg (Windows Subsystem for Linux GUI) to run the app. Just open a Linux tab on terminal and cd to the project directory (the drive is mounted on /mnt/drive, like /mnt/c) and run the app with dotnet run. You should have something like this:

As you can see, porting a WPF app to Avalonia requires some changes, but most of the code is completely portable. If you want to ease the process, you can move the non-UI code to .NET Standard libraries and use them as-is. We’ve used the DataGrid and the MVVM Community Toolkit with no problems.

All the source code for the project is at https://github.com/bsonnino/MvvmAvalonia

Sometime ago, I wrote this article for the MSDN Magazine, about Aspect Oriented Programming and how it could solve cross-cutting concerns in your application, like:

  • Authentication
  • Logging
  • Data audit
  • Data validation
  • Data caching
  • Performance measuring

The article shows how to use the Decorator pattern and the RealProxy class to create a Dynamic Proxy to solve these issues in a simple manner. If you want to have a full introduction on the subject, I suggest that you take a look at the article.
The time has passed, things changed a lot and we are now close to the introduction of .NET 7. The RealProxy class doesn’t exist anymore, as it’s based in Remoting, which was not ported to .NET Core. Fortunately, we still have the System.Reflection.DispatchProxy class that can solve the problem.

With this class, we can still write proxies that decorate our classes and allow us to implement AOP in our programs. In this article, we will use the DispatchProxy class to create a dynamic proxy that allows us to implement a filter for the methods to be executed and execute other functions before and after the method execution.

In the command prompt, create a new console app with:

dotnet new console -o DynamicProxy
cd DynamicProxy
code .

In Program.cs, we will define a Customer record (put it at the end of the code):

record Customer(string Id, string Name, string Address);

Then, add a new Repository.cs file and add an IRepository interface in it:

public interface IRepository<T>
{
    void Add(T entity);
    void Delete(T entity);
    IEnumerable<T> GetAll();
}

The next step is to create the generic class Repository that implements this interface:

public class Repository<T> : IRepository<T> 
{
    private readonly List<T> _entities = new List<T>();
    public void Add(T entity)
    {
        _entities.Add(entity);
        Console.WriteLine("Adding {0}", entity);
    }

    public void Delete(T entity)
    {
        _entities.Remove(entity);
        Console.WriteLine("Deleting {0}", entity);
    }

    public IEnumerable<T> GetAll()
    {
        Console.WriteLine("Getting entities");
        foreach (var entity in _entities)
        {
            Console.WriteLine($"  {entity}");
        }
        return _entities;
    }
}

As you can see, our repository class is a simple class that will store the entities in a list, delete and retrieve them.

With this class created, we can add the code to use it in Program.cs:

Console.WriteLine("***\r\n Begin program\r\n");
var customerRepository = new Repository<Customer>();
var customer = new Customer(1, "John Doe", "1 Main Street");
customerRepository.Add(customer);
customerRepository.GetAll();
customerRepository.Delete(customer);
customerRepository.GetAll();
Console.WriteLine("\r\nEnd program\r\n***");

If you run this program, you will see something like this:

Now, let’s say we want to implement logging to this class, and have a log entry for every time it enters a method and another entry when it exits. We could do that manually, but it would be cumbersome to add logging before and after every method.

Using the DispatchProxy class, we can implement a proxy that will add logging to any class that implements the IRepository interface. Create a new file RepositoryLoggerProxy.cs and add this code:

using System.Reflection;

class RepositoryLogger<T> : DispatchProxy where T : class
{
    T? _decorated;

    public T? Create(T decorated)
    {
        var proxy = Create<T, RepositoryLogger<T>>() as RepositoryLogger<T>;
        if (proxy != null)
        {
            proxy._decorated = decorated;
        }
        return proxy as T;
    }


    protected override object? Invoke(MethodInfo? methodInfo, object?[]? args)
    {
        if (methodInfo == null)
        {
            return null;
        }

        Log($"Entering {methodInfo.Name}");
        try
        {
            var result = methodInfo.Invoke(_decorated, args);
            Log($"Exiting {methodInfo.Name}");
            return result;
        }
        catch
        {
            Log($"Error {methodInfo.Name}");
            throw;
        }
    }

    private static void Log(string msg)
    {
        Console.ForegroundColor = msg.StartsWith("Entering") ? ConsoleColor.Blue :
            msg.StartsWith("Exiting") ? ConsoleColor.Green : ConsoleColor.Red;
        Console.WriteLine(msg);
        Console.ResetColor();
    }
}

The RepositoryLogger class inherits from DispatchProxy and has a Create method that will create an instance of a class that implements the interface that’s decorated. When we call the methods of this class, they are intercepted by the overriden Invoke method and we can add the logging before and after executing the method.

To use this new class, we can use something like:

Console.WriteLine("***\r\n Begin program\r\n");
var customerRepository = new Repository<Customer>();
var customerRepositoryLogger = new RepositoryLogger<IRepository<Customer>>().Create(customerRepository);
if (customerRepositoryLogger == null)
{
    return;
}
var customer = new Customer(1, "John Doe", "1 Main Street");
customerRepositoryLogger.Add(customer);
customerRepositoryLogger.GetAll();
customerRepositoryLogger.Delete(customer);
customerRepositoryLogger.GetAll();
Console.WriteLine("\r\nEnd program\r\n***");

Now, running the code, we get:

We have logging entering and exiting the class without having to change it. Remove logging is as simple as changing one line of code.

With this knowledge, we can extend our proxy class to do any action we want. To add actions before, after and on error is just a matter of passing them in the creation of the proxy. We can create a DynamicProxy class with this code:

using System.Reflection;

class DynamicProxy<T> : DispatchProxy where T : class
{
    T? _decorated;
    private Action<MethodInfo>? _beforeExecute;
    private Action<MethodInfo>? _afterExecute;
    private Action<MethodInfo>? _onError;
    private Predicate<MethodInfo> _shouldExecute;

    public T? Create(T decorated, Action<MethodInfo>? beforeExecute, 
        Action<MethodInfo>? afterExecute, Action<MethodInfo>? onError, 
        Predicate<MethodInfo>? shouldExecute)
    {
        var proxy = Create<T, DynamicProxy<T>>() as DynamicProxy<T>;
        if (proxy == null)
        {
            return null;
        }
        proxy._decorated = decorated;
        proxy._beforeExecute = beforeExecute;
        proxy._afterExecute = afterExecute;
        proxy._onError = onError;
        proxy._shouldExecute = shouldExecute ?? (s => true);
        return proxy as T;
    }

    protected override object? Invoke(MethodInfo? methodInfo, object?[]? args)
    {
        if (methodInfo == null)
        {
            return null;
        }
        if (!_shouldExecute(methodInfo))
        {
            return null;
        }
        _beforeExecute?.Invoke(methodInfo);
        try
        {
            var result = methodInfo.Invoke(_decorated, args);
            _afterExecute?.Invoke(methodInfo);
            return result;
        }
        catch
        {
            _onError?.Invoke(methodInfo);
            throw;
        }
    }
}

In the Create method, we pass the actions we want to execute before after and on error after each method. We can also pass a predicate to filter the methods we don’t want to execute. To use this new class, we can do something like this:

Console.WriteLine("***\r\n Begin program\r\n");
var customerRepository = new Repository<Customer>();
var customerRepositoryLogger = new DynamicProxy<IRepository<Customer>>().Create(customerRepository,
    s => Log($"Entering {s.Name}"),
    s => Log($"Exiting {s.Name}"),
    s => Log($"Error {s.Name}"),
    s => s.Name != "GetAll");
if (customerRepositoryLogger == null)
{
    return;
}
var customer = new Customer(1, "John Doe", "1 Main Street");
customerRepositoryLogger.Add(customer);
customerRepositoryLogger.GetAll();
customerRepositoryLogger.Delete(customer);
customerRepositoryLogger.GetAll();
Console.WriteLine("\r\nEnd program\r\n***");

static void Log(string msg)
{
    Console.ForegroundColor = msg.StartsWith("Entering") ? ConsoleColor.Blue :
        msg.StartsWith("Exiting") ? ConsoleColor.Green : ConsoleColor.Red;
    Console.WriteLine(msg);
    Console.ResetColor();
}

Executing this code will show something like:

Note that, with this code, the method GetAll isn’t executed, as it was filtered by the predicate.

As you can see, this is a very powerful class, as it can implement many different aspects for any interface (the DispatchProxy class only works with interfaces). For example, if I want to create my own mocking framework, where I don’t execute any method of a class, I can change the code of the Invoke method to

protected override object? Invoke(MethodInfo? methodInfo, object?[]? args)
{
    if (methodInfo == null)
    {
        return null;
    }
    _beforeExecute?.Invoke(methodInfo);
    try
    {
        object? result = null;
        if (_shouldExecute(methodInfo))
        {
            result = methodInfo.Invoke(_decorated, args);
        }
        _afterExecute?.Invoke(methodInfo);
        return result;
    }
    catch
    {
        _onError?.Invoke(methodInfo);
        throw;
    }
}

And create the proxy with something like this:

var customerRepositoryLogger = new DynamicProxy<IRepository<Customer>>().Create(customerRepository,
    s => Log($"Entering {s.Name}"),
    s => Log($"Exiting {s.Name}"),
    s => Log($"Error {s.Name}"),
    s => false);

In this case, the real functions won’t be called, just the methods before and after the call:

As you can see, the DispatchProxy class allows the creation of powerful classes that add aspects to your existing classes, without having to change them. With the DynamicProxy class you just have to add the actions to execute and the filter for the functions to be executed.

All the source code for this article is at https://github.com/bsonnino/DynamicProxy

When you are developing a new project and need to store settings for it, the first thing that comes to mind is to use the Appsettings.json file. With this file, you can store all settings in a single file and restore them easily.

For example, let’s create a console project that has three settings: Id, Name and Version. In a command line prompt, type:

dotnet new console -o SecretStorage
cd SecretStorage
code .

This will open VS Code in the folder for the new project. Add a new file named appSettings.json and add this code in it:

{
    "AppData": {
        "Id": "IdOfApp",
        "Name": "NameOfApp",
        "Version": "1.0.0.0"
    }
}

We will add a AppData class in the project:

public class AppData
{
    public string Id { get; init; }
    public string Name { get; init; }
    public string Version { get; init; }

    public override string ToString()
    {
        return $"Id: {Id}, Name: {Name}, Version: {Version}";
    }
}

To read the settings into the class, we could use JsonDeserializer from System.Text.Json:

using System.Text.Json;

var settingsText = File.ReadAllText("appsettings.json");
var settings = JsonSerializer.Deserialize<Settings>(settingsText);
Console.WriteLine(settings);

You need to define a class Settings to read the data:

public class Settings
{
    public AppData AppData { get; set; }
}

That’s fine, but let’s say you have different settings for development and production, you should have a copy of the appsettings.json with the modified values and some code like this one:

using System.Diagnostics;
using System.Text.Json;

string settingsText; 
if (Debugger.IsAttached)
{
    settingsText = File.ReadAllText("appsettings.development.json");
}
else
{
    settingsText = File.ReadAllText("appsettings.json");
}
var settings = JsonSerializer.Deserialize<Settings>(settingsText);
Console.WriteLine(settings);

Things start to become complicated. If there was a way to simplify this… In fact, yes there is. .NET provides us with the ConfigurationBuilder class. With it, you can read and merge several files to get the configuration. The following code will merge the appsettings.json and appsettings.development.json into a single class. In production, all you have to do is to remove the appsettings.development.json from the package and only the production file will be used.

To use the ConfigurationBuilder class you must add the NuGet packages Microsoft.Extensions.Configuration and Microsoft.Extensions.Configuration.Binder with

dotnet add package Microsoft.Extensions.Configuration
dotnet add package Microsoft.Extensions.Configuration.Binder

You will also have to add the Microsoft.Extensions.Configuration.Json package to use the AddJsonFile extension method.

One other thing that you will have to do is to tell msbuild to copy the settings file to the output directory. This is done by changing the csproj file, adding this clause

<ItemGroup>
  <Content Include="appsettings*.json">
    <CopyToOutputDirectory>Always</CopyToOutputDirectory>
  </Content>  
</ItemGroup>

Once you do that, this code will read the config files, merge them and print the settings:

IConfigurationRoot config = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json", false)
    .AddJsonFile("appsettings.development.json", true)
    .Build();
var appdata = config.GetSection(nameof(AppData)).Get<AppData>();
Console.WriteLine(appdata);

The second parameter in the AddJsonFile method tells that the appsettings.development.json file is optional and, if it’s not there, it wont be read. One other advantage is that I don’t need to duplicate all settings. You just need to add the overridden settings in the development file and the other ones will still be available.

