"Could not load file or assembly 'PresentationUI.Aero2' or one of its dependencies." Why not?

Question

In my WPF application, I get the following exception on startup:

A first chance exception of type 'System.IO.FileNotFoundException' occurred in mscorlib.dll

Additional information: Could not load file or assembly 
'PresentationUI.Aero2, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' 
or one of its dependencies. 

EDIT: Using fusion log, I get a little more valuable info than the call stack:

LOG: DisplayName = PresentationUI.Aero2, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35
 (Fully-specified)
LOG: Appbase = file:///[...]/bin/Debug/
LOG: Initial PrivatePath = NULL
LOG: Dynamic Base = NULL
LOG: Cache Base = NULL
LOG: AppName = EngideskLauncher.vshost.exe
Calling assembly : PresentationFramework, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35.
===
LOG: This bind starts in default load context.
LOG: Using application configuration file: [...]\bin\Debug\EngideskLauncher.vshost.exe.Config
LOG: Using host configuration file: 
LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework\v4.0.30319\config\machine.config.
LOG: Post-policy reference: PresentationUI.Aero2, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35
LOG: GAC Lookup was unsuccessful.
LOG: Attempting download of new URL file:///[...]/bin/Debug/PresentationUI.Aero2.DLL.
LOG: Attempting download of new URL file:///file:///[...]/bin/Debug/PresentationUI.Aero2/PresentationUI.Aero2.DLL.
LOG: Attempting download of new URL file:///file:///[...]/bin/Debug/PresentationUI.Aero2.EXE.
LOG: Attempting download of new URL file:///file:///[...]/bin/Debug/PresentationUI.Aero2/PresentationUI.Aero2.EXE.
LOG: All probing URLs attempted and failed.

What I find strange, is that the calling assembly is PresentationFramework, which is a .NET framework assembly, obviously. A .NET Framework assembly wouldn't call an assembly which is not a .NET framework assembly. Anyway, I can't find a PresentationUI.Aero2.DLL anywhere and not even Google seems to know anything about it??

Any ideas?

Additional information:

• .NET Framework 4.0
• Windows 8.1

Answer 1

If you're interested, this is a (benign) bug in WPF. The exception is first-chance and can be ignored.

WPF forgot to add Aero2.NormalColor.xaml to PresentationUI.dll. If you inspect PresentationUI.dll with your favorite reflector/decompiler, you'll find all sorts of themes, such as Aero.NormalColor.baml, etc. but no Aero2.NormalColor.xaml. This causes WPF to try and see if an external assembly exists:

This tries to load Aero2.NormalColor.baml from PresentationUI.dll and returns null: http://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/SystemResources.cs,773

This then goes to try an external assembly: http://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/SystemResources.cs,554

And this throws the actual exception: http://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/SystemResources.cs,706

This exception is commonly observed when using a FlowDocument or a FlowDocumentScrollViewer.

Answer 2

I was getting the same error and finally realized that it was simply stopping in the IDE because I had first chance exceptions turned on, the exception doesn't actually matter and you can ignore or continue past it.