Need of ClassInterfaceType.None?

Question

1. Didn't quite get the following from MSDN:

   ClassInterfaceType.None

   Indicates that no class interface is generated for the class. If no interfaces are implemented explicitly, the class can only provide late-bound access through the IDispatch interface. This is the recommended setting for ClassInterfaceAttribute. Using ClassInterfaceType.None is the only way to expose functionality through interfaces implemented explicitly by the class.

2. Is [ComVisible(true)] a must for COM visibility?

Answer 1

See this blog post for the expanation of the first problem. The point is that unless you specify ClassInterfaceType.None an extra interface is generated and this can cause problems with binary compatibility if you alter the order of methods, change their signatures or remove some of them. A much better alternative is to explicitly define an interface and inherit your class from it specifying ClassInterfaceType.None.

ComVisible(true) is not a must for COM visibility. By default all public methods of public classes and all public classes are COM visible, all other entities are not COM visible. If you need to alter this you use ComVisible attribute, usually to decrease the amount of COM visible classes since for every COM visible class registry entries and type library entries are generated and this causes unnecessary registry pollution and inflates the type library. IMO it's a good idea to explicitly mark all public entities with ComVisible.