C# DLL cannot affect value of a number passed by reference from a VB6 application

Question

I have a legacy VB6 application that calls a VB6 DLL, and I am trying to port the VB6 DLL to C# without yet touching the main VB6 application code. The old VB6 DLL had one interface that received a VB6 long (32-bit integer) by reference, and updated the value. In the C# DLL I have written, the updated value is never seen by the main VB6 application. It acts as though what was really marshalled to the C# DLL was a reference to a copy of the original data, not a reference to the original data. I can successfully pass arrays by reference, and update them, but single values aren't behaving.

The C# DLL code looks something like this:

[ComVisible(true)]
public interface IInteropDLL
{
   void Increment(ref Int32 num);
}
[ComVisible(true)]
public class InteropDLL : IInteropDLL
{
    public void Increment(ref Int32 num) { num++; }
}

The calling VB6 code looks something like this:

Private dll As IInteropDLL
Private Sub Form_Load()
    Set dll = New InteropDLL
End Sub
Private Sub TestLongReference()
    Dim num As Long
    num = 1
    dll.Increment( num )
    Debug.Print num      ' prints 1, not 2.  Why?
End Sub

What am I doing wrong? What would I have to do to fix it? Thanks in advance.

Answer 1

dll.Increment( num )

Because you are using parentheses, the value is forcibly passed by value, not by reference (the compiler creates a temporary copy and passes that by reference).

Remove the parentheses:

dll.Increment num

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EDIT: A more complete explanation by MarkJ.