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LINQ: Introducing The Take Last Operators

LINQ With C# (Portuguese)

Some time ago I needed to retrieve the last items of a sequence that satisfied some criteria and, looking at the operators available in the Enumerable class, I noticed that there wasn’t such operator.

The only way to achieve this was to reverse the sequence, take the items that satisfied the criteria and reverse the resulting sequence. Something like this:

sequence.Reverse().TakeWhile(criteria).Reverse();

Looks quite simple, right? First we call the Reverse method to produce a new sequence with the same items as the original sequence but in the reverse order, then we call the TakeWhile method to take the first items that satisfy the criteria and then call the Reverse method again to restore the original order of the items.

The problem with this approach is that the Reverse method buffers the entire sequence before iterating through its items in the reverse order - and the above code uses it twice. This means iterating over all items in the original sequence and buffer them all, iterating over first items of the resulting sequence that satisfy the criteria and buffer them all and, finally, iterate over that result to produce the final sequence.

If you’re counting, you’ve come to the conclusion that all items in the original sequence will be iterated over once and the ones in the resulting sequence will be iterated again three times. If the original sequence is large, this can take lots of memory and time.

There’s another issue if you’re using the variant the uses the index of the item in the original sequence in the evaluation of the selection criteria (>). When we reverse the order of the items, the indexes will be reversed and the predicate must take that in account, which might not be possible if you don’t know the number of items in the original sequence.

There must be a better way, and that’s why I implemented the Take Last Operators:

Name Description Example
TakeLast<TSource>(IEnumerable<TSource>)

Returns a specified number of contiguous elements from the end of a sequence.

int[] grades = { 59, 82, 70, 56, 92, 98, 85 };

var topThreeGrades = grades
                     .OrderBy(grade => grade)
                     .TakeLast(3);

Console.WriteLine("The top three grades are:");
foreach (int grade in topThreeGrades)
{
    Console.WriteLine(grade);
}
/*
This code produces the following output:

The top three grades are:
98
92
85
*/
TakeLastWhile<TSource>(IEnumerable<TSource>, Func<TSource, Boolean>)

Returns the elements from the end of a sequence as long as the specified condition is true.

string[] fruits =
    {
        "apple",
        "passionfruit",
        "banana",
        "mango",
        "orange",
        "blueberry",
        "grape",
        "strawberry"
    };

var query = fruits
            .TakeLastWhile(fruit => string.Compare("orange", fruit, true) != 0);

foreach (string fruit in query)
{
    Console.WriteLine(fruit);
}

/*
This code produces the following output:
blueberry
grape
strawberry
*/
TakeLastWhile<TSource>(IEnumerable<TSource>, Func<TSource, Int32, Boolean>)

Returns the elements from the end of a sequence as long as the specified condition is true.

string[] fruits =
    {
        "apple",
        "passionfruit",
        "banana",
        "mango",
        "orange",
        "blueberry",
        "grape",
        "strawberry"
    };

var query = fruits
            .TakeLastWhile((fruit, index) => fruit.Length >= index);

foreach (string fruit in query)
{
    Console.WriteLine(fruit);
}

/*
This code produces the following output:

strawberry
*/

You can find these (and more) operators in my CodePlex project for LINQ utilities and operators: PauloMorgado.Linq

PauloMorgado.Linq Project On CodePlex

LINQ With C# (Portuguese)

I’ve created a project on CodePlex to share my LINQ utilities and operators: PauloMorgado.Linq

In this project you can find the code from my previous posts about the Distinct operator:

In future posts, I’ll introduce other operators.

Patch For ASP.NET Vulnerability Available

Microsoft has published a Security Advisory (2416728) about a security vulnerability in ASP.NET on Saturday, September 18th. This vulnerability exists in all versions of ASP.NET and was publically disclosed late Friday at a security conference.

Scott Guthrie has provided information on workarounds (please see Important: ASP.NET Security Vulnerability and ASP.NET Security Vulnerability) to prevent attackers from using this vulnerability against their ASP.NET applications.

To help with Microsoft’s response to the new padding oracle vulnerability, a new forum was also set up: Security Vulnerability.

Microsoft has now announced the release of an out-of-band security update to address the ASP.NET Security Vulnerability.

