If you are using Visual Studio Fakes in your Unit Testing, and you created your test project from any of the below versions of Visual Studio, then if you upgrade to Visual Studio 2013 Update 4, you may start hitting build errors around Microsoft.QualityTools.Testing.Fakes.dll.

Visual Studio 2012

Visual Studio 2013 Update1

Visual Studio 2013 Update2

Visual Studio 2013 Update3

Exact error message : “The type ‘Microsoft.QualityTools.Testing.Fakes.Stubs.StubBase`1’ is defined in an assembly that is not referenced. You must add a reference to assembly ‘Microsoft.QualityTools.Testing.Fakes, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'”.

To overcome this build error : please follow any of the below workarounds :

1. In all your .proj files that refer this assembly, remove the version info from the reference. (it should not be having 11.0.0.0)

2. In all your .proj files that refer this assembly, turn specific version field of the reference to “false”.

Hope this helps.