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c#cominterface
Alex Marshall
Alex Marshall

Reputation: 10322

Explicit interface implementation for COM interfaces in C#

Some time I ago I was working on a major refactoring of an old Win32 program implemented with COM, and there were various parts that were implemented with C# (.NET). During my work on this project, I ran across a Microsoft page on COM programming in C# that recommended C# classes explicitly implement COM interfaces, rather than implicity. I recently tried to remember why, and I couldn't. I also couldn't find the page on the MSDN site again. Can anybody please tell me why Microsoft might recommend this ?

Upvotes: 4

Views: 1238

Answers (2)

Hans Passant
Hans Passant

Reputation: 942518

Hmm, that makes a wee bit of sense, COM is pure interface-based programming and the actual implementation of the interfaces should be hidden. Implementing interface methods explicitly gets you that automatically because they cannot be public.

Actually doing this is quite pointless, you could (and should) simply apply the [ClassInterface(ClassInterfaceType.None)] attribute to the class. That by itself ensures that the implementation isn't exposed, only the interfaces implemented by the class are visible. Implementing the interface methods explicitly isn't actually good enough. Because you cannot hide the fact that your class inherits System.Object. Which exposes the four public methods of Object and puts a reference to mscorlib.tlb in your type library, a reference that a real COM client will never use. It will almost always work because the odds that the compiler that uses your class runs on a machine that doesn't have .NET installed are pretty small. But very yucky nonetheless, it isn't actually required. Only the machine that uses the class needs it installed.

Just don't do this. Declare the interfaces you implement, give them the [InterfaceType(ComInterfaceType.InterfaceIsDual)] attribute to allow them to be used both early and late bound. And hide the actual implementation of them with [ClassInterface(ClassInterfaceType.None)]. Only sensible way.

Upvotes: 3

Jason Williams
Jason Williams

Reputation: 1283

It's old, but from here: http://msdn.microsoft.com/en-us/library/aa288461%28v=VS.71%29.aspx they mention implementing an interface explicitly so you can implement multiple interfaces that have the same member names.

This also requires that the user of your class cast an instance of your class to the appropriate interface.

As for why this is especially important for COM: my first guess is so that COM can call one set of methods while managed code may call another. However, I'm guessing here.

Upvotes: 1

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