Now, one more problem: let’s say we are using an API that requires a client Id and a client Secret. These values are very sensitive and they cannot be distributed. If you are using a public code repository, like GitHub, you cannot add something like this to appsettings.json and push your changes:

{
    "AppData": {
        "Id": "IdOfApp",
        "Name": "NameOfApp",
        "Version": "1.0.0.0"
    },
    "ApiData": {
        "ClientId": "ClientIdOfApp",
        "ClientSecret": "ClientSecretOfApp"
    }
}

That would be a real disaster, because your API codes would be open and you would end up with a massive bill at the end of the month. You could add these keys to appsettings.development.json and add it to the ignored files, so it wouldn’t be uploaded, but there is no guarantee that this won’t happen. Somebody could upload the file and things would be messy again.

The solution, in this case, would be to use the Secret Manager Tool. This tool allows you to store secrets in development mode, in a way that they cannot be shared to other users. This tool doesn’t encrypt any data and must only be used for development purposes. If you want to store the secrets in a safe encrypted way, you should use something like the Azure Key Vault.

To use it, you should initialize the storage with

dotnet user-secrets init

This will initialize the storage and generate a Guid for it and add it to the csproj file:

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <UserSecretsId>fc572277-3ded-4467-9c46-534a075f905b</UserSecretsId>
  </PropertyGroup>

Then, you need to add the package Microsoft.Extensions.Configuration.UserSecrets:

dotnet add package Microsoft.Extensions.Configuration.UserSecrets

We can now start utilizing the user secrets, by adding the new configuration type:

IConfigurationRoot config = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json", false)
    .AddJsonFile("appsettings.development.json", true)
    .AddUserSecrets<Settings>()
    .Build();

Then, we can add the secret data:

dotnet user-secrets set "ApiData:ClientId" "ClientIdOfApp"
dotnet user-secrets set "ApiData:ClientSecret" "ClientSecretOfApp"

As you can see, the data is flattened in order to be added to the user secrets. You can take a look at it by opening an Explorer window and going to %APPDATA%\Microsoft\UserSecrets\{guid}\secrets.json:

{
  "ApiData:ClientId": "ClientIdOfApp",
  "ApiData:ClientSecret": "ClientSecretOfApp"
}

As you can see, there isn’t any secret here, it’s just a way to store data with no possibility to share it in an open repository.

You can get the values stored with

dotnet user-secrets list

To remove some key from the store, you can use something like

dotnet user-secrets remove ClientId

And to clear all data, you can use

dotnet user-secrets clear

If you have some array data to store, you will have to flatten in the same way, using the index of the element as a part of the name. For example, if you have something like

public class Settings
{
    public AppData AppData { get; set; }
    public ApiData ApiData { get; set; }
    public string[] AllowedHosts { get; set; }
}

You can store the AllowedHosts data with

dotnet user-secrets set "AllowedHosts:0" "microsoft.com"
dotnet user-secrets set "AllowedHosts:1" "google.com"
dotnet user-secrets set "AllowedHosts:2" "amazon.com"

And you can read the settings with some code like this:

IConfigurationRoot config = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json", false)
    .AddJsonFile("appsettings.development.json", true)
    .AddUserSecrets<Settings>()
    .Build();
var settings = config.Get<Settings>();
foreach (var item in settings.AllowedHosts)
{
    Console.WriteLine(item);
}
Console.WriteLine(settings.AppData);
Console.WriteLine(settings.ApiData);

As you can see, if you need something to keep your development data safe from uploading to a public repository, you can use the user secrets in the same way you would do by using a json file. This simplifies a lot the storage of config files and allows every developer to have their own settings.

The full source code for this project is at https://github.com/bsonnino/SecretStorage

Sometimes, we need to get our display disposition to position windows on them in specific places. The usual way to do it in .NET is to use the Screen class, with a code like this one:

internal record Rect(int X, int Y, int Width, int Height);
internal record Display(string DeviceName, Rect Bounds, Rect WorkingArea, double ScalingFactor);

private void InitializeDisplayCanvas()
{
    
    var minX = 0;
    var minY = 0;
    var maxX = 0;
    var maxY = 0;
    foreach(var screen in Screen.AllScreens)
    {
        if (minX > screen.WorkingArea.X)
            minX = screen.WorkingArea.X;
        if (minY > screen.WorkingArea.Y)
            minY = screen.WorkingArea.Y;
        if (maxX < screen.WorkingArea.X+screen.WorkingArea.Width)
            maxX = screen.WorkingArea.X+screen.WorkingArea.Width;
        if (maxY < screen.WorkingArea.Y+screen.WorkingArea.Height)
            maxY = screen.WorkingArea.Y+screen.WorkingArea.Height;

        _displays.Add(new Display(screen.DeviceName, screen.Bounds, screen.WorkingArea, 1.0));
    }
    DisplayCanvas.Width = maxX - minX;
    DisplayCanvas.Height = maxY - minY;
    DisplayCanvas.RenderTransform = new TranslateTransform(-minX, -minY);
    var background = new System.Windows.Shapes.Rectangle
    {
        Width = DisplayCanvas.Width,
        Height = DisplayCanvas.Height,
        Fill = new SolidColorBrush(System.Windows.Media.Color.FromArgb(1,242,242,242)),
    };
    Canvas.SetLeft(background, minX);
    Canvas.SetTop(background, minY);
    DisplayCanvas.Children.Add(background);
    var numDisplay = 0;
    foreach (var display in _displays)
    {
        numDisplay++;
        var border = new Border
        {
            Width = display.WorkingArea.Width,
            Height = display.WorkingArea.Height,
            Background = System.Windows.Media.Brushes.DarkGray,
            CornerRadius = new CornerRadius(30)
        };
        var text = new TextBlock
        {
            Text = numDisplay.ToString(),
            FontSize = 200,
            FontWeight = FontWeights.Bold,  
            HorizontalAlignment = System.Windows.HorizontalAlignment.Center,
            VerticalAlignment = VerticalAlignment.Center,
        };
        border.Child = text;
        Canvas.SetLeft(border, display.WorkingArea.Left);
        Canvas.SetTop(border, display.WorkingArea.Top);
        DisplayCanvas.Children.Add(border);
    }

}

If you run the code, you will see something like this:

The monitors aren’t contiguous, as you would expect. But, the worst is that it doesn’t work well. If you try to position a window in the center of the middle monitor, with a code like this:

private void Button_Click(object sender, RoutedEventArgs e)
{
    var display = _displays[0];
    var window = new NewWindow
    {
        Top = display.WorkingArea.Top + (display.WorkingArea.Height - 200) / 2,
        Left = display.WorkingArea.Left + (display.WorkingArea.Width - 200) / 2,
    };
    window.Show();
}

You will get something like this:

As you can see, the window is far from centered. And why is that? The reason for these problems are the usage of high DPI. When you set the displays in you system, you set the resolution and the scaling factor:

In my setup, I have three monitors:

  • 1920×1080 125%
  • 3840×2160 150%
  • 1920×1080 100%

This scale factor is not taken in account when you are enumerating the displays and, when I am enumerating them, I have no way of getting this value. That way, everything is positioned in the wrong place. It would work fine if all monitors had a scaling factor of 100%, but most of the time that’s not true.

Researching for High DPI WPF, I came to this page, which shows the use of the appmanifest, so I gave it a try. I added a new item, Application Manifest and uncommented these lines:

<application xmlns="urn:schemas-microsoft-com:asm.v3">
    <windowsSettings>
      <dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">true</dpiAware>
      <longPathAware xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">true</longPathAware>
    </windowsSettings>
  </application>

And nothing happened. Then I added this line:

<windowsSettings>
  <dpiAwareness xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitor</dpiAwareness>
  <dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">true</dpiAware>
  <longPathAware xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">true</longPathAware>
</windowsSettings>

Running the project, I got the correct display settings:

But the secondary screen is positioned in the wrong place. If I change the dpiAwareness clause to Unaware, I get the wrong display disposition, but the window is positioned at the center!

We need to get the scale factor for each monitor, to get the correct values in both cases.

Before going further we must notice that this code has also another problem: it’s a WPF app that is using a Winforms class: Screen is declared in System.Windows.Forms and there is no equivalent in WPF. To use, it you must add UseWindowsForms in the csproj:

<Project Sdk="Microsoft.NET.Sdk">
	<PropertyGroup>
		<OutputType>WinExe</OutputType>
		<TargetFramework>net6.0-windows</TargetFramework>
		<Nullable>enable</Nullable>
		<UseWPF>true</UseWPF>
		<UseWindowsForms>true</UseWindowsForms>
	</PropertyGroup>
</Project>

That is something I really don’t like to do: use Winforms in a WPF project. If you want to check the project, the branch I’ve used is here.

So, I tried to find a way to enumerate the displays in WPF and have the right scaling factor, and I found two ways: query the registry or use Win32 API. Yes, the API that is available since the beginning of Windows, and it’s still there.

We could go to http://pinvoke.net/ to get the signatures we need for our project. This is a great site and a nice resource when you want to use Win32 APIs in C#, but we’ll use another resource: CsWin32, a nice source generator that generates the P/Invokes for us. I have already written an article about it, if you didn’t see it, you should check it out.

For that, we should install the NuGet package Microsoft.Windows.CsWin32 (be sure to check the pre-release box). Once installed, you must create a text file and name it NativeMethods.txt. There, we will add the names of all the methods and structures we need.

The first function we need is EnumDisplayMonitors, which we add there. With that, we can use it in our method:

private unsafe void InitializeDisplayCanvas()
{
    Windows.Win32.PInvoke.EnumDisplayMonitors(null, null, enumProc, IntPtr.Zero);
}

We are passing all parameters as null, except for the third one, which is the callback function. As I don’t know the parameters of this function, I will let Visual Studio create it for me. Just press Ctrl+. and Generate method enumProc and Visual Studio will generate the method for us:

private unsafe BOOL enumProc(HMONITOR param0, HDC param1, RECT* param2, LPARAM param3)
{
    throw new NotImplementedException();
}

We can change the names of the parameters and add the return value true to continue the enumeration. This function will be called for each monitor in the system and will pass in the first parameter the handle of the monitor. With that handle we can determine its properties with GetMonitorInfo (which we add in NativeMethods.txt) and use it to get the monitor information. This function uses a MONITORINFO struct as a parameter, and we must declare it before calling the function:

private unsafe BOOL enumProc(HMONITOR monitor, HDC hdc, RECT* clipRect, LPARAM data)
{
    var mi = new MONITORINFO
    {
        cbSize = (uint)Marshal.SizeOf(typeof(MONITORINFO))
    };
    if (Windows.Win32.PInvoke.GetMonitorInfo(monitor, ref mi))
    {

    }
    return true;
}

You have noticed that we’ve set the size of the structure before passing it to the function. This is common to many WinApi functions and forgetting to set this member or setting it with a wrong value is a common source of bugs.

