Did you have a VS2010 project (before installing VS2010 SP1 or RIA Service Pack 1) that used Silverlight 4 and WCF RIA Services and Entity Framework 4 (EF4) and now that VS2010 Service Pack 1 is installed (or maybe some other framework) your Silverlight Project no longer compiles. Chances are if you are using RIA Services you may have created files that you want to share (called Shared Code)on the Server and the SL Client. This is done in RIA by using the .shared.cs file extension on a filename.
After I installed either VS2010 SP1 or RIA Services SP1 and then I tried to compile my project and got a compiler error similar to:
error CS0102: The type 'MyApp.Model.MyClassHere' already contains a definition for 'MyPropertyHere'
It appears that this MAY be fixed now, but I’m not sure since I don’t have an issue exactly like what they had. I only have the issue if I have to shared files and one the classes (in one of the files) has a property or method that has a return type that is the other class type. If I get rid of that dependency and just return the object type instead then it compiles fine. While returning the type object, I have to cast every time I use it and that is silly and kind of a pain / ugly.
I hope that helps someone. If anyone else has this issue I’d love to know about it.