Monthly Archives: November 2014

Microsoft mechanism to load WPF resources is broken. (looking for a better title)

I’ve been using WPF along with .NET and have been a staunch supported of the technology for years now. The pattern of separating presentation from logic gained a lot of traction. Technologies such as iOS’s cocoa touch, JavaFX and others follow same model. Even HTML5 forced all the pure presentation/layout markup out from HTML and into CSS. So the idea is sound. But what about execution.

WPF implements this pattern using XAML. XAML markup is used to design the visual this xaml is stored in the xaml file, whose name usually matches the name of the class. For example, MyWindow.xaml. During compilation, an auto-generated file marries the code that defines the visual (MyWindow.xaml.cs) and the xaml file itself (MyWindow.xaml.cs). The autogenerate file would be named MyWindow.g.i.cs and it would contain the implementation of the InitializeComponent() method that’s referenced in the main MyWindow.xaml.cs class. The contents of the method looks something like this:

[System.Diagnostics.DebuggerNonUserCodeAttribute()]
[System.CodeDom.Compiler.GeneratedCodeAttribute("PresentationBuildTasks", "4.0.0.0")]
public void InitializeComponent() {
	if (_contentLoaded) {
		return;
	}
	_contentLoaded = true;
	System.Uri resourceLocater = new System.Uri("/MyLibrary;component/mainwindow.xaml", System.UriKind.Relative);
	
	#line 1 "..\..\MainWindow.xaml"
	System.Windows.Application.LoadComponent(this, resourceLocater);
	
	#line default
	#line hidden
}

So far so good right. OK, let’s talk about some of the advanced uses of .Net. Specifically support for having multiple versions of same assembly (dll) to be loaded in a single AppDomain. Such use case comes handy in a framework use case. A framework supports multiple plug-in modules, each can be developed by other developers who may rely on different versions of common libraries. I am not talking about things like log4net, I am talking about shared assemblies that define common logic and can be reused by each module. If 2 modules are loaded into a single app-domain and both depend on their ‘private’ version of the common assembly, a good framework should not force both modules to share latest version of the library, because

  • ‘latest’ does not mean backwards-compatible
  • ‘latest’ may have bugs that ‘earlier’ version did not
  • …I can go on for a while on this one. It’s 2014, dll hell is well known to software developers of the 20th century.

Now, let’s say that 2 different versions of the shared library called MyLibrary are loaded and one version is used by Module1 and another version is used by Module2. During compilation, assembly name along with the version are recorded in modules’ manifests so that when .NET encounters request to execute code defined in MyLibrary the source module will tell .Net which version of MyLibrary contains the code it wants executed. This is a basic building block of .NET and a very attractive feature.

Now, what about WPF? Well that’s not that clear is it? Look at Line #8 above: System.Uri resourceLocater = new System.Uri(“/MyLibrary;component/mainwindow.xaml”, System.UriKind.Relative). It defines what it calls a resourcelocator which the system will use to find the xaml resouce in an assembly called MyLibrary. I don’t know how WPF resolves the assembly, but if I were to write the logic for resolution, I would not have enough information to find a proper assembly MyLibrary, because I don’t know which version to load. I would have to guess. And as we know in any science guessing is bad. Computer Science is an exact… well… science. Guessing, hoping, holding fingers crossed or other techniques that do not produce determinate result will break at the worst possible moment! I can see how developers armed with inaccurate knowledge or with little experience make mistakes like this, but when Microsoft’s tools generated code that’s faulty, and make it impossible to intercept and fix, there is no other way to call it – they fucked up! How can they expect developers to trust their tooling if they, themselves, auto-generate code that sometimes breaks.

This breaks in a most bizarre way, too. MyLibrary version 2 was loading the MainWindow class, which then was running the InitializeComponent() method above, encountered the LoadComponent call, which asked to load the resource from the MyLibrary assembly without a version, and the code picked the wrong version of the MyLibrary assembly, which did not have the MyWindow class, nor the resource. What a way to blow up!

Such nonsense was reported to them too! This connect ticket marked as “Wont’t Fix” describes the problem: https://connect.microsoft.com/VisualStudio/feedback/details/668914/xaml-generated-code-uses-resource-uri-without-assembly-strong-name. StackOverflow also has references to this problem: http://stackoverflow.com/questions/1453107/how-to-force-wpf-to-use-resource-uris-that-use-assembly-strong-name-argh/6341678#6341678. Luckily one of the suggestions led me to the best solution.

The Fix


We were lucky to have our builds automated via scripts which rely on MSBuild to build the solutions. All I needed to do is to modify the script and pass the /p:AssemblyInfo=$version parameter to MSBuild. (http://stackoverflow.com/a/26689750/195275). This would tell the build system to add assembly version to the Resource reference in line 8. It now looks like the code below. Problem solved, but could have been avoided on the first place!

Shame on you, Microsoft!

[System.Diagnostics.DebuggerNonUserCodeAttribute()]
[System.CodeDom.Compiler.GeneratedCodeAttribute("PresentationBuildTasks", "4.0.0.0")]
public void InitializeComponent() {
	if (_contentLoaded) {
		return;
	}
	_contentLoaded = true;
	System.Uri resourceLocater = new System.Uri("/MyLibrary;V1.0.23;component/mainwindow.xaml", System.UriKind.Relative);
	
	#line 1 "..\..\MainWindow.xaml"
	System.Windows.Application.LoadComponent(this, resourceLocater);
	
	#line default
	#line hidden
}