Monthly Archives: April 2010

LINQ: Enhancing Distinct With The PredicateEqualityComparer

LINQ With C# (Portuguese)

Today I was writing a LINQ query and I needed to select distinct values based on a comparison criteria.

Fortunately, LINQ’s Distinct method allows an equality comparer to be supplied, but, unfortunately, sometimes, this means having to write custom equality comparer.

Because I was going to need more than one equality comparer for this set of tools I was building, I decided to build a generic equality comparer that would just take a custom predicate. Something like this:

public class PredicateEqualityComparer<T> : EqualityComparer<T>
{
    private Func<T, T, bool> predicate;

    public PredicateEqualityComparer(Func<T, T, bool> predicate)
        : base()
    {
        this.predicate = predicate;
    }

    public override bool Equals(T x, T y)
    {
        if (x != null)
        {
            return ((y != null) && this.predicate(x, y));
        }

        if (y != null)
        {
            return false;
        }

        return true;
    }

    public override int GetHashCode(T obj)
    {
        // Always return the same value to force the call to IEqualityComparer<T>.Equals
        return 0;
    }
}

Now I can write code like this:

.Distinct(new PredicateEqualityComparer<Item>((x, y) => x.Field == y.Field))

But I felt that I’d lost all conciseness and expressiveness of LINQ and it doesn’t support anonymous types. So I came up with another Distinct extension method:

public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource, bool> predicate)
{
    return source.Distinct(new PredicateEqualityComparer<TSource>(predicate));
}

And the query is now written like this:

.Distinct((x, y) => x.Field == y.Field)

Looks a lot better, doesn’t it? And it works wit anonymous types.

Update: I, accidently, had published the wrong version of the IEqualityComparer<T>.Equals method,

CodeIt.Right Code File Header Template For StyleCop Rules

I like to use both StyleCop and CodeIt.Right to validate my code – StyleCop because it’s free and CodeIt.Right because it’s really good.

While StyleCop provides only validation, CodeIt.Righ provides both validation and correction of violations.

Unfortunately, CodeIt.Right’s supplied template for code file headers does not conform to StyleCop rules.

Fortunately, CodeIt.Right allows us to define our own template. Here’s the one I use:

<#@ template language="C#" #>
//-----------------------------------------------------------------------
// <copyright file="<#= System.IO.Path.GetFileName(Context.DestinationFile) #>"
//            project="<#= Context.ProjectName #>"
//            assembly="<#= Context.AssemblyName #>"
//            solution="<#= Context.SolutionName #>"
//            company="<#= Context.GetGlobalProperty("CompanyName") #>">
//     Copyright (c) <#= Context.GetGlobalProperty("CompanyName") #>. All rights reserved.
// </copyright>
// <author id="<#= Context.GetGlobalProperty("UserID") #>"><#= Context.GetGlobalProperty("UserName") #></author>
// <summary></summary>
//-----------------------------------------------------------------------

Defining Document Compatibility In Internet Explorer 8

The procedures to define document compatibility in Internet Explorer 8 are well documented here, but I’ve seem many developers and system administrators that are not aware of this.

Although you can (and should) define the document compatibility your web pages were designed to, if you don’t, Internet Explorer 8 and the Web Browser Control will default to these compatibility modes:

Application

Intranet

Internet

Internet Explorer 8 IE7 mode IE8 mode
Application hosting the Web Browser Control IE7 mode IE7 mode

If you notice the table above, by default, only Internet Explore 8 will present itself to the as Internet Explorer 8 and only to Internet sites.

The way Internet Explorer (and any other browser) presents itself the web servers is using its user agent string:

Mode

User Agent String

IE7 Mozilla/4.0 (compatible; MSIE 7.0; …; Trident/4.0; …)
IE8 Mozilla/4.0 (compatible; MSIE 8.0; …; Trident/4.0; …)

(If you are curious about the history of the user-agent string, read the History of the user-agent string)

Microsoft did this to keep compatibility with legacy applications used by enterprises (large and small) but this brings a few issues to development and testing.

If you are building a public web site for Internet Explorer 8, you might see the right thing on your development machine, but the quality assurance team will see the site as if it were an Internet Explorer 7 if the version they are testing is on the intranet. If the the web site you are developing is going to be accessed from an application hosting the Web Browser Control and you don’t test on that application, you are not going to see the same thing.

To know how is your browser presenting itself to the web server in the internet, there are several web sites that will show information about the user-agent string (like http://www.useragents.org/) and it helps to have the same thing in your intranet. If you want to build such a web application using ASP.NET, you can use the UserAgent property of the HttpRequest class (or the Browser property for more detailed information).

This type of information is also available in Internet Explorer in the navigator object.

