XamlParseException Invalid URI: The format of the uri could not be determined

I'm getting this exception in my WPF app on one particular PC, on half a dozen others it runs fine. Here is the full exception trace:

System.Windows.Markup.XamlParseException: The type initializer for 'System.Windows.Window' threw an exception. 
---> System.TypeInitializationException: The type initializer for 'System.Windows.FrameworkElement' threw an exception. 
---> System.TypeInitializationException: The type initializer for 'System.Windows.Documents.TextElement' threw an exception. 
---> System.TypeInitializationException: The type initializer for 'MS.Internal.FontCache.Util' threw an exception. 
---> System.UriFormatException: Invalid URI: The format of the URI could not be determined.

   at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind)
    at System.Uri..ctor(String uriString, UriKind uriKind)
    at MS.Internal.FontCache.Util..cctor()

 --- End of inner exception stack trace ---

   at MS.Internal.FontCache.Util.get_Dpi()
   at System.Windows.SystemFonts.ConvertFontHeight(Int32 height)
   at System.Windows.Documents.TextElement..cctor()

   --- End of inner exception stack trace ---

   at System.Windows.FrameworkElement..cctor()

   --- End of inner exception stack trace ---

   at System.Windows.Window..cctor()

   --- End of inner exception stack trace ---

   at System.Windows.Markup.XamlReader.RewrapException(Exception e, IXamlLineInfo lineInfo, Uri baseUri)
   at System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, IXamlObjectWriterFactory writerFactory, Boolean skipJournaledProperties, Object rootObject, XamlObjectWriterSettings settings, Uri baseUri)
   at System.Windows.Markup.WpfXamlLoader.LoadBaml(XamlReader xamlReader, Boolean skipJournaledProperties, Object rootObject, XamlAccessLevel accessLevel, Uri baseUri)
   at System.Windows.Markup.XamlReader.LoadBaml(Stream stream, ParserContext parserContext, Object parent, Boolean closeStream)
   at System.Windows.Application.LoadBamlStreamWithSyncInfo(Stream stream, ParserContext pc)
   at Sy开发者_开发知识库stem.Windows.Application.LoadComponent(Uri resourceLocator, Boolean bSkipJournaledProperties)
   at System.Windows.Application.DoStartup()
   at System.Windows.Application.<.ctor>b__1(Object unused)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)

It looks like it's having a problem at this point:

---> System.TypeInitializationException: The type initializer for 'MS.Internal.FontCache.Util' threw an exception. 
---> System.UriFormatException: Invalid URI: The format of the URI could not be determined.

Seeing that the issue is not consistent between machines and there's mention of fonts, is it possible that you're referencing a font that isn't installed on this particular machine?

The problem is due to the WinDir environment variable getting messed up, which is used in the static initializer for internal Microsoft class FontCache.Util. This could be caused by a very long PATH environment variable, as documented here: https://superuser.com/questions/237268/missing-environment-variables-in-windows-7.

(FontCache.Util: http://referencesource.microsoft.com/#PresentationCore/Core/CSharp/MS/Internal/FontCache/FontCacheUtil.cs).