WPF Application fails on startup with TypeInitializationException

Anything wrong in the App.config file may cause the error, such as a typo of * at the end of a line, eg ...</startup> has an additional "*" at the end of the line ...</startup>*.

Do you use .config file? If so, check it for errors. Initialization errors of such sort are often triggered by invalid XML: if there are no errors in XAML, XML config is the first place to look.

Tracking the InnerExceptions deep down , you might find the following error:

"Only one <configSections> element allowed per config file and if present must be the first child of the root <configuration> element"

This order change happened after Visual Studio EntityFramework Wizard added the connectionStrings element to the top

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.