A C# add-in problem: The property Manager Page is declared in a separate DLL

December 2, 2009
Usually while creating a SolidWorks Add-in project, it is more convenient to move the Property Manager Page Interfaces to separate DLL-library.

However, sometimes an "Invalid Cast Exception" message appears in such .NET-applications.

The problem here is the visibility of the assembly. The DLL must be COM-visible. Go to Project Properties -> Assembly Information and then check the "Make Assembly COM-visible" box.

Another issue that can cause this exception is a class access modifier. The PMPage handler class must be declared 'Public' in order to avoid this problem.