MONITORINFO is declared in Windows.Win32.Graphics.Gdi, which is included in the usings for the file. Now, mi has the data for the monitor:

internal partial struct MONITORINFO
{
	internal uint cbSize;
	internal winmdroot.Foundation.RECT rcMonitor;
	internal winmdroot.Foundation.RECT rcWork;
	internal uint dwFlags;
}

But it doesn’t have the name of the monitor. This was solved by using the structure MONITORINFOEX, which has the name of the monitor. There is a catch, here: although GetMonitorInfo has an overload that uses the MONITORINFOEX structure, it’s not declared in CsWin32, so we must do a trick, here:

private unsafe BOOL enumProc(HMONITOR monitor, HDC hdc, RECT* clipRect, LPARAM data)
{
    var mi = new MONITORINFOEXW();
    mi.monitorInfo.cbSize = (uint)Marshal.SizeOf(typeof(MONITORINFOEXW));
    
    if (Windows.Win32.PInvoke.GetMonitorInfo(monitor, (MONITORINFO*) &mi))
    {

    }
    return true;
}

MONITORINFOEX is not declared automatically, you must use the explicit wide version, MONITORINFOEXW and add the impport to NativeMethods.txt. To use it, you must create the structure, initialize it and then cast the pointer to a pointer of MONITORINFO. It’s not the most beautiful code, but it works. Now we have the code to enumerate the displays:

public class DisplayList
{
    private List<Display> _displays = new List<Display>();

    public DisplayList()
    {
        QueryDisplayDevices();
    }

    public List<Display> Displays => _displays;

    private unsafe void QueryDisplayDevices()
    {
        PInvoke.EnumDisplayMonitors(null, null, enumProc, IntPtr.Zero);
    }

    private unsafe BOOL enumProc(HMONITOR monitor, HDC hdc, RECT* clipRect, LPARAM data)
    {
        var mi = new MONITORINFOEXW();
        mi.monitorInfo.cbSize = (uint)Marshal.SizeOf(typeof(MONITORINFOEXW));

        if (PInvoke.GetMonitorInfo(monitor, (MONITORINFO*)&mi))
        {
            var display = new Display(mi.szDevice.ToString(),
                new Rect(mi.monitorInfo.rcMonitor.left, mi.monitorInfo.rcMonitor.top, 
                    mi.monitorInfo.rcMonitor.Width, mi.monitorInfo.rcMonitor.Height),
                new Rect(mi.monitorInfo.rcWork.left, mi.monitorInfo.rcWork.top, 
                    mi.monitorInfo.rcWork.Width, mi.monitorInfo.rcWork.Height),
                1);
            _displays.Add(display);
        }
        return true;
    }
}

private void InitializeDisplayCanvas()
{
    var displayList = new DisplayList();
    _displays = displayList.Displays;
    InitializeCanvasWithDisplays();
}

private void InitializeCanvasWithDisplays()
{
    var minX = 0;
    var minY = 0;
    var maxX = 0;
    var maxY = 0;
    foreach (var display in _displays)
    {
        if (minX > display.WorkingArea.X)
            minX = display.WorkingArea.X;
        if (minY > display.WorkingArea.Y)
            minY = display.WorkingArea.Y;
        if (maxX < display.WorkingArea.X + display.WorkingArea.Width)
            maxX = display.WorkingArea.X + display.WorkingArea.Width;
        if (maxY < display.WorkingArea.Y + display.WorkingArea.Height)
            maxY = display.WorkingArea.Y + display.WorkingArea.Height;
    }
    DisplayCanvas.Width = maxX - minX;
    DisplayCanvas.Height = maxY - minY;
    DisplayCanvas.RenderTransform = new TranslateTransform(-minX, -minY);
    var background = new System.Windows.Shapes.Rectangle
    {
        Width = DisplayCanvas.Width,
        Height = DisplayCanvas.Height,
        Fill = new SolidColorBrush(System.Windows.Media.Color.FromArgb(1, 242, 242, 242)),
    };
    Canvas.SetLeft(background, minX);
    Canvas.SetTop(background, minY);
    DisplayCanvas.Children.Add(background);
    var numDisplay = 0;
    foreach (var display in _displays)
    {
        numDisplay++;
        var border = new Border
        {
            Width = display.WorkingArea.Width,
            Height = display.WorkingArea.Height,
            Background = System.Windows.Media.Brushes.DarkGray,
            CornerRadius = new CornerRadius(30)
        };
        var text = new TextBlock
        {
            Text = numDisplay.ToString(),
            FontSize = 200,
            FontWeight = FontWeights.Bold,
            HorizontalAlignment = System.Windows.HorizontalAlignment.Center,
            VerticalAlignment = VerticalAlignment.Center,
        };
        border.Child = text;
        Canvas.SetLeft(border, display.WorkingArea.X);
        Canvas.SetTop(border, display.WorkingArea.Y);
        DisplayCanvas.Children.Add(border);
    }
}
private void Button_Click(object sender, RoutedEventArgs e)
{
    var display = _displays[0];
    var window = new NewWindow
    {
        Top = display.Bounds.Y + (display.Bounds.Height - 200) / display.ScalingFactor / 2,
        Left = display.Bounds.X + (display.Bounds.Width - 200) / display.ScalingFactor / 2,
    };
    window.Show();
}

If you run this code, you’ll get the same result we’ve got with the previous version, but at least we don’t have to include WinForms here. But we can go a step further and get the scale for the monitors. For that, we must use the EnumDisplaySettings function, like this:

if (PInvoke.GetMonitorInfo(monitor, (MONITORINFO*)&mi))
{
    var dm = new DEVMODEW
    {
        dmSize = (ushort)Marshal.SizeOf(typeof(DEVMODEW))
    };
    PInvoke.EnumDisplaySettings(mi.szDevice.ToString(), ENUM_DISPLAY_SETTINGS_MODE.ENUM_CURRENT_SETTINGS, ref dm);

    var scalingFactor = Math.Round((double)dm.dmPelsWidth / mi.monitorInfo.rcMonitor.Width, 2);
    var display = new Display(mi.szDevice.ToString(),
        new Rect(mi.monitorInfo.rcMonitor.left, mi.monitorInfo.rcMonitor.top, 
            (int)(mi.monitorInfo.rcMonitor.Width * scalingFactor), (int)(mi.monitorInfo.rcMonitor.Height * scalingFactor)),
        new Rect(mi.monitorInfo.rcWork.left, mi.monitorInfo.rcWork.top, 
            (int)(mi.monitorInfo.rcWork.Width * scalingFactor), (int)(mi.monitorInfo.rcWork.Height * scalingFactor)),
        scalingFactor);
    _displays.Add(display);
}

Now, if you run the code, you’ll see that it runs fine and positions the window in the center of the display.

We now have a WPF application that can detect all installed displays and position a window correctly in any display. The key to that is, besides using the Win32 API to get the display information, to use the App Manifest dpiAwareness setting to Unaware. If you change to any other setting, you will get the wrong positions, because the scaling factors will only be correct when the Unaware setting is used.

The full source code for this article is at https://github.com/bsonnino/EnumeratingDisplays

When you are using the MVVM pattern, at some time, you have to send data between ViewModels. For example, a detail ViewModel must tell the collection ViewModel that the current item should be deleted, or when you have a main view that opens details views and keeps track of them and must be acknowledged of any of them closing.
This is a very common pattern and usually is solved by coupling the two ViewModels.
That’s a solution, but not the best one, because coupling makes difficult maintenance and testing. For example, we’ll use the WPF project at https://github.com/bsonnino/MessengerToolkit/tree/Original.

This is a WPF project with a main window that shows a grid with all the customers:

When you click the button at the right, it opens a secondary window with the details:

This project is composed by two ViewModels, MainViewModel and CustomerViewModel. MainViewModel serves the main view and performs all actions for the buttons: Filter the data, Add a new customer and Save the data:

public partial class MainViewModel : ObservableObject
{
    private readonly ICustomerRepository _customerRepository;
    private readonly INavigationService _navigationService;
    
    public MainViewModel(ICustomerRepository customerRepository, INavigationService navigationService)
    {
        _customerRepository = customerRepository ?? 
                              throw new ArgumentNullException("customerRepository");
        Customers = new ObservableCollection<CustomerViewModel>(
            _customerRepository.Customers.Select(c => new CustomerViewModel(c)));
        _navigationService = navigationService;
    }

    [ObservableProperty]
    private ObservableCollection<CustomerViewModel> _customers;

    [ObservableProperty]
    private int _windowCount;

    [RelayCommand]
    private void Add()
    {
        var customer = new Customer();
        _customerRepository.Add(customer);
        var vm = new CustomerViewModel(customer);
        Customers.Add(vm);
        _navigationService.Navigate(vm);
    }

    [RelayCommand]
    private void Save()
    {
        _customerRepository.Commit();
    }

    [RelayCommand]
    private void Search(string textToSearch)
    {
        var coll = CollectionViewSource.GetDefaultView(Customers);
        if (!string.IsNullOrWhiteSpace(textToSearch))
            coll.Filter = c => ((CustomerViewModel)c).Country?.ToLower().Contains(textToSearch.ToLower()) == true;
        else
            coll.Filter = null;
    }

    [RelayCommand]
    private void ShowDetails(CustomerViewModel vm)
    {
        _navigationService.Navigate(vm);
        WindowCount++;
    }
}

It uses the features available in version 8 of the MVVM toolkit. In this code, we can see two things:

  • The ViewModel maintains the open windows count. When you click on the button to show the details, it will increase the window count. The problem here is to decrease the count once the window is closed. Right now, the count only increases 😃
  • To open the detail window, we could use something like:
private void ShowDetails(CustomerViewModel vm)
{
    var detailWindow = new Detail { DataContext = vm };
    detailWindow.Show();
    WindowCount++;
}

This approach has two flaws:

  • We are coupling the ViewModel to the Detail view
  • We are calling View details in the ViewModel

This makes this code to be completely untestable and defeats all purpose of the MVVM pattern. So, we must find another solution. What I devised is a NavigationService, that will open a Window, depending on the type of the ViewModel that is passed to the Navigate method. This service is very simple, and has only one method,Navigate:

public interface INavigationService
{
    void Navigate(object arg);
}

public class NavigationService : INavigationService
{
    private readonly Dictionary<Type, Type> viewMapping = new()
    {
        [typeof(MainViewModel)] = typeof(MainWindow),
        [typeof(CustomerViewModel)] = typeof(Detail),
    };

    public void Navigate(object arg)
    {
        Type vmType = arg.GetType();
        if (viewMapping.ContainsKey(vmType))
        {
            var windowType = viewMapping[vmType];
            var window = (System.Windows.Window)Activator.CreateInstance(windowType);
            window.DataContext = arg;
            window.Show();
        }
    }
}

We’ve defined an interface, INavigationService, and created a class that implements it, NavigationService. This class has a dictionary with all the ViewModel types that have a corresponding window type, with their corresponding window type as the value of the item.

When we call Navigate and pass the instance of the ViewModel, the method will check its type, verify if it exists in the dictionary and instantiate the window, setting the VM as its DataContext and opening the Window. That way, we decouple the ViewModel from the views. The navigation service is injected in the constructor of MainViewModel by the use of dependency injection.

In App.xaml.cs we are registering the services and the VM. That way, we get the right data injected to the main VM:

public IServiceProvider Services { get; }

private static IServiceProvider ConfigureServices()
{
    var services = new ServiceCollection();

    services.AddSingleton<INavigationService, NavigationService>();
    services.AddSingleton<ICustomerRepository, CustomerRepository>();
    services.AddSingleton<MainViewModel>();

    return services.BuildServiceProvider();
}

One note is ShowDetails, which in fact is a command that should be sent to the CustomerViewModel, because it’s tied to every CustomerViewModel in the grid. What we did is to tie it to the ViewModel of the main view with:

<DataGridTemplateColumn>
    <DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
            <Button Command="{Binding DataContext.ShowDetailsCommand, 
                RelativeSource={RelativeSource AncestorType=Window}}" 
                    CommandParameter="{Binding}" ToolTip="Details">
                <TextBlock Text="" FontFamily="Segoe UI Symbol" />
            </Button>
        </DataTemplate>
    </DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>

We set the command to ShowDetailsCommand in the datacontext of the parent window, and pass the current CustomerViewModel as a parameter. That suffices to do the trick.

Now, onto our problems:

  • The Window Closing event is just that – an event, not a command and there is no direct way to handle it in the ViewModel
  • Even if we can handle it in the ViewModel, there is no direct way to warn the main viewmodel about the change
  • When we click the Remove button in the Detail view, we cannot remove the item from the list, because the list is on the Main ViewModel, which is not accessible from the Customer ViewModel

For the first problem, we could handle the Closing event in the code behind of the Detail view:

private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
    var vm = DataContext as CustomerViewModel;
    vm.ClosingCommand.Execute(null);
}

or, better, send the closing signal directly to the main viewmodel:

private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
    var vm = App.Current.MainVM;
    vm.ClosingCommand.Execute(null);
}

This would work fine, but purists would say that it’s a code smell. So, let’s do it in the MVVM way. For that, we must install the Microsoft.Xaml.Behaviors.WPF NuGet package, that has some behaviors we will need in the detail window:

<Window x:Class="CustomerApp.Detail"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:b="http://schemas.microsoft.com/xaml/behaviors"
        mc:Ignorable="d"
        Height="440"
        Width="520"
        Background="AliceBlue">
    <b:Interaction.Triggers>
        <b:EventTrigger EventName="Closing">
            <b:InvokeCommandAction Command="{Binding ClosingCommand}" />
        </b:EventTrigger>
    </b:Interaction.Triggers>

There, we have Interaction.Triggers, to add the triggers we want. We want to use the EventTrigger, that is triggered whe an event is fired. In our case, it’s the Closing event. When it’s fired, it will invoke the command ClosingCommand in the detail window. Problem solved. No more code behind.