Applying the update addresses the ASP.NET Security Vulnerability, and once the update is applied to your system the workarounds Scott has previously blogged about will no longer be required. But, until the update has been installed, those workarounds must be used.

You can learn more about this security update release from this reading the Microsoft Security Response Center Blog Post as well as the official Advance Notification Bulletin.

Important Links:

Hydrating Objects With Expression Trees – Part III

LINQ With C# (Portuguese)

To finalize this series on object hydration, I’ll show some performance comparisons between the different methods of hydrating objects.

For the purpose of this exercise, I’ll use this class:

class SomeType
{
    public int Id { get; set; }
    public string Name { get; set; }
    public DateTimeOffset CreationTime { get; set; }
    public Guid UniqueId { get; set; }
}

and this set of data:

var data = (
    from i in Enumerable.Range(1, ObjectCount)
    select new object[] { i, i.ToString(), DateTimeOffset.Now, Guid.NewGuid() }
).ToArray();

The data bellow shows the time (in seconds) for different runs for different values of ObjectCount (in the same machine with approximately the same load) as well as it’s availability for different version of the .NET Framework and the C# programming language:

10000 100000 1000000 Valid for
Setup Hydrate Total Setup Hydrate Total Setup Hydrate Total Framework version C# Version
Activation and Reflection setter 0.060 0.101 0.161 0.055 0.736 0.791 0.054 6.822 6.876 1.0, 1.1, 2.0, 3.5, 4.0 1.0, 2.0, 3.0, 4.0
Activation and Expression Tree setter 0.300 0.003 0.303 0.313 0.049 0.359 0.293 0.578 0.871 4.0 none
Member Initializer 0.035 0.001 0.036 0.039 0.027 0.066 0.041 0.518 0.559 3.5, 4.0 3.0, 4.0

These values will vary with the number of the objects being hydrated and the number of its properties, but the method using the Member Initializer will be the most performant.

Code samples for this series of posts (and the one about object dumping with expression trees) can be found on my MSDN Code Gallery: Dump And Hydrate Objects With Expression Trees

Hydrating Objects With Expression Trees – Part II

LINQ With C# (Portuguese)

In my previous post I showed how to hydrate objects by creating instances and setting properties in those instances.

But, if the intent is to hydrate the objects from data, why not having an expression that does just that? That’s what the member initialization expression is for.

To create such an expression we need the constructor expression and the property binding expressions:

var properties = objectType.GetProperties();
var bindings = new MemberBinding[properties.Length];
var valuesArrayExpression = Expression.Parameter(typeof(object[]), "v");

for (int p = 0; p < properties.Length; p++)
{
    var property = properties[p];

    bindings[p] = Expression.Bind(
        property,
        Expression.Convert(
            Expression.ArrayAccess(
                valuesArrayExpression,
                Expression.Constant(p, typeof(int))
            ),
            property.PropertyType
        )
    );
}

var memberInitExpression = Expression.MemberInit(
    Expression.New(objectType),
    bindings
);

var objectHidrationExpression = Expression.Lambda<Func<object[], object>>(memberInitExpression, valuesArrayExpression);

var compiledObjectHidrationExpression = objectHidrationExpression.Compile();

This might seem more complex than the previous solution, but using it is a lot more simple:

for (int o = 0; o < objects.Length; o++)
{
    newObjects[o] = compiledObjectHidrationExpression(objects[o]);
}

Hydrating Objects With Expression Trees – Part I

LINQ With C# (Portuguese)

After my post about dumping objects using expression trees, I’ve been asked if the same could be done for hydrating objects.

Sure it can, but it might not be that easy.

What we are looking for is a way to set properties on objects of an unknown type. For that, we need to generate methods to set each property of the objects.

Such methods would look like this expression:

Expression<Action<object, object>> expression = (o, v) => ((SomeType)o).Property1 = (PropertyType)v;

Unfortunately, we cannot use the .NET Reflector trick because, if you try to compile this, you’ll get this error:

error CS0832: An expression tree may not contain an assignment operator

Fortunately, that corresponds to a valid .NET expression tree. We just have to build it by hand.

