COM Interop Not Detecting .NET Functions Via Interface

Question

I have a .NET assembly that I have executed regasm and gacutil. I also have a COM interop that I am trying to get to work with the .NET assembly. However, through my pDotNetCOMPtr I am not able to "detect" any of the methods on my .NET public interface. The MFC COM DLL keeps saying that there is not method called Encrypt in _SslTcpClientPtr when I try to compile with Visual Studio 2010. I am using the .NET 4.0 Framework. Thoughts?

COM / MFC

extern "C" __declspec(dllexport) BSTR __stdcall Encrypt(BSTR encryptString)
{   
    CoInitialize(NULL);

    ICVTnsClient::_SslTcpClientPtr pDotNetCOMPtr;

    HRESULT hRes = pDotNetCOMPtr.CreateInstance(ICVTnsClient::CLSID_SslTcpClient);

    if (hRes == S_OK)
    {
        BSTR str;

        hRes = pDotNetCOMPtr->Encrypt(encryptString, &str);     

        if (str == NULL) {
            return SysAllocString(L"EEncryptionError");
        }
        else return str;    
    }

    pDotNetCOMPtr = NULL;

    return SysAllocString(L"EDLLError");

    CoUninitialize ();
}

.NET

namespace ICVTnsClient
{
    [Guid("D6F80E95-8A27-4ae6-B6DE-0542A0FC7039")]
    [ComVisible(true)]
    public interface _SslTcpClient
    {
        string Encrypt(string requestContent);
        string Decrypt(string requestContent);        
    }

    [Guid("13FE33AD-4BF8-495f-AB4D-6C61BD463EA4")]
    [ClassInterface(ClassInterfaceType.None)]
    public class SslTcpClient : _SslTcpClient
    {

       ...
       public string Encrypt(string requestContent) { // do something }

       public string Decrypt(string requestContent) { // do something }

    }
  }
}

Is it Visual C++ one the C++ side or some other C++ implementation? — sharptooth
– sharptooth, Commented Jun 13, 2012 at 7:23
Related: stackoverflow.com/questions/2631120/com-interface-guid?rq=1 — user195488
– user195488, Commented Jun 13, 2012 at 12:16

Hans Passant · Accepted Answer · 2012-06-13 07:56:14Z

4

That's because you forgot the [InterfaceType] attribute so that the interface can be early-bound and the method names appear in the type library. Fix:

[Guid("D6F80E95-8A27-4ae6-B6DE-0542A0FC7039")]
[ComVisible(true)]
[InterfaceType(ComInterfaceType.InterfaceIsDual)]
public interface _SslTcpClient
{
    // etc..
}

ComInterfaceType.InterfaceIsDual allows it to be both early and late bound. Microsoft prefers the default, IsIDispatch, less ways to shoot your foot with late binding.

answered Jun 13, 2012 at 7:56

Hans Passant

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