Question

I followed this tuorial to create a COM dll in Visual Basic. http://www.codeproject.com/KB/COM/Basics_of_Idl_file.aspx

I now want to use this dll in a C++ project. I used OLE/COM Viewer to create an .idl file as is described in the second half this tutorial.
http://www.codeproject.com/KB/COM/vb_from_vc.aspx

I compiled the .idl with the midl compiler and included the .h file that was created in my c++ project.

Here is my Visual Basic Code

<ComClass(MyComClass.ClassId, MyComClass.InterfaceId, MyComClass.EventsId)> _
Public Class MyComClass

#Region "COM GUIDs"
    ' These  GUIDs provide the COM identity for this class 
    ' and its COM interfaces. If you change them, existing 
    ' clients will no longer be able to access the class.
    Public Const ClassId As String = "46604f8a-85a2-4027-9728-0390534c9209"
    Public Const InterfaceId As String = "30274029-711d-459a-9270-f9d73ad8737f"
    Public Const EventsId As String = "5e234d69-5263-4001-86ff-c475b113a77d"
#End Region

    ' A creatable COM class must have a Public Sub New() 
    ' with no parameters, otherwise, the class will not be 
    ' registered in the COM registry and cannot be created 
    ' via CreateObject.
    Public Sub New()
        MyBase.New()
    End Sub
    Protected Overrides Sub Finalize()
        MyBase.Finalize()
    End Sub
    Public Sub DisplayMessage()
        MsgBox("Hello from MyComClass!")
    End Sub
End Class

Here is my c++ Code

    // Declare an HRESULT and a pointer to the clsVBTestClass interface

HRESULT     hr;
_MyComClass *IVBTestClass = NULL;

// Now we will intilize COM

hr = CoInitialize(0);

// Use the SUCCEEDED macro and see if we can get a pointer 

// to the interface

if(SUCCEEDED(hr))
{
    hr = CoCreateInstance( CLSID_MyComClass,
                NULL,
                CLSCTX_INPROC_SERVER,
                IID__MyComClass, 
                (void**) &IVBTestClass);

    // If we succeeded then call the CountStringLength method, 

    // if it failed then display an appropriate message to the user.

    if(SUCCEEDED(hr))
    {
        long        ReturnValue;
        _bstr_t     bstrValue("Hello World");

        // We can test this HR as well if we wanted to

        hr = IVBTestClass->DisplayMessage();

        hr = IVBTestClass->Release();
    }
    else
    {

    }
}
// Uninitialize COM

CoUninitialize();

I receive the following errors when I compile my c++ project

error LNK2001: unresolved external symbol _CLSID_MyComClass
error LNK2001: unresolved external symbol IID_MyComClass

Could someone help me figure out what I am doing wrong?

Answer 1

If you did not get to the last part of creating a Type Library, that is important.

You then need an #import statement in your C++ code to use the .tlb file (or .dll if the type library is embedded in the dll, which is common).

#import is the equivalent to including a header file with COM, but generates a .tlh file (header) and a .tli (implementation) automatically.