Passing parameters between C++ app and C# COM wrapper

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C++

C#

VB

I have made a COM interface to wrap my .Net libraries as they are given data from some VB6 programs. However, now I'm unsuccessfully trying to communicate with a C++ using the same interface.

It all works fine in VB6, but I can't make it work in that .Net COM interface.

The C++ program calls the ProcessData method, expecting it to return a Long and alter the value of byteOut.

This is how it works ok in VB6:

Public Function ProcessData(ByRef bytDataIn() As Byte, ByRef bytDataOut() As Byte) As Long
On Error GoTo ErrPoint
     bytDataOut = Str2ArrByte(proxy.ProcessData(bytDataIn))  '
    ProcessData = 1
    Exit Function       
ErrPoint:
    ProcessData = 0
End Function

And this is how it does NOT work in C#:

public interface IComClass1
    {
        [DispId(1)]
        string ProcessXML(ref byte[] byteIn); // this is the OK working method called from VB as seen above
        //... another methods for VB, ...
        [DispId(5)]
        long ProcessData(ref byte[] byteIn, ref byte[] byteOut); //but the direct calls from C++ do NOT work
    }

public class ComClass1 : IComClass1
    {
      ...     
      public long ProcessData(ref byte[] byteIn, ref byte[] byteOut)
        {  
             long LRet = 0;
             try
             { byteOut = processByteArray(byteIn); /* any method returning byte[] */}
              catch
             { LRet = 1; }            
             return LRet;
         }
      }

So far, I can bypass it by using the VB library as another in-between, but although working I don't find this solution optimal. Should I use marshalling? I tried already, but with no positive result...