.Net Interop Problem with LPArray : COM Client throws Exception

I need to know how to properly marshal the data from my legacy app to a .Net component. The integrity of the data must be kept. No memory leaks must occur in the solution. The solution should never fault. My legacy app uses the following IDL interface definition:
 
interface IMyInterface2 : IUnknown
{
 HRESULT doit( [in, size_is(msgLength)]UCHAR* msg, [in] ULONG msgLength );
};
 
When my legacy COM Client calls my .Net component that implements this interface, an access violation (0xC0000005) exception is thrown. I've tried everything and can not figure out why the exception is thrown in
the COM Client.
 
Here is the COM Client:
 
#include "stdafx.h"
#include "IMyInterface2_h.h"
#include "IMyInterface2_i.c"
 
int _tmain(int argc, _TCHAR* argv[])
{
HRESULT hr = S_OK;
hr = ::CoInitialize( NULL );
CLSID rcid = { 0x5F850215, 0x7AC2, 0x48b8, { 0xA8, 0xCF, 0xA0, 0x08, 0xF7, 0x33, 0xD6, 0x61 } };
 
{
CComPtr<IMyInterface2> iMyInterface2;
hr = iMyInterface2.CoCreateInstance( rcid, NULL, CLSCTX_INPROC_SERVER
);
UCHAR* pBytes = (UCHAR*)::CoTaskMemAlloc( 20 );
::ZeroMemory( pBytes, 20 );
iMyInterface2->doit( pBytes, 20 );
}
::CoUninitialize();
 
return 0;
}
 
The exception is thrown on the statement:
iMyInterface2->doit( pBytes, 20 );
The call never gets to the .Net component's method
 
And here is the .Net service component. The .Net component must be copied in the same directory as the COM client.
 
using System;
using System.Runtime.InteropServices;
 
namespace ComSrv
{
 [GuidAttribute("5F850215-7AC2-48b8-A8CF-A008F733D661")]
 public class MyClass : IMyInterface2
 {
  public void doit( byte[] msg, ulong msgLength )
  {
  }
 }
 
 [ComImport]
 [GuidAttribute("B55AFAEB-BC7D-4ca9-8065-F9A45FE378B6")]
 [InterfaceTypeAttribute  
ComInterfaceType.InterfaceIsIUnknown)]
 public interface IMyInterface2
 {
  void doit(
   [In, MarshalAs( UnmanagedType.LPArray, SizeParamIndex=1, ArraySubType=UnmanagedType.U1 )]byte[] msg, [In]ulong msgLength );
 }
}