The designer could not be shown for this file because none of the classes within it can be designed. The designer inspected the following classes in the file: DerivedControl --- The base class 'GenericControlBase' could not be loaded. Ensure the assembly has been referenced and that all projects have been built. I can understand – but it’s still a bit annoying! I have some windows forms user controls that inherit from a class that implements generics. Problem is the designer can’t reflect those classes and in turn spits out the wonderful error above.