COM interop, C#, Visual Studio 2010 -> embed interop types

Question

My C# program accesses SAP via Nco3 (sapnco.dll). This program also needs to work with Delphi. Some of my methods return types from the sapnco.dll:

public void IRfcTable table(...) { ... }

in Delphi this method shows up as

function table(...): IUnknown { ... }

I suppose this IUnknown is because my TLB does not include the sapnco.dll. I tried "Embed Interop Types = true" in Visual Studio, but then this error occurs:

Error Interoptypen aus Assembly "C:\..." können nicht eingebettet werden, weil das ImportedFromTypeLibAttribute-Attribut oder das PrimaryInteropAssemblyAttribute-Attribut fehlt. c:...\sapnco.dll

(Interop Types could not be embedded because some attributes are missing).

Is this the right way? If so, where to put these attributes?

Answer 1

sapnco.dll is a .NET dll, so it is not exposed to COM, so you cannot directly use this types in a COM environment. The solution to your problem is to create a library to wrap the sapnco.dll in COM exposed classes:

As an example:

[ComVisible(true)]
public interface IComRfcTable
{
    public void DoSomething();
}

[ComVisible(true)]
public class ComRfcTable : IComRfcTable
{
    private _rfcTable; // object to wrap
    public ComRfcTable(IRfcTable rfcTable)
    {
        _rfcTable = rfcTable
    }

    public void DoSomething()
    {
        _rfcTable.DoSomething();
    }
}

Then your method must be implemented like:

 public IComRfcTable table(...) { ... }