To solve the second and third problem, we would have to couple our Detail ViewModel to the Main ViewModel and have a variable that stores its instance. One way would be to pass the VM as parameter:

private readonly MainViewModel _mainvm;

public CustomerViewModel(Customer customer, MainViewModel mainvm)
{
    _customer = customer;
    _mainvm = mainvm;
    CustomerId = _customer.CustomerId;
    CompanyName = _customer.CompanyName;
    ContactName = _customer.ContactName;
    ContactTitle = _customer.ContactTitle;
    Address = _customer.Address;
    City = _customer.City;
    Region = _customer.Region;
    PostalCode = _customer.PostalCode;
    Country = _customer.Country;
    Phone = _customer.Phone;
    Fax = _customer.Fax;
}

One other way would be to get the App property:

private readonly MainViewModel _mainvm = App.Current.MainVM;

or to use the Service Locator to get the instance:

private readonly MainViewModel _mainvm = (MainViewModel)App.Current.Services.GetService(typeof(MainViewModel));

No matter with way we are using it, we are coupling the two viewmodels and that will make testing difficult. Passing the VM as a parameter for the constructor will be the more testable way, the other two would get us in trouble: we should have an App created for the second option and have an App created and a Service Locator in place to get our instance. But wouldn’t it be better if we could get rid of the instance of Main ViewModel and do not have them coupled?

In fact there is: Messages. Messaging is a mechanism that decouples the sender and receiver of a message, there is not even need to have a receiver for the message, or there may be many receivers from it – the sender doesn’t care.

The messenger is based on the Mediator pattern, where the Messenger class acts as the mediator for the messages. The potential receivers subscribe for receiving some kind of messages and, when some sender sends a message, the messenger will send it to the subscribers that are willing to receive that kind of message.

In the MVVM Toolkit, you can define messengers that implement the IMessenger interface, but the class WeakReferenceMessenger exposes a Default property, which offers a thread safe implementation for this interface, which makes it easier to implement. This class has a Register method, which will register the client for some message type and will pass the callback function that will be called when a message of the desired type is received. If you wat toknow more about that, you can take a look at https://docs.microsoft.com/en-us/windows/communitytoolkit/mvvm/messenger.

For our needs, we have to define two types of different messages: a message for when a customer should be deleted and the other one, when the window is closed:

public class WindowClosedMessage: ValueChangedMessage<CustomerViewModel>
{
    public WindowClosedMessage(CustomerViewModel vm) : base(vm)
    {

    }
}

public class ViewModelDeletedMessage : ValueChangedMessage<CustomerViewModel>
{
    public ViewModelDeletedMessage(CustomerViewModel vm) : base(vm)
    {

    }
}

The messages inherit from ValueChangedMessage. This kind of messages are sent to the recipients and the execution goes on. If, on the other way, the Sender needs an answer from the receiver, it will send a message of the RequestMessage type. In this case, the sender will wait the response that the receiver will send using the Reply method of the message. In our case, both messages don’t need a reply, so we use the ValueChangedMessage type. We must create two different types of message, because we want different processing for each one.

Now, we must send the messages in CustomerViewModel, in the Closing and Delete commands:

[RelayCommand]
private void Delete()
{
    WeakReferenceMessenger.Default.Send(new ViewModelDeletedMessage(this));
}

[RelayCommand]
private void Closing()
{
    WeakReferenceMessenger.Default.Send(new WindowClosedMessage(this));
}

The processing for these messages is done in the MainViewModel class:

public MainViewModel(ICustomerRepository customerRepository, INavigationService navigationService)
{
    _customerRepository = customerRepository ?? 
                          throw new ArgumentNullException("customerRepository");
    Customers = new ObservableCollection<CustomerViewModel>(
        _customerRepository.Customers.Select(c => new CustomerViewModel(c)));
    _navigationService = navigationService;
    WeakReferenceMessenger.Default.Register<WindowClosedMessage>(this, (r, m) =>
    {
        WindowCount--;
    });
    WeakReferenceMessenger.Default.Register<ViewModelDeletedMessage>(this, (r, m) =>
    {
        DeleteCustomer(m.Value);
    });
}

private void DeleteCustomer(CustomerViewModel vm)
{
    Customers.Remove(vm);
    var deletedCustomer = _customerRepository.Customers.FirstOrDefault(c => c.CustomerId == vm.CustomerId);
    if (deletedCustomer != null)
    {
        _customerRepository.Remove(deletedCustomer);
    }
}

Now, when you run the app, you will see that when you close the detail window, the window count decreases. We could even keep a list of open windows, with some code like this:

[ObservableProperty]
private ObservableCollection<CustomerViewModel> _openWindows = new ObservableCollection<CustomerViewModel>();

public MainViewModel(ICustomerRepository customerRepository, INavigationService navigationService)
{
    _customerRepository = customerRepository ?? 
                          throw new ArgumentNullException("customerRepository");
    Customers = new ObservableCollection<CustomerViewModel>(
        _customerRepository.Customers.Select(c => new CustomerViewModel(c)));
    _navigationService = navigationService;
    WeakReferenceMessenger.Default.Register<WindowClosedMessage>(this, (r, m) =>
    {
        WindowCount--;
        _openWindows.Remove(m.Value);
    });
    WeakReferenceMessenger.Default.Register<ViewModelDeletedMessage>(this, (r, m) =>
    {
        DeleteCustomer(m.Value);
    });
}

....

[RelayCommand]
private void ShowDetails(CustomerViewModel vm)
{
    _navigationService.Navigate(vm);
    WindowCount++;
    _openWindows.Add(vm);
}

When you click the Delete button in the detail window, the corresponding item is deleted in the main list, but the window remains open, and it should not ne there anymore, as the corresponding customer does not exist anymore. We could use some code behind to close the window, but I’d prefer another approach: change the navigation service to allow closing the windows. For that, we should keep the lists of open windows and have another method, Close, to close the selected window:

public interface INavigationService
{
    void Navigate(object arg);
    void Close(object arg);
}

public class NavigationService : INavigationService
{
    private readonly Dictionary<Type, Type> viewMapping = new()
    {
        [typeof(MainViewModel)] = typeof(MainWindow),
        [typeof(CustomerViewModel)] = typeof(Detail),
    };

    private readonly Dictionary<object, List<System.Windows.Window>> _openWindows = new();

    public void Navigate(object arg)
    {
        Type vmType = arg.GetType();
        if (viewMapping.ContainsKey(vmType))
        {
            var windowType = viewMapping[vmType];
            var window = (System.Windows.Window)Activator.CreateInstance(windowType);
            window.DataContext = arg;
            if (!_openWindows.ContainsKey(arg))
            {
                _openWindows.Add(arg, new List<System.Windows.Window>());
            }
            _openWindows[arg].Add(window);
            window.Show();
        }
    }

    public void Close(object arg)
    {
        if (_openWindows.ContainsKey(arg))
        {
            foreach(var window in _openWindows[arg])
            {
                window.Close();
            }
        }
    }
}

In this case, we keep a list of windows open for each customer. If we try to open another window for the same customer, the corresponding window will be added to the corresponding list. When the close method is called, all windows for that customer will be closed at once. The DeleteCustomer in MainViewModel becomes

private void DeleteCustomer(CustomerViewModel vm)
{
    Customers.Remove(vm);
    var deletedCustomer = _customerRepository.Customers.FirstOrDefault(c => c.CustomerId == vm.CustomerId);
    if (deletedCustomer != null)
    {
        _customerRepository.Remove(deletedCustomer);
    }
    _navigationService.Close(vm);
}

And now everything works fine.

But, what about testing? We did all this to ease testing, let’s test the VMs. In the Test project, let’s create a class CustomerViewModelTests.cs and add some tests:

[TestClass]
public class CustomerViewModelTests
{
    [TestMethod]
    public void Constructor_NullCustomer_ShouldThrow()
    {
        Action act = () => new CustomerViewModel(null);

        act.Should().Throw<ArgumentNullException>()
            .Where(e => e.Message.Contains("customer"));
    }
}

If we run this test, we’ll see it doesn’t pass. We haven’t made a check for null customers, so we have to change that in the code:

public CustomerViewModel(Customer customer)
{
    if (customer == null)
        throw new ArgumentNullException("customer");
    _customer = customer;
    CustomerId = _customer.CustomerId;
    CompanyName = _customer.CompanyName;
    ContactName = _customer.ContactName;
    ContactTitle = _customer.ContactTitle;
    Address = _customer.Address;
    City = _customer.City;
    Region = _customer.Region;
    PostalCode = _customer.PostalCode;
    Country = _customer.Country;
    Phone = _customer.Phone;
    Fax = _customer.Fax;
}

The test now passes and we can continue. For the next test, I’d like to have a customer filled with data, and FakeItEasy (which we are using for our test mocks) fills the data with nulls and that’s not what we want. In this article I show how to use Bogus to create fake data. We can use AutoBogus.FakeItEasy to integrate it with FakeItEasy. Install the package AutoBogus.FakeItEasy and let’s create the second test

[TestMethod]
public void Constructor_ShouldSetFields()
{
    var customer = AutoFaker.Generate<Customer>();
    var customerVM = new CustomerViewModel(customer);
    customerVM.CustomerId.Should().Be(customer.CustomerId);
    customerVM.CompanyName.Should().Be(customer.CompanyName);
    customerVM.ContactName.Should().Be(customer.ContactName);
    customerVM.ContactTitle.Should().Be(customer.ContactTitle);
    customerVM.Address.Should().Be(customer.Address);
    customerVM.City.Should().Be(customer.City);
    customerVM.Region.Should().Be(customer.Region);
    customerVM.PostalCode.Should().Be(customer.PostalCode);
    customerVM.Country.Should().Be(customer.Country);
    customerVM.Phone.Should().Be(customer.Phone);
    customerVM.Fax.Should().Be(customer.Fax);
}

For the third test, we will be testing the Delete command, and we want to check if if sends the correct message. For that, we must install the _Community.Toolkit.MVVM in the test project. Then, we can create the tests for the two commands:

[TestMethod]
public void DeleteCommand_ShouldSendMessageWithVM()
{
    var customer = AutoFaker.Generate<Customer>();
    var customerVM = new CustomerViewModel(customer);
    object callbackResponse = null;
    var waitEvent = new AutoResetEvent(false);
    WeakReferenceMessenger.Default.Register<ViewModelDeletedMessage>(this, (r, m) =>
    {
        callbackResponse = customerVM;
        waitEvent.Set();
    });
    customerVM.DeleteCommand.Execute(null);
    waitEvent.WaitOne(100);
    callbackResponse.Should().Be(customerVM);
}

[TestMethod]
public void CloseCommand_ShouldSendMessageWithVM()
{
    var customer = AutoFaker.Generate<Customer>();
    var customerVM = new CustomerViewModel(customer);
    object callbackResponse = null;
    var waitEvent = new AutoResetEvent(false);
    WeakReferenceMessenger.Default.Register<WindowClosedMessage>(this, (r, m) =>
    {
        callbackResponse = customerVM;
        waitEvent.Set();
    });
    customerVM.ClosingCommand.Execute(null);
    waitEvent.WaitOne(100);
    callbackResponse.Should().Be(customerVM);
}

These two tests have a particularity: the result is set in the callback, so we may not have it immediately, so we can have a flaky test if we don’t wait some time before testing the value – sometimes it may pass and sometimes not. For this issue, I’ve used an AutoResetEvent to be set when the callback is called and wait 100ms to see if it’s called. That makes the trick.

