After a recent upgrade from Silverlight 4.0 to Silverlight 5.0, I started getting this error from Visual Studio 2010
Error 21 CA0055 : Could not unify the platforms (mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, mscorlib, Version=5.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e) for '[path to Silverlight project dll]'.
Took me a while to track it down, and I found the solution to my problem in this post.
Being that we don’t use Code Analysis, I just turned it off on my Silverlight project.
But if you do use it, you’re going to have to run the command line arguments as the post suggests.