Question

I am using a COM visible C# library that I call from VBA in Excel. The C# method that is called includes arguments passed by ref. I am using VS2010, Office 2010, .Net framework 4.

• In an earlier version of my Excel file, the call to the library works fine and still does.

• In the newer version (the call itself to the C# library has not changed), VBA manages to call the library, however when the C# method finishes, I get a runtime Error 430 “Class doesn’t support Automation or does not support expected interface”. I used the DEBUG mode to see that. In that same file, VBA manages to call a deployed .NET library.

What I tried:
– Re-adding the reference to the OLE Automation library in VBA.
– Re-compiling the C# library

I am a bit puzzled as to what could be wrong and where. Excel itself does not seem to be the problem, since my older file works. The C# library is called from my older file too so the problem should not come from the library. As for the ability of my file to call COM visible libraries, well, the call to another C# library works (that one is deployed).

Any help would be much appreciated!

---

The VBA call to csharp is as follows:

Dim csharptoolsDispatch As Object
Set csharptoolsDispatch = CreateObject("Dispatch.Caller")
Dim a as string
ReDim input(0 to 5) as single 
ReDim output(0 to 5) as single 
a = csharptoolsDispatch.solveDispatch(2, False, input, output)

The C# looks like:

-Interface

namespace Dispatch
{
   public interface iCaller
   {
       string solveDispatch(int a, bool flag, float[] input, ref float[] output);
   }
}

-Class

namespace Dispatch
{
    [ClassInterface(ClassInterfaceType.None)]
    [ComVisible(true)]
    public class Caller : iCaller
    {
        public string solveDispatch(int a, bool flag, float[] input, ref float[] output)
        {    
           //code
           return "ok";
        }
    }
}

The call to the Csharp method works fine, the Csharp runs fine and just after the ‘return “ok”‘, when the “focus” is back on VBA I get the error 430 message.

Answer 1

Ok, I managed to sort it out.

In hindsight, it was a pretty obvious mistake but weird behaviour from the COM interop interface between VBA and C# made it difficult to spot.

In the VBA call to C#, among the arguments, there was a table of single which was defined as a table of int in C#. That argument was passed in ref.

In such a case, I would expect VBA to crash saying ‘invalid argument’ or something. Instead, there seem to be an implicit cast of the single to an int that makes the call to C# possible.
Then, when the csharp method finishes, VBA don’t seem able to make sense of the argument it passed in ref and that changed type. That causes the 430 error.

My earlier version did not have that mistake and therefore worked.

I have to remenber VBA is very weakly typed!

Anyway, mystery solved!