We switched Autofac 3.0 to be a Portable Class Library so we can target multiple platforms. In consuming the updated Autofac, I’ve noticed some folks receive errors like this at runtime:

Test 'MyNamespace.MyFixture.MyTest' failed: System.IO.FileLoadException : Could not load file or assembly 'System.Core, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Retargetable=Yes' or one of its dependencies. The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)
    at Autofac.Builder.RegistrationData..ctor(Service defaultService)
    at Autofac.Builder.RegistrationBuilder`3..ctor(Service defaultService, TActivatorData activatorData, TRegistrationStyle style)
    at Autofac.RegistrationExtensions.RegisterInstance[T](ContainerBuilder builder, T instance)
    MyProject\MyFixture.cs(49,0): at MyNamespace.MyFixture.MyTest()

You can solve this by getting the latest .NET framework patches from Windows Update. Article KB2468871 and the associated patch specifically address this, however in practice I’ve noticed that specific patch doesn’t always fix it for everyone. I actually don’t know if it’s a different patch that fixes the 100% case or if it’s different patches for different people. What I do know is that once the machine is patched up with the latest, the problem goes away.

If you’re on a domain where there’s an internal Windows Server Update Services instance running, you may think you have the latest but you might not. If it looks like you’re all patched up but still seeing the error, make sure you’ve checked online for updates.

See here, it looks like I’ve got the latest…

…but really, I’m missing several updates.

A note on the version 2.0.5.0 you see in the error message: That’s the version of the framework Autofac references as an artifact of being a Portable Class Library. 2.0.5.0 is Silverlight, which is one of the platforms we target. You won’t see it installed on your machine, don’t go trying to manually hack around and install it. But don’t freak out, that version is expected and it’s OK. The .NET framework patches should allow you to dynamically redirect that reference at runtime to the appropriate version (e.g., 4.0.0.0) and the error will go away.