Giorgio Sardo has a few functions to detect Internet Explorer 8 but you might also want to develop a diagnostics page (or part) to show the web browser features, something like this:

<fieldset id="webBrowserInfo">
    <legend>Web Browser</legend>
    <table border="1">
        <tr>
            <td class="label" style="width: 100px"><label for="webBrowser$userAgent">userAgent</label> </td>
            <td class="value" colspan="3"><span id="webBrowser$userAgent"></span></td>
        </tr>
        <tr>
            <td class="label" style="width: 100px"><label for="webBrowser$appVersion">appVersion</label></td>
            <td class="value"><span id="webBrowser$appVersion"></span></td>
            <td class="label"><label for="webBrowser$appMinorVersion">appMinorVersion</label></td>
            <td class="value"><span id="webBrowser$appMinorVersion"></span></td>
        </tr>
        <tr>
            <td class="label" style="width: 100px"><label for="webBrowser$appCodeName" style="width: 600px">appCodeName</label></td>
            <td class="value" colspan="3"><span id="webBrowser$appCodeName"></span></td>
        </tr>
        <tr>
            <td class="label"><label for="webBrowser$appName">appName</label></td>
            <td class="value" colspan="3"><span id="webBrowser$appName"></span></td>
        </tr>
        <tr>
            <td class="label" style="width: 100px"><label for="webBrowser$userLanguage">userLanguage</label></td>
            <td class="value"><span id="webBrowser$userLanguage"></span></td>
            <td class="label" style="width: 100px"><label for="webBrowser$cpuClass">cpuClass</label></td>
            <td class="value"><span id="webBrowser$cpuClass"></span></td>
        </tr>
        <tr>
            <td class="label"><label for="webBrowser$systemLanguage">systemLanguage</label></td>
            <td class="value"><span id="webBrowser$systemLanguage"></span></td>
            <td class="label"><label for="webBrowser$platform">platform</label></td>
            <td class="value"><span id="webBrowser$platform"></span></td>
        </tr>
        <tr>
            <td class="label"><label for="webBrowser$browserLanguage">browserLanguage</label></td>
            <td class="value"><span id="webBrowser$browserLanguage"></span></td>
            <td class="label" style="width: 100px"><label for="webBrowser$cookieEnabled">cookieEnabled</label></td>
            <td class="value" colspan="5"><span id="webBrowser$cookieEnabled"></span></td>
        </tr>
    </table>
</fieldset>

<script type="text/javascript">
    document.getElementById("webBrowser$userAgent").innerHTML = window.navigator.userAgent;
    document.getElementById("webBrowser$appCodeName").innerHTML = window.navigator.appCodeName;
    document.getElementById("webBrowser$appMinorVersion").innerHTML = window.navigator.appMinorVersion;
    document.getElementById("webBrowser$appName").innerHTML = window.navigator.appName;
    document.getElementById("webBrowser$appVersion").innerHTML = window.navigator.appVersion;
    document.getElementById("webBrowser$browserLanguage").innerHTML = window.navigator.browserLanguage;
    document.getElementById("webBrowser$cookieEnabled").innerHTML = window.navigator.cookieEnabled;
    document.getElementById("webBrowser$cpuClass").innerHTML = window.navigator.cpuClass;
    document.getElementById("webBrowser$platform").innerHTML = window.navigator.platform;
    document.getElementById("webBrowser$systemLanguage").innerHTML = window.navigator.systemLanguage;
    document.getElementById("webBrowser$userLanguage").innerHTML = window.navigator.userLanguage;
</script>

Windows Home Server "Unknown network error has occurred during PC Restore" Problem

This week I had to restore my son’s PC due to a game he had installed that rendered Windows 7 in a state it was unable to reboot or recover from.

Fortunately we have a Windows Home Server, so I was not worried. All it would take was to restore to the previous backup – automatically made the day before.

Well, it turned out it wasn’t so easy.

Windows Home Server stores drivers with the backup but the restore program is a 32bit program and this machine had a 64bit version of Windows 7.

After adding 32bit drivers for the wireless network card, I tried to restore the PC, but the restore program was unable to find the server with the wireless network card and was unable to detect the on-board Ethernet card.

I downloaded 32bit drivers for the on-board Ethernet card, grabbed a long Ethernet cable and tried again. Now the restore program was able to find the server and start the restore.

The problem was that, after a while, the restore would end with an Unknown network error has occurred during PC Restore error.

After a few tries, I figured out that the error occurred about the time the the router assigned IP lease expired – 1 hour. After extending the lease time, the restore was able to complete.

I still find Windows Home Server great, but it could be better if:

  • Client PCs could boot directly from the server through a network boot.
    I’m not sure if this is possible, but it would be a very nice thing to have.
  • The restore program should have a 64bit version and be able to use the 64bit drivers of the client PC.
    Or, alternatively, ask for 32bit drivers when the connector is installed in the client PC.
  • Improved network support.
    I still don’t know what the real problems were, because I eventually was able to restore the PC and I’m not a networking expert.

Comprehensive instructions on what to do when networking is not working with the restore CD environment by Olaf, can be found here. It doesn’t mention the lease problem, though.