For the MainViewModel tests, we can do this:

[TestClass]
public class MainViewModelTests
{
    [TestMethod]
    public void Constructor_NullRepository_ShouldThrow()
    {
        Action act = () => new MainViewModel(null, null);

        act.Should().Throw<ArgumentNullException>()
            .Where(e => e.Message.Contains("customerRepository"));
    }

    [TestMethod]
    public void Constructor_Customers_ShouldHaveValue()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);

        vm.Customers.Count.Should().Be(customers.Count);
    }

    [TestMethod]
    public void AddCommand_ShouldAddInRepository()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        vm.AddCommand.Execute(null);
        A.CallTo(() => repository.Add(A<Customer>._)).MustHaveHappened();
    }

    [TestMethod]
    public void AddCommand_ShouldAddInCollection()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        vm.AddCommand.Execute(null);
        vm.Customers.Count.Should().Be(11);
    }

    [TestMethod]
    public void AddCommand_ShouldCallNavigate()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        vm.AddCommand.Execute(null);
        A.CallTo(() => navigationService.Navigate(A<CustomerViewModel>.Ignored)).MustHaveHappened();
    }

    [TestMethod]
    public void SaveCommand_ShouldCommitInRepository()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var vm = new MainViewModel(repository, navigationService);
        vm.SaveCommand.Execute(null);
        A.CallTo(() => repository.Commit()).MustHaveHappened();
    }

    [TestMethod]
    public void SearchCommand_WithText_ShouldSetFilter()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        vm.SearchCommand.Execute("text");
        var coll = CollectionViewSource.GetDefaultView(vm.Customers);
        coll.Filter.Should().NotBeNull();
    }

    [TestMethod]
    public void SearchCommand_WithoutText_ShouldSetFilter()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        vm.SearchCommand.Execute("");
        var coll = CollectionViewSource.GetDefaultView(vm.Customers);
        coll.Filter.Should().BeNull();
    }

    [TestMethod]
    public void ShowDetailsCommand_ShouldCallNavigate()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        CustomerViewModel customerVM = vm.Customers[1];
        vm.ShowDetailsCommand.Execute(customerVM);
        A.CallTo(() => navigationService.Navigate(customerVM)).MustHaveHappened();
    }

    [TestMethod]
    public void ShowDetailsCommand_ShouldIncrementWindowCount()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        CustomerViewModel customerVM = vm.Customers[1];
        vm.ShowDetailsCommand.Execute(customerVM);
        vm.WindowCount.Should().Be(1);
    }

    [TestMethod]
    public void ShowDetailsCommand_ShouldAddToOpenWindows()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        CustomerViewModel customerVM = vm.Customers[1];
        vm.ShowDetailsCommand.Execute(customerVM);
        vm.OpenWindows.Count.Should().Be(1);
        vm.OpenWindows[0].Should().Be(customerVM);
    }

    [TestMethod]
    public void CustomerCloseCommand_ShouldDecreaseWindowCount()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        CustomerViewModel customerVM = vm.Customers[1];
        vm.ShowDetailsCommand.Execute(customerVM);
        customerVM.ClosingCommand.Execute(null);
        vm.WindowCount.Should().Be(0);
    }

    [TestMethod]
    public void CustomerCloseCommand_ShouldRemoveFromOpenWindows()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        CustomerViewModel customerVM = vm.Customers[1];
        vm.ShowDetailsCommand.Execute(customerVM);
        customerVM.ClosingCommand.Execute(null);
        vm.OpenWindows.Count.Should().Be(0);
    }

    [TestMethod]
    public void CustomerDeleteCommand_ShouldCallNavigationClose()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        CustomerViewModel customerVM = vm.Customers[1];
        vm.ShowDetailsCommand.Execute(customerVM);
        customerVM.DeleteCommand.Execute(null);
        A.CallTo(() => navigationService.Close(customerVM)).MustHaveHappened();
    }

    [TestMethod]
    public void CustomerDeleteCommand_ShouldRemoveCustomer()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        CustomerViewModel customerVM = vm.Customers[1];
        vm.ShowDetailsCommand.Execute(customerVM);
        customerVM.DeleteCommand.Execute(null);
        vm.Customers.Count.Should().Be(9);
        vm.Customers.Should().NotContain(customerVM);
    }

    [TestMethod]
    public void CustomerDeleteCommand_ShouldCallRemoveFromRepository()
    {
        var repository = A.Fake<ICustomerRepository>();
        var navigationService = A.Fake<INavigationService>();
        var customers = A.CollectionOfFake<Customer>(10);
        A.CallTo(() => repository.Customers).Returns(customers);
        var vm = new MainViewModel(repository, navigationService);
        CustomerViewModel customerVM = vm.Customers[1];
        vm.ShowDetailsCommand.Execute(customerVM);
        customerVM.DeleteCommand.Execute(null);
        A.CallTo(() => repository.Remove(A<Customer>.Ignored)).MustHaveHappened();
    }
}

If you run the tests, all will pass.

As you can see, we have decoupled the two ViewModels with the Messenger implemented in the MVVM Toolkit, our code is completely testable and it runs fine. You can check this code here.

We can still go one step further (yes, we can always do that 😃): inject the IMessenger in the constructor of the ViewModels.

In App.xaml.cs we add the registration for IMessenger:

private static IServiceProvider ConfigureServices()
{
    var services = new ServiceCollection();

    services.AddSingleton<INavigationService, NavigationService>();
    services.AddSingleton<ICustomerRepository, CustomerRepository>();
    services.AddSingleton<IMessenger>(WeakReferenceMessenger.Default);
    services.AddSingleton<MainViewModel>();

    return services.BuildServiceProvider();
}

And inject it in the constructor of MainViewModel and CustomerViewModel:

public MainViewModel(ICustomerRepository customerRepository, 
    INavigationService navigationService,
    IMessenger messenger)
{
    _customerRepository = customerRepository ?? 
                          throw new ArgumentNullException("customerRepository");
    _navigationService = navigationService ?? 
        throw new ArgumentNullException("navigationService"); 
    _messenger = messenger ??
        throw new ArgumentNullException("messenger");
    Customers = new ObservableCollection<CustomerViewModel>(
        _customerRepository.Customers.Select(c => new CustomerViewModel(c, messenger)));
    messenger.Register<WindowClosedMessage>(this, (r, m) =>
    {
        WindowCount--;
        _openWindows.Remove(m.Value);
    });
    messenger.Register<ViewModelDeletedMessage>(this, (r, m) =>
    {
        DeleteCustomer(m.Value);
    });
}
private readonly IMessenger _messenger;

public CustomerViewModel(Customer customer, IMessenger messenger)
{
    if (customer == null)
        throw new ArgumentNullException("customer");
    if (messenger == null)
        throw new ArgumentNullException("messenger");
    _messenger = messenger;
    _customer = customer;
    CustomerId = _customer.CustomerId;
    CompanyName = _customer.CompanyName;
    ContactName = _customer.ContactName;
    ContactTitle = _customer.ContactTitle;
    Address = _customer.Address;
    City = _customer.City;
    Region = _customer.Region;
    PostalCode = _customer.PostalCode;
    Country = _customer.Country;
    Phone = _customer.Phone;
    Fax = _customer.Fax;
}

We have decoupled the ViewModels from the default messenger. There are some changes to do in the code and in the tests, but you can check them in the final project, here. The code now is decoupled, testable and works fine 😃

All the source code for this article is at https://github.com/bsonnino/MessengerToolkit

Sometime ago, I’ve written this article introducing the MVVM Community Toolkit and developing a CRUD application to show how to use the MVVM pattern with the Community toolkit.

The time has passed and version 8.0 of the MVVM Community toolkit has been released and, with it, a rewrite using incremental generators. This may seem a minor update, but it’s a huge move, as the MVVM pattern is full of boilerplate: implementing the INotifyPropertyChanged interface for the viewmodels, binding commands that implement the ICommand interface, using the RelayCommand class and implementing observable properties that raise the PropertyChanged event when changed. All that make the code cumbersome and repetitive, but that’s what we had to do to implement the MVVM pattern in our apps. Until now.

With the use of source generators, the toolkit removes a lot of the boilerplate and makes the code easier to create and read. In this article, we’ll take the project that we developed in the previous article and will change it to use the new toolkit.

You can clone the code in https://github.com/bsonnino/MvvmApp and open the CustomerApp – Mvvm app in Visual Studio 2022. As the original project is targeted to .NET 5.0, we’ll upgrade it to .NET 6.0. This is an easy task: just open CustomerApp.csproj and change the TargetFramework to .net6.0-windows:

<PropertyGroup>
	<OutputType>WinExe</OutputType>
	<TargetFramework>net6.0-windows</TargetFramework>
	<UseWPF>true</UseWPF>
	<GenerateAssemblyInfo>false</GenerateAssemblyInfo>
</PropertyGroup>

Do the same with CustomerApp.Tests.csproj. There is no need to do it on the CustomerLib project, as it’s a Net Standard project.

The next step is to update the NuGet packages. The package Microsoft.Extensions.DependencyInjection must be upgraded to version 6.0.0. If you try to upgrade the Microsoft.Toolkit.Mvvm package, you’ll see that there is no upgrade to version 8.0. That’s because the package name has changed and you must uninstall this one and install the CommunityToolkit.Mvvm package.

Now, the project is ready to build and, when you do that, you’ll see that it doesn’t work 😦. This is because the namespaces have changed and we need to update the using clauses in MainViewModel.cs. We have to remove the old using clauses and replace with the new ones:

using Microsoft.Toolkit.Mvvm.ComponentModel;
using Microsoft.Toolkit.Mvvm.Input;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

Once you do that and recompile the project, you’ll see that it compiles fine and runs in the same way the original project did. Not bad for a project upgraded from .NET 5 to .NET 6, with a new version of the MVVM framework.

But did I say that with the new toolkit you can remove the boilerplate? We can start doing that now. The first thing is to remove the properties and their getters and setters. We decorate the _selectedCustomer field with the [ObservableProperty] attribute:

[ObservableProperty]
private Customer _selectedCustomer;

When you do that, you’ll see that the class name has a red underline:

That’s because when we add the attribute, the toolkit generates a partial class, and we need to add the partial keyword to the class:

public partial class MainViewModel : ObservableObject

When we add that, SelectedCustomer is underlined:

That’s because we have declared the property in our code and the toolkit has also declared the same property in the generated code. We can now remove the property declaration from our code:

public Customer SelectedCustomer
{
    get => _selectedCustomer;
    set
    {
        SetProperty(ref _selectedCustomer, value);
        RemoveCommand.NotifyCanExecuteChanged();
    }
}

As you can see from the code we are removing, the setter notifies the RemoveCommand. To have the same effect, we add the NotifyCanExecuteChangedFor attribute to the field:

[ObservableProperty]
[NotifyCanExecuteChangedFor(nameof(RemoveCommand))]
private Customer _selectedCustomer;

If you compile the code, you will see that it runs the same way it did before, and we are still using the property name (SelectedCommand) in the commands, even if it’s not explicitly defined in the code. That’s because the toolkit is generating the property in its partial part of the class.

The next steps are to remove the boilerplate from the commands in the code. For that, we must remove all declarations and leave only the command methods, changing their name to the command name (without Command at the end) and adding the [RelayCommand] attribute for the method. For the Add command, we have to change this code:

public IRelayCommand AddCommand { get; }

private void DoAdd()
{
    var customer = new Customer();
    _customerRepository.Add(customer);
    SelectedCustomer = customer;
    OnPropertyChanged("Customers");
}

To this code:

[RelayCommand]
private void Add()
{
    var customer = new Customer();
    _customerRepository.Add(customer);
    SelectedCustomer = customer;
    OnPropertyChanged("Customers");
}

We also have to remove the initialization code:

AddCommand = new RelayCommand(DoAdd);
RemoveCommand = new RelayCommand(DoRemove, () => SelectedCustomer != null);
SaveCommand = new RelayCommand(DoSave);
SearchCommand = new RelayCommand<string>(DoSearch);

If you notice the removed code, you’ll see that the RemoveCommand has a CanExecute method. To solve that, we have to have to add a parameter to RelayCommand:

[RelayCommand(CanExecute = "HasSelectedCustomer")]
private void Remove()
{

The parameter points to HasSelectedCustomer, a method that should be defined in the code:

private bool HasSelectedCustomer() => SelectedCustomer != null;

With that, we have completed our code and now the project runs in the same way it did before. The code is simpler and with no boilerplate:

public partial class MainViewModel : ObservableObject
{
    private readonly ICustomerRepository _customerRepository;

    [ObservableProperty]
    [NotifyCanExecuteChangedFor(nameof(RemoveCommand))]
    private Customer _selectedCustomer;

    public MainViewModel(ICustomerRepository customerRepository)
    {
        _customerRepository = customerRepository ??
                              throw new ArgumentNullException("customerRepository");
    }

    public IEnumerable<Customer> Customers => _customerRepository.Customers;

    [RelayCommand]
    private void Add()
    {
        var customer = new Customer();
        _customerRepository.Add(customer);
        SelectedCustomer = customer;
        OnPropertyChanged("Customers");
    }

    [RelayCommand(CanExecute = "HasSelectedCustomer")]
    private void Remove()
    {
        if (SelectedCustomer != null)
        {
            _customerRepository.Remove(SelectedCustomer);
            SelectedCustomer = null;
            OnPropertyChanged("Customers");
        }
    }

    private bool HasSelectedCustomer() => SelectedCustomer != null;

    [RelayCommand]
    private void Save()
    {
        _customerRepository.Commit();
    }

    [RelayCommand]
    private void Search(string textToSearch)
    {
        var coll = CollectionViewSource.GetDefaultView(Customers);
        if (!string.IsNullOrWhiteSpace(textToSearch))
            coll.Filter = c => ((Customer)c).Country.ToLower().Contains(textToSearch.ToLower());
        else
            coll.Filter = null;
    }
}

As you can see, this new version brought a huge improvement. We can use the MVVM pattern with no issues, there is no extra code related to the pattern (except for the attributes) and the code is easier to read and follow. And all tests still run, with no change at all.

