Prism cannot load modules from XAML when using MEF because the first char of the file name will be truncated and a file not found exception is raised.
 
The line in LoadModuleType() in MefFileModuleTypeLoader.Desktop.cs:
 
string path = moduleInfo.Ref.Substring(RefFilePrefix.Length + 1);
 
should be:
 
string path = moduleInfo.Ref.Substring(RefFilePrefix.Length);
 
I.e. Ref = "file://TestModule.dll"
 
After URI parsing:
 
RefFilePrefix = "file://" (Length = 7)
 
In the current release the path ends up with:
 
Ref.Substring(8) =>
 
path = "estModule.dll"
 
Workaround: Write Ref="file://_TestModule.dll" (with an extra character at the beginning) in XAML module catalog.