Oopppss, sorry, I didn't clean the original code enought.
Here is is again fixed.

The method at hand is
USDALib.PhysicalLocation_t[] GetROI(int partID, string roiName);

The USDALib is the USDALib.dll interop for the COM object USDA.dll

The VB6 code is referencing the USDA.ll (the native COM object) that defines
the PhysicalLocation_t
and also referencing the 1000 Gates

///////////////////////////////////
/// File 1000GatesInterface.cs

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

namespace MultiBScanDisplay
{
   [ComVisible(true),
 Guid(GUIDs.IID_I1000Gates),
    InterfaceType(ComInterfaceType.InterfaceIsDual)]
   public interface I1000Gates
   {
    // The USDALib is the USDALib.dll interop for the native (unmanaged)
COM object USDA.dll
       USDALib.PhysicalLocation_t[] GetROI(int partID, string roiName);
   }
}

///////////////////////////////////
/// File COMBridge.cs

using System;
using System.Drawing;
using System.Runtime.InteropServices;

namespace MultiBScanDisplay
{
   /// <summary>COMBridgeFor1000Gates is the bridge between the application
   /// interfaces and the actual implementation of the methods.</summary>
[Guid(GUIDs.CLASS_1000Gates)]
[ClassInterface(ClassInterfaceType.None)]
[ProgId("MultiBScan.1000Gates")]
[ComVisible(true)]
   public class COMBridgeFor1000Gates : ReferenceCountedObjectBase,
I1000Gates
   {
 [DllImport("msvcr70.dll", CallingConvention = CallingConvention.Cdecl)]
 public static extern int _controlfp(int n, int mask);

 [DllImport("kernel32.dll", CallingConvention = CallingConvention.Cdecl)]
 public static extern bool IsWow64Process(IntPtr hProcess, out bool
Wow64Process);

 [DllImport("kernel32.dll", CharSet=CharSet.Ansi, ExactSpelling=true)]
 public static extern UIntPtr GetProcAddress(IntPtr hModule, string
procName);

 public COMBridgeFor1000Gates()
 {
  // README: according to Microsoft knowledge base!
  UIntPtr pIsWow64Process = UIntPtr.Zero;
  bool Is64 = false;

  // Query for IsWow64Process function, if doesn't exist then not a 64bit
system.
  pIsWow64Process =
GetProcAddress(System.Diagnostics.Process.GetCurrentProcess().Handle,"IsWow64Process");
  if( pIsWow64Process != UIntPtr.Zero )
  {
   // Function exists, check it for values.
   IsWow64Process(System.Diagnostics.Process.GetCurrentProcess().Handle,out
Is64);
  }
  // Perform if not a 64bit system.
           if( !Is64 )
           {   // this flag should be set in 32bit system in order to avoid
an exception in the drawing.
               _controlfp(0x9001F, 0xFFFFF);
           }
       }

 ~COMBridgeFor1000Gates() {}

       #region I1000Gates Members

         USDALib.PhysicalLocation_t[] I1000Gates.GetROI(int partID, string
roiName)
        {
                USDALib.PhysicalLocation_t[] jp = new
USDALib.PhysicalLocation_t[10];
                for( int i=0; i < 10; ++i )
                {
                    jp[i].Pixel = i;
                    jp[i].ScanLine = i;
                    jp[i].Value = i;
                }
                return jp;
        }

       #endregion I1000Gates Members
   }

   #region Factory Class for COM object

class BridgeClassFactoryFor1000Gates : ClassFactoryBase
{
 public override void virtual_CreateInstance(IntPtr pUnkOuter, ref Guid
riid, out IntPtr ppvObject)
 {
           if( riid == Marshal.GenerateGuidForType(typeof(I1000Gates)) ||
               riid == Program.IID_IDispatch ||
               riid == Program.IID_IUnknown )
  {
   COMBridgeFor1000Gates COMBridge_New = new COMBridgeFor1000Gates();

               ppvObject = Marshal.GetComInterfaceForObject(COMBridge_New,
typeof(I1000Gates));
               EvaluationConfiguration.ExecutionMode =
EvaluationConfiguration.RunMode.ActiveX;
           }
  else
  {
   throw new COMException("No interface",  unchecked((int) 0x80004002));
  }
 }
   }

   #endregion Factory Class for COM object
}

//////////////////////////////////////////
//// File IClassFactory.cs

using System;
using System.Runtime.InteropServices;

namespace MultiBScanDisplay
{
///<summary>
/// Interface IClassFactory is here to provide a C#
   /// definition of the COM IClassFactory interface.
///</summary>
[ComImport, // This interface originated from COM.
 InterfaceType(ComInterfaceType.InterfaceIsIUnknown),// Indicate that this
interface is not IDispatch-based.
    Guid("00000001-0000-0000-C000-000000000046")]// This GUID is the actual
GUID of IClassFactory.
public interface IClassFactory
{
 /// <summary>CreateInstance.</summary>
 /// <param name="pUnkOuter">Unknown outer.</param>
 /// <param name="riid">RI ID.</param>
 /// <param name="ppvObject">PPV Object.</param>
 void CreateInstance(IntPtr pUnkOuter, ref Guid riid, out IntPtr ppvObject);

 /// <param name="fLock">Lock flag.</param>
 void LockServer(bool fLock);
}    
}

//////////////////////////////////////////
//// File GUIDs.cs

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

namespace MultiBScanDisplay
{
   public class GUIDs
   {
       internal const string CLASS_1000Gates =
"1B4363BF-33DF-49c7-B0D0-A201E631C100";
       internal const string IID_I1000Gates  =
"BE8E6044-5B18-4d56-AA75-9E5120F27884";
   }
}

//////////////////////////////////////////////////////

Why the BV6 client is getting an exception?
If I define a struct of my own in the C# application exposoed to COM, then
it works perfectly.
But I can not do that beacuse of design architecture issue.

Any idea?