The full source code for this article is at https://github.com/bsonnino/MVVMToolkit8

Nowadays, a very common scenario is that you have your WPF app ready and running for some time, and your boss tells you that it’s time to go further and port this app to the web, to have a larger market and be run in multiple platforms.

This is not a simple thing, because WPF is not multi-platform and, although it was ported to .NET Core, it’s still a Windows Platform.

You can think of rewriting the whole app, but even if you decide to use Asp.Net Core and Razor pages, or Blazor, you will have a huge effort to do it, because writing the UI in these platforms is completely different than using XAML.

Xamarin is a viable alternative, it’s close to WPF and it uses XAML, but it’s not a web platform, you can only write native apps for iOs, Android or Mac.

But things are not lost, at all. The guys at Uno Platform (https://www.platform.uno) created a nice project that uses WebAssembly to run the code you’ve created on the browser.

And what is WebAssembly? WebAssembly is a technology that allow browsers to run non-javascript code. When people hear this, they usually think of browsers plugins, like Silverlight or Adobe Flash. In fact, these are completely different. While the plugins were apps created to run in the browser, they were installed and supported by the vendors, and could suffer all sort of security vulnerabilities and were, at some point, abandoned by the browsers.

WebAssembly, on the other side, is an open standard fully supported by the browsers and a web standard. You can create code in may languages and compile it into a wasm module, thus running your C++/Rust code in a browser. This link shows a list of languages that are used in WebAssembly.

Uno Platform uses WebAssembly to run UWP code in the web. So, if you have an UWP program, you can compile it with Uno and run it on the web, with minimal changes. That’s great when you want to port your UWP app to the web.

You may have noticed that in the previous paragraph I didn’t mention WPF, but only UWP/WinUI. Uno Platform works with UWP projects, and not WPF. But UWP/WinUI is still XAML and has many similarities with WPF. So, while it’s not a direct port, it’s not a complete rewrite either. Let’s see what must be done to port a WPF program to the web using Uno Platform.

For this article, I will be using the WPF project that shows how to use MVVM with the MVVM Toolkit, which I’ve shown in this article. The source code for this article is here.

Installing UNO Platform

To install the UNO Platform in Visual Studio, you need to have the UWP, Xamarin and Asp.NET workloads installed. You can open the Visual Studio installer and verify if the three workloads are installed:

One other prerequisite is to have .NET 5 SDK or later installed. If you haven’t done so, you can install .NET 5 from https://dotnet.microsoft.com/download/dotnet-core/5.0 or .NET 6 from https://dotnet.microsoft.com/en-us/download/dotnet/6.0.

Once they are installed, in Visual Studio, go to Extensions > Manage Extensions and install Uno Platform Templates.

With the extension installed, you can create a new solution using Uno:

We will create a new Multi-Platform App (Uno Platform|.net 6).

When you create the app, you will see multiple projects there:

As you can see, there are projects for Mobile (Android, iOS and Mac, GTK, WPF, UWP and Wasm). We will be using the Wasm project, so you should select it as the startup project. When you run it, it will open a browser window with the app:

It doesn’t seem too much, but when you think that the underlying code is XAML, that’s pretty nice!

Converting the WPF Project

As you can see in the Solution Explorer, there is a project .Shared, that contains the shared resources for our project. Putting your pages here will share the pages for all projects in the solution.

Before we port the solution, we should port the customer lib, that contains the classes and the repository. In fact this is an easy port, we don’t have to do anything, as it’s already a .NET Standard library.

Just add the files for the project in the same folder of the other projects and add the new project to the solution.

After that, you can add the project as a reference to the other projects you are creating.

Then, you should create the ViewModel for the project. Before that, we will have to add the package Microsoft.Toolkit.Mvvm to all the projects you are using. In the Solution Explorer, right-click the dependencies node in the Wasm project and select Manage Nuget Packages and add the Microsoft.Toolkit.Mvvm package. You must also add the reference to the other projects.

Then, create a folder named ViewModel in the Shared project and add the file MainViewModel.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using CustomerLib;
using Microsoft.Toolkit.Mvvm.ComponentModel;
using Microsoft.Toolkit.Mvvm.Input;

namespace MVVMUnoApp.ViewModel
{
    public class MainViewModel : ObservableObject
    {
        private readonly ICustomerRepository _customerRepository;
        private readonly IEnumerable<Customer> _allCustomers;
        private Customer _selectedCustomer;

        public MainViewModel(ICustomerRepository customerRepository)
        {
            _customerRepository = customerRepository ??
                                  throw new ArgumentNullException("customerRepository");
            _allCustomers = _customerRepository.Customers;
            AddCommand = new RelayCommand(DoAdd);
            RemoveCommand = new RelayCommand(DoRemove, () => SelectedCustomer != null);
            SaveCommand = new RelayCommand(DoSave);
            SearchCommand = new RelayCommand<string>(DoSearch);
        }

        public IEnumerable<Customer> Customers {get; private set;}

        public Customer SelectedCustomer
        {
            get => _selectedCustomer;
            set
            {
                SetProperty(ref _selectedCustomer, value);
                RemoveCommand.NotifyCanExecuteChanged();
            }
        }

        public IRelayCommand AddCommand { get; }
        public IRelayCommand RemoveCommand { get; }
        public IRelayCommand SaveCommand { get; }
        public IRelayCommand<string> SearchCommand { get; }

        private void DoAdd()
        {
            var customer = new Customer();
            _customerRepository.Add(customer);
            SelectedCustomer = customer;
            OnPropertyChanged("Customers");
        }

        private void DoRemove()
        {
            if (SelectedCustomer != null)
            {
                _customerRepository.Remove(SelectedCustomer);
                SelectedCustomer = null;
                OnPropertyChanged("Customers");
            }
        }

        private void DoSave()
        {
            _customerRepository.Commit();
        }

        private void DoSearch(string textToSearch)
        {
            if (!string.IsNullOrWhiteSpace(textToSearch))
                Customers = _allCustomers.Where(c => ((Customer)c).Country.ToLower().Contains(textToSearch.ToLower()));
            else
                Customers = _allCustomers;
            OnPropertyChanged("Customers");
        }
    }
}

This file is almost the same as the one in the original project, with a change in DoSearch due to the fact that the CollectionViewSource works in a different way in UWP/WinUI than in WPF. For that, we removed the code that filters CollectionViewSource:

private void DoSearch(string textToSearch)
{
    var coll = CollectionViewSource.GetDefaultView(Customers);
    if (!string.IsNullOrWhiteSpace(textToSearch))
        coll.Filter = c =>
            ((Customer)c).Country.ToLower().Contains(textToSearch.ToLower());
    else
        coll.Filter = null;
}

And did an explicit filter on the customer list returned by the repository.

The next step is to add the views to the Shared project. The main view uses a DataGrid that doesn’t exist in UWP/WinUI, but there is a solution, here: add the DataGrid of the Windows Control Toolkit, that works fine in UWP/WinUI. For that, add the NuGet reference to Uno.Microsoft.Toolkit.Uwp.UI.Controls.Datagrid to all the used projects. In the UWP project, you should not add this reference, but add Microsoft.Toolkit.Uwp.UI.Controls.Datagrid.

Then, change the code in MainPage.xaml in the Shared project to:

<Page
    x:Class="MVVMUnoApp.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:controls="using:Microsoft.Toolkit.Uwp.UI.Controls"
    xmlns:local="using:MVVMUnoApp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"    
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="40" />
            <RowDefinition Height="*" />
            <RowDefinition Height="2*" />
            <RowDefinition Height="50" />
        </Grid.RowDefinitions>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Country" VerticalAlignment="Center" Margin="5"/>
            <TextBox x:Name="searchText" VerticalAlignment="Center" Margin="5,3" Width="250" VerticalContentAlignment="Center"/>
            <Button x:Name="PesqBtn" Content="Find" Width="75" Margin="10,5" VerticalAlignment="Center" 
                    Command="{Binding SearchCommand}" CommandParameter="{Binding ElementName=searchText,Path=Text}"/>
        </StackPanel>
        <controls:DataGrid AutoGenerateColumns="False" x:Name="master" Grid.Row="1" 
                  ItemsSource="{Binding Customers}" SelectedItem="{Binding SelectedCustomer, Mode=TwoWay}">
            <controls:DataGrid.Columns>
                <controls:DataGridTextColumn x:Name="customerIDColumn" Binding="{Binding CustomerId}" Header="Customer ID" />
                <controls:DataGridTextColumn x:Name="companyNameColumn" Binding="{Binding CompanyName}" Header="Company Name" Width="160" />
                <controls:DataGridTextColumn x:Name="contactNameColumn" Binding="{Binding ContactName}" Header="Contact Name" Width="160" />
                <controls:DataGridTextColumn x:Name="contactTitleColumn" Binding="{Binding ContactTitle, Mode=TwoWay}" Header="Contact Title"  />
                <controls:DataGridTextColumn x:Name="addressColumn" Binding="{Binding Address}" Header="Address" Width="130" />
                <controls:DataGridTextColumn x:Name="cityColumn" Binding="{Binding City}" Header="City" />
                <controls:DataGridTextColumn x:Name="regionColumn" Binding="{Binding Region}" Header="Region" />
                <controls:DataGridTextColumn x:Name="postalCodeColumn" Binding="{Binding PostalCode}" Header="Postal Code" />
                <controls:DataGridTextColumn x:Name="countryColumn" Binding="{Binding Country}" Header="Country" />
                <controls:DataGridTextColumn x:Name="faxColumn" Binding="{Binding Fax}" Header="Fax" Width="100" />
                <controls:DataGridTextColumn x:Name="phoneColumn" Binding="{Binding Phone}" Header="Phone" Width="100" />
            </controls:DataGrid.Columns>
        </controls:DataGrid>
        <local:Detail Grid.Row="2" DataContext="{Binding ElementName=master, Path=SelectedItem, Mode=OneWay}" Margin="5" x:Name="detail"/>
        <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" Margin="5" Grid.Row="3">
            <Button Width="75" Margin="5,0" Content="Add" Command="{Binding AddCommand}" />
            <Button Width="75" Margin="5,0" Content="Remove" Command="{Binding RemoveCommand}" />
            <Button Width="75" Margin="5,0" Content="Save" Command="{Binding SaveCommand}" />
        </StackPanel>

    </Grid>
</Page>

In this case, there were almost no changes to do: the Toolkit DataGrid is very similar to the one in WPF. We must set the DataContext property in MainPage.xaml.cs:

public MainPage()
{
    this.InitializeComponent();
    DataContext = App.Current.MainVM;
}

We are using the same code we’ve used in the WPF project. For that, we must use the Dependency Injection provided in the MVVM toolkit in App.xaml.cs:

public App()
{
    InitializeLogging();

    this.InitializeComponent();
    
    var services = new ServiceCollection();

    services.AddSingleton<ICustomerRepository, CustomerRepository>();
    services.AddSingleton<MainViewModel>();

    Services = services.BuildServiceProvider();

#if HAS_UNO || NETFX_CORE
    this.Suspending += OnSuspending;
#endif
}
public new static App Current => (App)Application.Current;
public IServiceProvider Services { get; }
public MainViewModel MainVM => Services.GetService<MainViewModel>();

As you can see, there are no changes here.

