COM Interop registration

Question

I have a .NET assembly which I am exposing to COM. The assembly has two public interfaces and one public class. When I build the assembly I get this warning:

(assemblyName.dll) does not contain any types that can be registered for COM Interop.

My assembly information includes the following line.

[assembly: ComVisible(true)]

Most people having this problem on the web, that I have found, fixed it with the above line in their assembly information. This has not helped for me.

I also tried adding [ComVisible(true)] to the class and interface definitions, and it also did not help.

Answer 1

ComVisible classes generally need to have a public default constructor. Its members should typically also reference only ComVisible types.

You don't need to specify ComVisible(true) on the class if you have specified it at the assembly level.

However, the usual way to generate an assembly with ComVisible classes is:

• Specify ComVisible(false) at assembly-level. Thus only classes that are explicitly marked with ComVisible(true) are exposed to COM.

• Define an explicit ComVisible interface :

e.g.

[
ComVisible(true),
GuidAttribute("..."),
Description("...")
]
public interface IMyComVisibleType
{
        // members...
     }

• Your ComVisible class should specify ClassInterfaceType.None, and should implement the ComVisible interface:

e.g.

     [
     ComVisible(true),
     GuidAttribute("..."),
     ClassInterface(ClassInterfaceType.None)
     ]
     public sealed class MyComVisibleType : IMyComVisibleType
     {
        // implementation ...
     }

Note that the Guid and Description attributes are not required, but useful to give you more control of the COM generation.

If the above doesn't help, try posting some sample code and I'm sure someone will be able to help.

Answer 2

I ran into the default constructor problem. What fooled me was that the type library file will contain the class GUID reference even though that class is not being registered. A quick way to see what will be registered is to create a registry file ('assembly.reg') like this:

regasm assembly.dll /regfile:assembly.reg /codebase

There's a good discussion of exposing interfaces in COM Interop: Base class properties not exposed to COM. Some example code is here: Exposing .NET Components to COM.