It seems that we currently do not take into account versioning issues while determining validity of an assignment of different types. Take a look at the example in the attached solution. The method in the first project takes a parameter of type CommandBar defined in the assembly Office version X. This method is called in the second project with an argument of type CommandBar defined in the same assembly but of the version Y. The code compiles fine regardless of if X is less than Y or vice versa. However, ReSharper complains about type incompatibility in the method call.