As of .NET 1.1 Service Pack 1, the COM Interop layer will only allow a COM
object to talk to a .NET object if the .NET object has public visibility and
is marked [ComVisible(true)].  Prior to .NET 1.1 SP1, you could pass a class
instance to COM even if the class was internal or didn't have
[ComVisible(true)].  The only requirement used to be that the class
implement a COM interface.  Now the COM Interop layer also ensures that you've
explicitly made the class visible to COM.

If you forget to do this, the COM Interop layer will throw a
System.InvalidCastException at run-time, with a message of "No such
interface supported".  This error is very misleading because your class does
implement the correct interface or the code wouldn't have compiled.  The
problem is that at run-time the QueryInterface from COM fails because the
COM Interop layer won't give access to a .NET object unless it is public and
marked [ComVisible(true)].

Unfortunately, this wasn't documented as part of any SP1 change that I could
find.  It broke my application, and I spent a couple of frustrating days
tracking down the problem.  I'm posting about it here so that maybe
Microsoft can document it and so that people searching newsgroups for this
problem can find a solution.

FWIW, I've also written an FxCop rule to check for this case in all my
assemblies.  Perhaps Microsoft's FxCop team should include a similar rule.
The logic in my implementation of ITypeRule's Check method is:

 if (IsComVisible(type))
 {
   if (!type.IsPublic)
   {
     return "COM types must have public visibility.";
   }
 }
 else if (ImplementsComInterface(type) &&
!type.IsSubclassOf(typeof(Control)))
 {
   //The type implements a COM interface, but it isn't marked
   //as visible to COM.  That's almost certainly an error.
   if (type.IsPublic)
   {
     return "Types that implement COM interfaces should use
[ComVisible(true)].";
   }
   else
   {
     return "Types that implement COM interfaces should be public and use
[ComVisible(true)].";
   }
 }

 return null;