Calling ATL COM method from C# fails with HRESULT: 0x80004002 (E_NOINTERFACE)

I am currently trying to port some C++ sample code for an ATL COM Component to C#. The component was developed in C++. It compiles fine with VC6 and VC8 and the console-based sample that calls the COM is also working fine.

The C# code compiles fine, however, when I execute it I get the following runtime error:

System.InvalidCastException was unhandled
Message="Unable to cast COM object of type 'SIMPLE_ATLLib.First_ATLClass' to interface type 'SIMPLE_ATLLib.IFirst_ATL'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{C8F6E230-2672-11D3-A8A8-00105AA943DF}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE))."
  Source="Interop.SIMPLE_ATLLib"
  StackTrace:
       at SIMPLE_ATLLib.First_ATLClass.AddNumbers(Int32 Num1, Int32 Num2)
       at Test_ATL.Program.Main(String[] args) in C:\develop\samples\COM_ATL tutorial\COM_ATL_Tutorial_CS_Source\Test_ATL.cs:line 29
       at System.AppDomain.nExecuteAssembly(Assembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()

here is the source code:
  SIMPLE_ATLLib.First_ATLClass testClass = new SIMPLE_ATLLib.First_ATLClass();
  if (testClass == null)
   {
      Console.WriteLine("ERROR: can't create instance of testClass SIMPLE_ATLLib.First_ATLClass()");
   }
   Console.WriteLine("Getting a long value ..");
   Int32 iValue;
   iValue=testClass.AddNumbers(1, 2); // this is the part that I can't get to work


// ================== WORKING C++ code =====================
/**
 * This test calls the AddNumbers() method. It adds two numbers. The result
 * is returned bu reference.
*/
void test_addNumbers(IFirst_ATL *comClass)
{
  HRESULT hr;
  long ReturnValue;
  hr = comClass->AddNumbers(5, 7, &ReturnValue);
  cout << "================================================================" << endl;
  cout << "calculating 5 + 7 ... ";
  if(SUCCEEDED(hr))
  {
    cout << "result is: " << dec << ReturnValue << endl;
  }
  else
  {
    cout << "ERROR " << hr << endl;
  }
}

NOTE:
the sample code and ATL COM was derived from "A Simple COM tutorial using ATL" by  C. Lung which contains the COM sources and VB and VC sample code. Complete original sources are available at: http://www.codeguru.com/Cpp/COM-Tech/atl/tutorials/article.php/c17