So, for a given type, the set of property setters would be built this way:

var compiledExpressions = (from property in objectType.GetProperties()
                           let objectParameterExpression = Expression.Parameter(typeof(object), "o")
                           let convertedObjectParameteExpressionr = Expression.ConvertChecked(objectParameter, objectType)
                           let valueParameter = Expression.Parameter(propertyType, "v")
                           let convertedValueParameter = Expression.ConvertChecked(valueParameter, property.PropertyType)
                           let propertyExpression = Expression.Property(convertedObjectParameter, property)
                           select
                                Expression.Lambda<Action<object, object>>(
                                    Expression.Assign(
                                        propertyExpression,
                                        convertedValueParameter
                                    ),
                                    objectParameter,
                                    valueParameter
                                ).Compile()).ToArray();

And hydrating objects would be like this:

for (int o = 0; o < objects.Length; o++)
{
    var objectProperties = objects[o];

    var newObject = newObjects[o] = Activator.CreateInstance(objectType);

    for (int p = 0; p < compiledExpressions.Length; p++)
    {
        compiledExpressions[p](newObject, objectProperties[p]);
    }
}

Mastering Expression Trees With .NET Reflector

Following my last post, I received lots of enquiries about how got to master the creation of expression trees.

The answer is: .NET Reflector

On that post I needed to to generate an expression tree for this expression:

Expression<Func<object, object>> expression = o => ((object)((SomeType)o).Property1);

I just compiled that code in Visual Studio 2010, loaded the assembly in .NET Reflector, and disassembled it to C# without optimizations (View –> Options –> Disassembler –> Optimization: None).

The disassembled code looked like this:

Expression<Func<object, object>> expression;
ParameterExpression CS$0$0000;
ParameterExpression[] CS$0$0001;
expression = Expression.Lambda<Func<object, object>>(Expression.Convert(Expression.Property(Expression.Convert(CS$0$0000 = Expression.Parameter(typeof(object), "o"), typeof(SomeType)), (MethodInfo) methodof(SomeType.get_Property1)), typeof(object)), new ParameterExpression[] { CS$0$0000 });

After giving valid C# names to the variables and tidy up the code a bit, I came up with this:

ParameterExpression parameter = Expression.Parameter(typeof(object), "o");
Expression<Func<object, object>> expression =
    Expression.Lambda<Func<object, object>>(
        Expression.Convert(
            Expression.Property(
                Expression.Convert(
                    parameter,
                    typeof(SomeType)
                ),
                "Property1"
            ),
            typeof(object)
        ),
        parameter
    );

Easy! Isn’t it?

Dumping Objects Using Expression Trees

LINQ With C# (Portuguese)

No. I’m not proposing to get rid of objects.

A colleague of mine was asked if I knew a way to dump a list of objects of unknown type into a DataTable with better performance than the way he was using.

The objects being dumped usually have over a dozen of properties, but, for the sake of this post, let’s assume they look like this:

class SomeClass
{
    public int Property1 { get; set; }
    public long Property2 { get; set; }
    public string Property3 { get; set; }
    public object Property4 { get; set; }
    public DateTimeOffset Property5 { get; set; }
}

The code he was using was something like this:

var properties = objectType.GetProperties();

foreach (object obj in objects)
{
    foreach (var property in properties)
    {
        property.GetValue(obj, null);
    }
}

For a list of one million objects, this is takes a little over 6000 milliseconds on my machine.

I immediately thought: Expression Trees!

If the type of the objects was know at compile time, it would be something like this:

Expression<Func<SomeClass, int>> expression = o => o.Property1;
var compiled = expression.Compile();
var propertyValue = compiled.Invoke(obj);

But, at compile time, the type of the object and, consequently, the type of its properties, is unknown. So, we’ll need, for each property, an expression tree like this:

Expression<Func<object, object>> expression = o => ((SomeClass)o).Property1;

The previous expression gets the value of a property of the conversion of the parameter of type object to the type of the object. The result must also be converted to type object because the type of the result must match the type of the return value of the expression.