The next step is to add the Details page, which has almost not changes from the WPF project:

<UserControl
    x:Class="MVVMUnoApp.Detail"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Name="detailControl"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300"
    d:DesignWidth="400">

    <Grid>
       
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto" />
                <ColumnDefinition Width="*" />
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>
            <TextBlock Text="Customer Id:" Grid.Column="0" Grid.Row="0"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="0"   Margin="3" Name="customerIdTextBox" Text="{Binding CustomerId, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Company Name:" Grid.Column="0" Grid.Row="1"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="1"   Margin="3" Name="companyNameTextBox" Text="{Binding CompanyName, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Contact Name:" Grid.Column="0" Grid.Row="2"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="2"   Margin="3" Name="contactNameTextBox" Text="{Binding ContactName, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Contact Title:" Grid.Column="0" Grid.Row="3"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="3"   Margin="3" Name="contactTitleTextBox" Text="{Binding ContactTitle, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Address:" Grid.Column="0" Grid.Row="4" HorizontalAlignment="Left" Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="4" Margin="3" Name="addressTextBox" Text="{Binding Address, Mode=TwoWay}" VerticalAlignment="Center" />
            <TextBlock Text="City:" Grid.Column="0" Grid.Row="5"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="5"   Margin="3" Name="cityTextBox" Text="{Binding City, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Postal Code:" Grid.Column="0" Grid.Row="6"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="6"   Margin="3" Name="postalCodeTextBox" Text="{Binding PostalCode, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Region:" Grid.Column="0" Grid.Row="7"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="7"   Margin="3" Name="regionTextBox" Text="{Binding Region, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Country:" Grid.Column="0" Grid.Row="8"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="8"   Margin="3" Name="countryTextBox" Text="{Binding Country, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Phone:" Grid.Column="0" Grid.Row="9"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="9"   Margin="3" Name="phoneTextBox" Text="{Binding Phone, Mode=TwoWay}" VerticalAlignment="Center"  />
            <TextBlock Text="Fax:" Grid.Column="0" Grid.Row="10"  Margin="3" VerticalAlignment="Center" />
            <TextBox Grid.Column="1" Grid.Row="10"   Margin="3" Name="faxTextBox" Text="{Binding Fax, Mode=TwoWay}" VerticalAlignment="Center"  />
        </Grid>
</UserControl>

The only change is to remove the ValidatesOnExceptions=true and NotifyOnValidationError=true from the textboxes, because UWP/WinUI doesn’t have native validation. If you want to add validation to UWP/WinUI, you should check the Template10 Validation project.

Now, the project is ok and ready to be run. When you run it, it shows the main window, but doesn’t show any data:

That’s really strange. If you change the startup project to UWP and run it, it runs fine:

We should investigate a little more. When using the Wasm project, we have a web application. In this case, do the web tools work ? Run the Wasm project again and press F12 to open the dev tools:

As you can see, we have the diagnostic tools available and they show us that Customers.xml is not found. That makes sense, as we’re not running a local app, but a web app, and local files aren’t available. We should use another mechanism to get our data. Searching in the internet, we come to this site, which says that the Storage files available in UWP are also available in Wasm, so we should change our code from

var doc = XDocument.Load("Customers.xml");

To

var file = await Windows.Storage.StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///Customers.xml"));
var content = await FileIO.ReadTextAsync(file);
var doc = XDocument.Parse(content);

Unfortunately, this way is not supported in Wasm, when using a library file. We have two solutions here:

  • Create a server that will serve the data and use REST calls to the server in the repository
  • Add the repository files to the Shared project and use the UWP storage method

There is no doubt that in a normal project I would go to the first solution. It’s the way to go when you are creating web applications, it’s more flexible and extensible. But, as I’m only showing you how to port a small WPF project to the web and I don’t want to create a server and change the repository now (it would be beyond the scope of the article), I won’t do it (if you want to see that solution, write in the comments, if there are enough requests, I will do it 😃).

So, what we’ll do is to create a new folder named Repository in the Shared project and add all the files that are in the library to this folder. After that, you can delete the library from the solution.

If you run the application, you still won’t see the data, but the error will disappear. The issue now is that we have a synchronization problem: the data now is obtained in async mode (in the original project it was obtained in sync mode). While that is happening, the ViewModel is being built and asks for the customers, which haven’t been read, so the _allCustomers field will be empty and nothing will be shown.

To fix this issue, we must have a function named GetCustomersAsync that retrieves the customers and use it in the ViewModel.

The ICustomerRepository interface will be:

public interface ICustomerRepository
{
    bool Add(Customer customer);
    bool Remove(Customer customer);
    bool Commit();
    Task<IEnumerable<Customer>> GetCustomersAsync();
}

And the CustomerRepository class becomes:

public class CustomerRepository : ICustomerRepository
{
    private IList<Customer> customers;

    public async Task<IEnumerable<Customer>> GetCustomersAsync()
    {
        var file = await Windows.Storage.StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///Customers.xml"));
        var content = await FileIO.ReadTextAsync(file);
        var doc = XDocument.Parse(content);
        customers = new ObservableCollection<Customer>((from c in doc.Descendants("Customer")
                                                        select new Customer
                                                        {
                                                            CustomerId = GetValueOrDefault(c, "CustomerID"),
                                                            CompanyName = GetValueOrDefault(c, "CompanyName"),
                                                            ContactName = GetValueOrDefault(c, "ContactName"),
                                                            ContactTitle = GetValueOrDefault(c, "ContactTitle"),
                                                            Address = GetValueOrDefault(c, "Address"),
                                                            City = GetValueOrDefault(c, "City"),
                                                            Region = GetValueOrDefault(c, "Region"),
                                                            PostalCode = GetValueOrDefault(c, "PostalCode"),
                                                            Country = GetValueOrDefault(c, "Country"),
                                                            Phone = GetValueOrDefault(c, "Phone"),
                                                            Fax = GetValueOrDefault(c, "Fax")
                                                        }).ToList());
        return customers;
    }

    #region ICustomerRepository Members

    public bool Add(Customer customer)
    {
        if (customers.IndexOf(customer) < 0)
        {
            customers.Add(customer);
            return true;
        }
        return false;
    }

    public bool Remove(Customer customer)
    {
        if (customers.IndexOf(customer) >= 0)
        {
            customers.Remove(customer);
            return true;
        }
        return false;
    }

    public bool Commit()
    {
        try
        {
            var doc = new XDocument(new XDeclaration("1.0", "utf-8", "yes"));
            var root = new XElement("Customers");
            foreach (Customer customer in customers)
            {
                root.Add(new XElement("Customer",
                                      new XElement("CustomerID", customer.CustomerId),
                                      new XElement("CompanyName", customer.CompanyName),
                                      new XElement("ContactName", customer.ContactName),
                                      new XElement("ContactTitle", customer.ContactTitle),
                                      new XElement("Address", customer.Address),
                                      new XElement("City", customer.City),
                                      new XElement("Region", customer.Region),
                                      new XElement("PostalCode", customer.PostalCode),
                                      new XElement("Country", customer.Country),
                                      new XElement("Phone", customer.Phone),
                                      new XElement("Fax", customer.Fax)
                             ));
            }
            doc.Add(root);
            doc.Save("Customers.xml");
            return true;
        }
        catch (Exception)
        {
            return false;
        }
    }

    #endregion

    private static string GetValueOrDefault(XContainer el, string propertyName)
    {
        return el.Element(propertyName)?.Value ?? string.Empty;
    }
}

To use this repository, we should change the ViewModel to:

private readonly ICustomerRepository _customerRepository;
private IEnumerable<Customer> _allCustomers;
private Customer _selectedCustomer;

public MainViewModel(ICustomerRepository customerRepository)
{
    _customerRepository = customerRepository ??
                          throw new ArgumentNullException("customerRepository");
    GetCustomers();
    AddCommand = new RelayCommand(DoAdd);
    RemoveCommand = new RelayCommand(DoRemove, () => SelectedCustomer != null);
    SaveCommand = new RelayCommand(DoSave);
    SearchCommand = new RelayCommand<string>(DoSearch);
}

private async void GetCustomers()
{
    _allCustomers = await _customerRepository.GetCustomersAsync();
    Customers = _allCustomers;
    OnPropertyChanged("Customers");
}

We must call the async method in the ViewModel constructor and, in this case, we cannot await for the call, so we fire the call and, when the data is ready, it fires the INotifyPropertyChanged event and populates the data.

If we run the program, we see that it opens a new browser window with the program:

We’ve ported our WPF program to the web using Uno Platform. Nice, no ? We can leverage our XAML expertise and port it to the web. It’s not a direct change but, nevertheless, it’s way less trouble than a complete rewrite. Uno Platform did a great work to port UWP to WebAssembly! And you get some extra bonuses: you still have an UWP and a WPF project running with the same code. The original Uno template also allows you to run the code in Android or iOs (but I removed these from the code for this article).

The full code for this article is in https://github.com/bsonnino/MVVMUnoApp

There are some times when you need to get the disk information in your system, to check what’s happening, for inventory check, or even to know the free and available space.
Getting the free and available space is fairly easy in .NET, just use the GetDrives method of the DriveInfo class:

DriveInfo[] drives = DriveInfo.GetDrives();

foreach (DriveInfo drive in drives)
{
    Console.WriteLine($"Name: {drive.Name}");
    Console.WriteLine($"VolumeLabel: {drive.VolumeLabel}");
    Console.WriteLine($"RootDirectory: {drive.RootDirectory}");
    Console.WriteLine($"DriveType: {drive.DriveType}");
    Console.WriteLine($"DriveFormat: {drive.DriveFormat}");
    Console.WriteLine($"IsReady: {drive.IsReady}");
    Console.WriteLine($"TotalSize: {drive.TotalSize}");
    Console.WriteLine($"TotalFreeSpace: {drive.TotalFreeSpace}");
    Console.WriteLine($"AvailableFreeSpace: {drive.AvailableFreeSpace}");
    Console.WriteLine();
}

If you run this code, you will get the info for all drives in your system:

This is ok for most apps, but sometimes you want more than that. If, for example, you want an inventory of your physical disks (not the logical disks, like DriveInfo gives) ?

The physical and logical structure of your disks can differ a lot. For example, if you have a disk with three partitions, you will have one physical disk and three logical disks.

On the other side, you can mount a drive in an empty folder and have two physical disks and one logical disk. And we are not considering multiple OSs, here.

.NET doesn’t have any dedicated class for the physical disk structure, so we should find another way to do it. In fact, there is a way to get this information in Windows, called WMI (Windows Management Instrumentation).

It’s the infrastructure for data management in Windows and you can use it to manage your local and remote systems. You can access it with C++, C# or even PowerShell, to get info and manage your computer.

With WMI, you can use SQL-like queries to query information in your system, using the available classes. For example, to get all the drives in your system, you would use:

SELECT * FROM Win32_DiskDrive

That would return a key-value set with all the properties of the system disks, and you can check the values you want. You can even filter the properties and drives that you want. Something like:

SELECT DeviceID, Model, Name, Size, Status, Partitions, 
  TotalTracks, TotalSectors, BytesPerSector, SectorsPerTrack, TotalCylinders, TotalHeads, 
  TracksPerCylinder, CapabilityDescriptions 
  FROM Win32_DiskDrive WHERE MediaType = 'Fixed hard disk media'

Will get only the device data and geometry for the fixed disks in your system (it won’t get info for the external HDs).

Now that we already know how to get the disk information, let’s see how to do it in C#.

To use WMI in a C# program, we must add the package System.Management to the project and start using it.

Open a new terminal window and create a new console application with

dotnet new console -o DiskInfoCSharp

Once you do that, a new console project will be created in the DiskInfoCSharp folder. You can cd to it and add the new package to the project with

dotnet add package System.Management

This will add the package to the project and will allow you to use WMI in it. You can open VS Code with code . and start editing the project.

The first step is to create a scope and connect to it:

using System.Management;

ManagementScope scope = new ManagementScope("\\\\.\\root\\cimv2");
scope.Connect();

This will allow you to query data in the local computer. If you want to connect to a remote computer, just change the \\. to the name of the remote computer.

Then, you must create the query for getting the disk information:

ObjectQuery query = new ObjectQuery(@"SELECT DeviceID, Model, Name, Size, Status, Partitions, 
  TotalTracks, TotalSectors, BytesPerSector, SectorsPerTrack, TotalCylinders, TotalHeads, 
  TracksPerCylinder, CapabilityDescriptions 
  FROM Win32_DiskDrive WHERE MediaType = 'Fixed hard disk media'");

As you can see, the query has a syntax similar to SQL, you can change it to get different fields (or use * to get all fields) or change (or remove) the WHERE clause. This query will only return the selected fields for the fixed hard disks.

