How to pass a BSTR string as a parameter from C++ to a managed C# DLL

While working on com interop ,i followed the tutorial on this link.The code runs fine but the problem comes while dealing with the string.Although i have some knowledge about BSTR string type but unable to execute code on c++ section where the fuctions are being called. On c# side, the fuction is as follows:

public void ShowDialog([MarshalAs(UnmanagedType.BStr)] string stringToPrint)

how do i call this fuction from c++ while passing the string value from here?

here is the c++ code section

int _tmain(int argc, _TCHAR* argv[])
{



    CoInitialize(NULL);

    MyInterop::IMyDotNetInterfacePtr pDotNetCOMPtr;

    HRESULT hRes = pDotNetCOMPtr.CreateInstance(MyInterop::CLSID_MyDotNetClass);
    if (hRes == S_OK)
    {

        BSTR s;
        pDotNetCOMPtr->ShowDialog(s);

        }

    CoUninitialize();


}

the requirement is the that string needs to to passed from c++ just like e.g

string abc

cin>abc

now this abc String needs to passed in the calling fuction ie

ShowDialog()

but i am unable to do that..any help would be appricated.thankyou.