WPF Application fails on startup with TypeInitializationException

Question

I have a simple WPF application which I am trying to start. I am following the Microsoft Patterns and Practices "Composite Application Guidance for WPF". I've followed their instructions however my WPF application fails immediately with a "TypeInitializationException".

The InnerException property reveals that "The type initializer for 'System.Windows.Navigation.BaseUriHelper' threw an exception."

Here is my app.xaml:

<Application x:Class="MyNamespace.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Application.Resources>         
    </Application.Resources>
</Application>

And here is my app.xaml.cs (exception thrown at "public App()"):

public partial class App : Application
{
    public App()
    {
        Bootstrapper bootStrapper = new Bootstrapper();
        bootStrapper.Run();
    }
}

I have set the "App" class as the startup object in the project.

What is going astray?

Answer 1

Thanks @ima, your answer pointed me in the right direction. I was using an app.config file and it contained this:

<configuration>
  <startup>
    <supportedRuntime version="v2.0.50727" sku="Client"/>
  </startup>
  <configSections>
    <section name="modules" type="Microsoft.Practices.Composite.Modularity.ModulesConfigurationSection, Microsoft.Practices.Composite"/>
  </configSections>
  <modules>
    <module assemblyFile="Modules/MyNamespace.Modules.ModuleName.dll" moduleType="MyNamespace.Modules.ModuleName.ModuleClass" moduleName="Name"/>
  </modules>
</configuration>

It seems the problem was the <startup> element because when I removed it the application ran fine. I was confused because Visual Studio 2008 added that when I checked the box to utilise the "Client Profile" available in 3.5 SP1.

After some mucking about checking and un-checking the box I ended up with a configuration file like this:

<configuration>
  <configSections>
    <section name="modules" type="Microsoft.Practices.Composite.Modularity.ModulesConfigurationSection, Microsoft.Practices.Composite"/>
  </configSections>
  <modules>
    <module assemblyFile="Modules/MyNamespace.Modules.ModuleName.dll" moduleType="MyNamespace.Modules.ModuleName.ModuleClass" moduleName="Name"/>
  </modules>
  <startup>
    <supportedRuntime version="v2.0.50727" sku="Client"/>
  </startup>
</configuration>

Which works!

I'm not sure why the order of elements in the app.config is important - but it seems it is.