How to pass strings from VC++.exe to C#.dll

Hi all,

I have to pass a string from VC++.exe to C#.dll

But in C#.dll the string is null.Can anyone help me in this
regard.Thanks in advance

VC++.exe

void main()
{
               CoInitialize(NULL);
    MyInterop::IMyDotNetInterfacePtr pDotNetCOMPtr;
    WCHAR str[] = _T("C:\\Debugging\\MyInterop\\MyInterop\\bin\\Debug\
\rag.xml");
    HRESULT hRes =
pDotNetCOMPtr.CreateInstance(MyInterop::CLSID_MyDotNetClass);

    if (hRes == S_OK)
    {
        pDotNetCOMPtr->ShowDialog (str);
    }

    CoUninitialize ();
    return 0;
}

C#.dll

namespace MyInterop
{
    /// <summary>
    /// Summary description for Class1.
    /// </summary>
    ///
    [Guid("0E1889D6-98B0-44b1-AF45-0439994A0229")]
    public interface IMyDotNetInterface
    {
        void ShowDialog(string str);
    }

    [ClassInterface(ClassInterfaceType.None)]
    [Guid("7FAC7C73-7513-4703-8695-769CF4E61547")]
    public class MyDotNetClass : IMyDotNetInterface
    {
        public MyDotNetClass()
        {
        }

        public void ShowDialog(string str)--->str i am receiving as
NULL
        {
            string rag = str;
            Console.WriteLine(rag);
            MessageBox.Show("I am a Managed DotNET C# COM Object
Dialog");
        }
    }
}