Access methods from ATL dll file in C#

Question

I've been working with trying to link up some c++ code and wrap it inside a COM object to access via C#. I created an atl project and added a simple method such as Add(double a, double b). The following is the code from my atl.h file:

// atl.h : Declaration of the Catl
#pragma once
#include "resource.h"       // main symbols

#include "com_i.h"


#if defined(_WIN32_WCE) && !defined(_CE_DCOM) && !defined(_CE_ALLOW_SINGLE_THREADED_OBJECTS_IN_MTA)
#error "Single-threaded COM objects are not properly supported on Windows CE platform, such as the Windows Mobile platforms that do not include full DCOM support. Define _CE_ALLOW_SINGLE_THREADED_OBJECTS_IN_MTA to force ATL to support creating single-thread COM object's and allow use of it's single-threaded COM object implementations. The threading model in your rgs file was set to 'Free' as that is the only threading model supported in non DCOM Windows CE platforms."
#endif



// Catl

class ATL_NO_VTABLE Catl :
public CComObjectRootEx<CComSingleThreadModel>,
public CComCoClass<Catl, &CLSID_atl>,
public Iatl
{
public:
Catl()
{
}

DECLARE_REGISTRY_RESOURCEID(IDR_ATL)

DECLARE_NOT_AGGREGATABLE(Catl)

BEGIN_COM_MAP(Catl)
COM_INTERFACE_ENTRY(Iatl)
END_COM_MAP()



DECLARE_PROTECT_FINAL_CONSTRUCT()

HRESULT FinalConstruct()
{
    return S_OK;
}

void FinalRelease()
{
}

public:

STDMETHOD(Add)(DOUBLE a, DOUBLE b);
};

OBJECT_ENTRY_AUTO(__uuidof(atl), Catl)

The following is from the atl.cpp file

// atl.cpp : Implementation of Catl

#include "stdafx.h"
#include "atl.h"

STDMETHODIMP Catl::Add(DOUBLE a, DOUBLE b)
{
// TODO: Add your implementation code here

return a + b;
}

Inside my c# file I'm calling the dll... after i referenced it... it sees the dll but not the methods assigned. which is my problem. Heres the code from program.cs

sing System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;

namespace sharpdll
{
class Program
{
    [DllImport("com.dll")]
    public static extern double Add(double a, double b);

    static void Main(string[] args)
    {
        Add(2, 3);
    }
}
}

Debugging breaks at Add(2, 3); Says "Unable to find an entry point named 'Add' in DLL 'com.dll'." Any ideas?

Answer 1

DllImport is for PInvoke (to native Win32 dlls).
You want COM Interop.

Register your ATL com object, then Add a reference to it, as you would to any .Net or COM component.

An alternative to ATL, you can expose your C++ functionality through C++/CLI.

Hope this helps,