Then, we should instantiate a ManagementObjectSearcher object and start iterating on all instances of fixed disks in your system:

ManagementObjectSearcher searcher = new ManagementObjectSearcher(scope, query);
foreach (ManagementObject wmi_HD in searcher.Get())
{
    foreach (PropertyData property in wmi_HD.Properties)
        Console.WriteLine($"{property.Name} = {property.Value}");
    var capabilities = wmi_HD["CapabilityDescriptions"] as string[];
    if (capabilities != null)
    {
        Console.WriteLine("Capabilities");
        foreach (var capability in capabilities)
            Console.WriteLine($"  {capability}");
    }
    Console.WriteLine("-----------------------------------");
}

For each disk, we get all the properties and then we iterate on the capability descriptions, as this is a property that returns an array of strings.

If we run this program, we will get something like

As you can see, it’s very easy to get disk information using WMI in C#. There are some caveats in using this method:

  • You don’t have the properties in advance. Yes, you could create a class with the properties and populate it, but it’s not something that is built in
  • This method only works in Windows. If you are creating a multi-platform program, this method isn’t for you

On the other side, this is a very easy method to get the information in local and remote computers, you can easily filter data and use it as you want.

If you go to the Using WMI page, you will see a note:

As you can see, the System.Management namespace is not recommended anymore, and it must be replaced by the Microsoft.Management.Infrastructure, especially for .NET Core apps.

To port our app, the first thing to do is to ass the package Microsoft.Management.Infrastructure with

dotnet add package Microsoft.Management.Infrastructure

Once you do that, you should change the classes you are using to access the data. We create a CimSession and query the data with QueryInstances:

using Microsoft.Management.Infrastructure;

var instances = CimSession.Create(null)
                .QueryInstances(@"root\cimv2", "WQL", @"SELECT DeviceID, Model, Name, Size, Status, Partitions, 
   TotalTracks, TotalSectors, BytesPerSector, SectorsPerTrack, TotalCylinders, TotalHeads, 
   TracksPerCylinder, CapabilityDescriptions 
   FROM Win32_DiskDrive WHERE MediaType = 'Fixed hard disk media'");

Once you do that, you can iterate on the instances and get the properties with:

foreach (var instance in instances)
{
    foreach (var property in instance.CimInstanceProperties)
        Console.WriteLine($"{property.Name} = {property.Value}");
    var capabilities = instance.CimInstanceProperties["CapabilityDescriptions"].Value as string[];
    if (capabilities != null)
    {
        Console.WriteLine("Capabilities");
        foreach (var capability in capabilities)
            Console.WriteLine($"  {capability}");
    }
    Console.WriteLine("-----------------------------------");
}

If you run the program, you will get properties similar to the ones in the previous figure.

As you can see, WMI can ease the retrieval of information in a Windows machine. Unfortunately, that can’t be used in a Linux machine, that may be a subject for another article!

All the code for this article is at https://github.com/bsonnino/WMICsharp

Once there was a time where Microsoft and Linux were enemies and who was developing in Windows didn’t develop for Linux and vice-versa. Time has passed and things have changed. And how they have changed!

Multi-platform development has evolved, Microsoft released .NET open source, and you can run it both on Linux and Mac (besides Windows, of course). You can use VS Code to develop your code in any of these platforms, and VS Code isn’t restricted to .NET Languages: you can develop in Python, Perl or even Clipper/Harbour, Cobol or Fortran. What a change!

Besides that, you can run a Linux distro directly in Windows, with no need of any VM, dual boot or Live CD. The key for that is WSL, the Windows Subsystem for Linux, that allows you to run a full Linux environment, including any Linux application without modification (you can even run Linux graphical apps with WSLg, that’s in test – the image below shows GIMP running on Windows)

And, just to make sure, it’s not an emulation of Linux. You are running the full distro directly on Windows. WSL uses a translation layer between Linux and Windows, to translate the calls between both OSs and WSL2 uses a Linux kernel for its magic. You can compare both versions of WSL here.

To develop in Linux, the first thing is to get Windows Terminal. This is the successor of the old Windows command line prompt and offers many improvements: customization, tabbed interface and it allows you to use multiple shell types: Windows command, Powershell, Linux prompt or even an Azure Cloud Shell. If you are using Windows 11, you already have it, as it’s installed by default. If you are still using Windows 10, you can get it from the Microsoft Store.

Once you have Windows Terminal installed, you must install WSL. Just open a command prompt window and type

wsl --install

This will install WSL in your machine. If that doesn’t work, you will have to install it manually, using the installation steps listed here.

With WSL installed, you need to install your preferred Linux distro. You can get it from the Microsoft Store:

Or you can install it directly from the command prompt. wsl --list lists all the available distros in your machine (you can install and use many distros) and wsl --list --online lists the available distros online:

To install an online distro, you must use

wsl --install -d <Distro>

If the distro isn’t available online or in the store, you still can install it, by getting the appx packages an manually installing it, as described here. One other way is to check if there is an alternate way to install it in WSL. For example, to install Linux Mint, you can go to the LinuxMintWSL Github and download the installer for it.

When you have WLS and the distros installed in your machine, you can click on the down arrow in the Terminal title bar and it will show them in the list, so you can select and open a command prompt for that distro:

The next step is to install dotnet 6.0. To do that, you should follow the instructions for your distro in

For Linux Mint (based on Ubuntu), you should run these commands on the bash prompt:

wget https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb
sudo dpkg -i packages-microsoft-prod.deb
rm packages-microsoft-prod.deb

sudo apt-get update; \
  sudo apt-get install -y apt-transport-https && \
  sudo apt-get update && \
  sudo apt-get install -y dotnet-sdk-6.0

Once installed, you can get the installed version with

dotnet --version

Now, you can create and run a dotnet program in Linux. To show the available app types to create, you can run

dotnet new

This will list the common templates for a new app. To list all templates, you should use

dotnet new --list

We can create a new console app using this command:

dotnet new console -o simple-console

This command will create a new console app in the simple-console folder. We can change to this folder with cd simple-console and run the program with

dotnet run

This is already great but, unless you are already a VI or Emacs guru (which I am not 😃), you will have some trouble to modify it. I personally, think that the best way to modify a Linux program in WSL is to use VSCode. By typing code . in the command prompt, VS Code server will be downloaded and installed automatically and VSCode will open in your desktop

In this case, you won’t have VS Code for Linux running. When you are using VS Code from WSL, something different happens: VS Code will be split into two parts, a server (which runs on Linux) and a client (that runs on Windows). You will be editing the files on Windows, but saving them in Linux. You can see in VS Code status bar, at the bottom WSL: Mint, indicating that we are editing files in WSL. If you open a terminal in VS Code with Ctrl+`, you will see that it’s a bash prompt and not a command line or a Powershell prompt.

We can edit program.cs to show the OS version with:

Console.WriteLine($@"Hello World! 
Operating System: {System.Runtime.InteropServices.RuntimeInformation.OSDescription}");

When we save and run the program we will get

This is still great, but I doubt that’s what you want to do when your are developing a Linux app. Many times, we’ll want to develop a web api to serve our data. For that, we must change to the parent folder and create a new app with

dotnet new webapi -o webapi

If you change to the webapi folder and run it with dotnet run, you will see the endpoints for the API:

In our case, if we open a browser window and type

https://localhost:7219/swagger

We will see the swagger interface for our api:

We can test it and see that it works fine:

But we can also use the curl command to get the data. In the bash prompt, we can type

curl -X 'GET' \
  'https://localhost:7219/WeatherForecast' \
  -H 'accept: text/plain'

to get the api data (I got this command line from the swagget test window). If you do that, you will see a certificate error. We didn’t set the certificate for the web site, and we can bypass that with the -k flag:

curl -k -X 'GET' \
  'https://localhost:7219/WeatherForecast' \
  -H 'accept: text/plain'

As you can see, we are getting the json data for the weather forecast sample. You can still consume the api from our console app. All we have to do is change the program.cs file to

using System.Text.Json;

var handler = new HttpClientHandler();
handler.ServerCertificateCustomValidationCallback =
              (message, cert, chain, errors) => 
              { return true; };
using var client = new HttpClient(handler);
var json = await client.GetStringAsync("https://localhost:7219/weatherforecast");
var forecasts = JsonSerializer.Deserialize<List<WeatherForecast>>(json);
if (forecasts == null)
    return;
foreach (var forecast in forecasts)
    Console.WriteLine($"{forecast.date:dd/MM/yyyy}  {forecast.temperatureC}  {forecast.summary}");

public class WeatherForecast
{
    public DateTime date { get; set; }
    public int temperatureC { get; set; }
    public int temperatureF => 32 + (int)(temperatureC / 0.5556);
    public string? summary { get; set; }
} 

If you run the program, you will see something like this:

We are consuming the API from our console program in Linux, but we are running .NET, we can run the same program in Windows. One nice thing in WSL is that we can run Windows apps from Linux. If you type

explorer.exe .

It will open Windows Explorer with the current path open. If you copy the address in the address bar, you will see:

\\wsl.localhost\Mint\home\bruno\LinuxProg\simple-console

As you can see, the Linux folder is considered a new network. We can map the drive in Windows to use it, with a command like this one:

net use y: \\wsl.localhost\Mint

If you open a command prompt and type this command, you will have the y: drive available to use. You can then change to the program folder and type dotnet run to run the same program in Windows. If you do that, you will see that we can run the same program we’ve created in Linux in the Windows command prompt and consume the API that is still running in Linux.

As you can see, with .NET you can run the same program in Linux or in Windows and you can interact with the APIs that are running in Linux. You will be using the tools that you still know. You can even access one file system from the other seamlessly (the Windows file system is mounted in /mnt/c/ in Linux). You will be using the same C# language that you know and, most of the time, you don’t even need to know Linux APIs – .NET will take care of that for you.

The full code of this article will be in https://github.com/bsonnino/LinuxProg

Sometime ago I posted an article about Dotnet Try. This is a wonderful tool to document your code but, since then, a lot has changed.

.NET 6  and C#10 are here, Visual Studio Code is a nice environment for editing your code, and you can even edit your code in  the web. And a new feature has appeared to document your code: .NET Interactive. With it, you can create interactive notebooks and create interactive pieces of code, where you can share and try sample code.

Its use is very simple: just install the >NET Interactive extension in VS Code and you’re ready to go. Just press Ctrl+Shift+Alt+N and open a new notebook (you can choose C#, F# or even Powershell as the default language) and a new notebook is ready to use. If you want to open an existing notebook, just press Ctrl+Shift+P and select .NET Interactive: Open Notebook and open the existing notebook.

When you create a new notebook, you can add markdown or code cells. The markdown cells have text formatted with the Markdown syntax and the code cells can have code, in the selected language. You can run the code by clicking the arrow icon next to the cell or by pressing the Ctrl+Alt+Enter keys.

To display the values, you can use Console.WriteLineor simply declare a variable and put it in a single line like in

if you want to display more than  a single value, you can use the display function, like in

You can even mix languages in the notebook. If you add !#fsharp in the first line, you can add F# code, like in:

If you want to use javascript, you can use something like:

Getting data from an URL

You can get some data from an URL, using the command

#!value --from-url https://raw.githubusercontent.com/ageron/handson-ml2/master/datasets/housing/housing.csv --name housingPath

Once you have the data, you can display it with SandDance, with this code:

using Microsoft.Data.Analysis;
using Microsoft.ML;
using System.Collections.Generic;

var housingData = DataFrame.LoadCsv(housingPath);
housingData.ExploreWithSandDance().Display();

Using Nuget Packages

You can also use Nuget packages with the #r directive. For example, we can get the suppliers dataset as a Json file from the url “https://northwind.vercel.app/api/suppliers”, with this code:

#!value --from-url https://northwind.vercel.app/api/suppliers --name suppliersJson

Then we can show the data as C# classes with:

#!share --from value suppliersJson
#r "nuget:Newtonsoft.Json"

using Newtonsoft.Json;

public record Address(string Street, string City, string Region, string PostalCode, string Country, string Phone);
public record Supplier(string Id, string CompanyName, string ContactName, string ContactTitle, Address Address);
var suppliers = JsonConvert.DeserializeObject<List<Supplier>>(suppliersJson);
suppliers

As you can see there are multiple possibilities. I even created the notebook for this article (you can download it at https://github.com/bsonnino/CSharpNotebooks) based on my last article, about Linq in .NET 6. You can use it as a notebook for you, as a learning tool, as a prototype tool, or even as a tool for sharing knowledge.

The notebook for this article is at https://github.com/bsonnino/CSharpNotebooks