Hi,
I've encountered a very basic problem when dealing with User Defined Types (UDT) in a COM class.
If a COM method returns a UDT in a VARIANT, the calling .NET application receives an
exception "The specified record cannot be mapped to a managed value class.".

In order to reproduce this problem, create a new C++ ATL project that implements an inproc-server (dll) component.
The name of the project is API.
Then create a simple C# console application 'TestApp' and add the COM object as a reference.

-- This is the contents of the idl file API.idl:

import "oaidl.idl";
import "ocidl.idl";

[
   object,
   uuid(4DA8E86C-C8D2-443D-8940-B52C2151E37C),
   dual,
   nonextensible,
   helpstring("ITest Interface"),
   pointer_default(unique)
]
interface ITest : IDispatch {
   HRESULT Test([in] BSTR message, [out,retval] VARIANT* result);
};
[
   uuid(1ECB0F02-F896-473D-ADFE-44BB1602F08D),
   version(1.0),
   helpstring("API 1.0 Type Library")
]
library APILib
{
   importlib("stdole2.tlb");

   typedef [uuid(5523d721-5fe5-4a40-892e-8824b14aea0d)] struct MyStruct
   {
       double X;
       double Y;
       double Z;
   } MyStruct;

   [
       uuid(C0A1CB57-1726-4729-874A-3C607E90F0EC),
   ]
   coclass Test
   {
       [default] interface ITest;
   };
}

-- This is the implementation of COM object Test (file Test.cpp)

#include "stdafx.h"
#include "Test.h"
#include ".\test.h"

void convertStructToVariant(VARIANT& var, const MyStruct& s)
{
   CComPtr<IRecordInfo> pRecInfo;
   HRESULT hr;
   GUID structUuid;
   hr = CLSIDFromString(L"{5523d721-5fe5-4a40-892e-8824b14aea0d}", &structUuid);
   hr = GetRecordInfoFromGuids(LIBID_APILib, 1, 0, GetUserDefaultLCID(), structUuid, &pRecInfo);

   if (pRecInfo)
   {
       VariantClear(&var);

       var.vt = VT_RECORD;
       var.pvRecord = pRecInfo->RecordCreate();
       var.pRecInfo = pRecInfo.Detach();

       *((MyStruct*)var.pvRecord) = s;
   }
}

STDMETHODIMP CTest::Test(BSTR message, VARIANT* result)
{
   USES_CONVERSION;

   CComVariant var;
   if (!wcsicmp(message, L"RETURN-STRING"))
   {
       var = CComVariant("TEST-RESULT");
   }
   else if (!wcsicmp(message, L"RETURN-STRUCT"))
   {
       MyStruct s;
       s.X = 1;
       s.Y = 2;
       s.Z = 3;

       convertStructToVariant(var, s);
   }

   var.Detach(result);

   return S_OK;
}

-- TestApp.cs
using System;
using System.Windows.Forms;

namespace TestApp
{
   class Application
   {
       [STAThread]
       static void Main(string[] args)
       {
           APILib.Test test = new APILib.Test();
           object result;

           try
           {
               result = test.Test("RETURN-STRING");

               MessageBox.Show(result.ToString(), "String Test");
           }
           catch (Exception e)
           {
               MessageBox.Show("Caught exception \"" + e.ToString() + "\"", "Struct Test");
           }

           try
           {
               result = test.Test("RETURN-STRUCT");

               MessageBox.Show(result.ToString(), "Struct Test");
           }
           catch (Exception e)
           {
               MessageBox.Show("Caught exception \"" + e.ToString() + "\"", "Struct Test");
           }
       }
   }
}

If I run TestApp.exe, 2 MessageBoxes appear:
The first one for the string test prints "TEST-RESULT".
The second one shows:
Caught exception "System.ArgumentException: The specified record cannot be mapped to a managed value class.
  at APILib.TestClass.Test(String message)
  at TestApp.Application.Main(String[] args) in c:\temp\udt\testapp\testapp.cs:line 28"

Any help would be greatly appreciated.
Thanks,
- J?rgen