.net - Can't instantiate a COM object written in C# from VBA (VB6 ok)

Question

Using VS 2008, here is my COM object

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace TestCom
{    
    [Guid("9E5E5FB2-219D-4ee7-AB27-E4DBED8E123E")]
    [ClassInterface(ClassInterfaceType.AutoDual)]
    [ProgId("Test9.COMINT")]
    public class TestComClass  
    { 
        public void Init(string userid, string password)
        {
            MessageBox.Show(string.Format("{0}/{1}", userid, password));
        }       
    }
}

If I build this and register it on a production machine as follows

REGASM /CODEBASE TESTCOM.DLL

From a simple VB6 app this works fine

Private Sub Form_Load()
  Dim o As Object
  Set o = CreateObject("Test9.COMINT")
  o.Init "A", "B" 
End Sub

This exact same code called from VBA in Excel gives

"automation error" (0x80131700)

Everything works fine on a development machine, just not on a production machine with just .NET and MS Office installed.

Update

I think this is something to do with the .NET framework not being initialized properly, when running under Excel. If I use Filemon I can see it skip around looking for MSCORWKS.DLL. When I call the same object from VBScript, it finds MSCorwks.dll fine.

When I called CorBindToCurrentRunTime from VBA to try to forcibly load the CLR, interestingly I get the exact same HRESULT (0x80131700) as when I do CreateObject() in VBA.

Therefore I think it is a framework initialization issue.

Answer 1

I'm going to answer my own question, hopefully to spare others the hours of tedious drudgery I have just endured.

If you get this, it is because the .NET based COM assembly can't find the .NET framework

The solution is simple. Create a file containing the following

<?xml version="1.0"?>
<configuration>
  <startup>
   <supportedRuntime version="v2.0.50727"/>
  </startup>
</configuration>

Call it "Excel.Exe.Config" and place it in the same directory as "EXCEL.EXE"

Problem solved!