For the same type of objects, the collection of property accessors would be built this way:

var compiledExpressions = (from property in properties
                           let objectParameter = Expression.Parameter(typeof(object), "o")
                           select
                             Expression.Lambda<Func<object, object>>(
                                 Expression.Convert(
                                     Expression.Property(
                                         Expression.Convert(
                                             objectParameter,
                                             objectType
                                         ),
                                         property
                                     ),
                                     typeof(object)
                                 ),
                                 objectParameter
                             ).Compile()).ToArray();

Looks bit overcomplicated, but reading all properties of all objects for the same object set with this code:

foreach (object obj in objects)
{
    foreach (var compiledExpression in compiledExpressions)
    {
        compiledExpression (obj);
    }
}

takes a little over 150 milliseconds on my machine.

That’s right. 2.5% of the previous time.

C#: More On Array Variance

In a previous post, I went through how arrays have are covariant in relation to the type of its elements, but not safely covariant.

In the following example, the second assignment is invalid at run time because, although the type of the objectArray variable is array of object, the real type of the array is array of string and an object cannot be assigned to a string.

object[] objectArray = new string[] { "string 1", "string 2" };
objectArray[0] = "string 3";
objectArray[1] = new object();

On the other hand, because arrays are not contravariant in relation to the type of its elements, in the following code, the second line will fail at run time because string[] ≤ object[].

object[] objectArray = new object[] { "string 1", "string 2" };
string[] stringArray = (string[])objectArray;

The fact that all elements in the object array are strings doesn’t make it convertible to an array of string. To convert this object array of strings into a string array, you’ll need to create a new string array and copy each converted element.

The conversion can be as easily as this:

string[] stringArray = objectArray.Cast<string>().ToArray();

The above code is just shorthand to traversing the whole array while converting its elements to string and creating an string array with all elements.

Arrays are a good storage structure because they are the only structure provided by the runtime to store groups of items of the same type. However, because of limitations like the above, its use in APIs should be very carefully considered.

If all you need is to traverse all elements of some collection, you should use an IEnumerable<T> (IEnumerable<out T> in .NET 4.0). This way, the cost of using Enumerable.Cast<T>() is minimal.

Is Your ASP.NET Development Server Not Working?

Since Visual Studio 2005, Visual Studio comes with a development web server: the ASP.NET Development Server.

I’ve been using this web server for simple test projects since than with Visual Studio 2005 and Visual Studio 2008 in Windows XP Professional on my work laptop and Windows XP Professional, Windows Vista 64bit Ultimate and Windows 7 64bit Ultimate at my home desktop without any problems (apart the known custom identity problem, that is).

When I received my new work laptop, I installed Windows Vista 64bit Enterprise and Visual Studio 2008 and, for my surprise, the ASP.NET Development Server wasn’t working.

I started looking for differences between the laptop environment and the desktop environment and the most notorious differences were:

System

Laptop

Desktop

SKU

Windows Vista 64bit Enterprise

Windows Vista 64bit Ultimate

Joined to a Domain

Yes

No

Anti-Virus

McAffe

ESET

After asserting that no domain policies were being applied to my laptop and domain user and nothing was being logged by the ant-virus, my suspicions turned to the fact that the laptop was running an Enterprise SKU and the desktop was running an Ultimate SKU. After having problems with other applications I was sure that problem was the Enterprise SKU, but never found a solution to the problem. Because I wasn’t doing any web development at the time, I left it alone.

After upgrading to Windows 7, the problem persisted but, because I wasn’t doing any web development at the time, once again, I left it alone.

Now that I installed Visual Studio 2010 I had to solve this. After searching around forums and blogs that either didn’t offer an answer or offered very complicated workarounds that, sometimes, involved messing with the registry, I came to the conclusion that the solution is, in fact, very simple.

When Windows Vista is installed, hosts file, according to this contains this definition:

127.0.0.1       localhost
::1             localhost

This was not what I had on my laptop hosts file. What I had was this:

#127.0.0.1       localhost
#::1             localhost

I might have changed it myself, but from the amount of people that I found complaining about this problem on Windows Vista, this was probably the way it was.

The installation of Windows 7 leaves the hosts file like this:

#127.0.0.1       localhost
#::1             localhost

And although the ASP.NET Development Server works fine on Windows 7 64bit Ultimate, on Windows 7 64bit Enterprise it needs to be change to this:

127.0.0.1       localhost
::1             localhost

And I suspect it’s the same with Windows Vista 64bit Enterprise.