About calling into VC++ DLL

[sujk2k]

Hello,
my problem is related to marshalling of C data type 'BSTR from the .NET code

steps:
1. I have a c# program which calls one function from a VC++ dll file as
follows:

[DllImport(@"c:\Documents and Settings\skudale\My Documents\Visual Studio 2005\Projects\TestString\debug\CppExpLib.dll")]
        //internal static extern int addnum(int a, int b);
        internal static extern int testBSTR([In] InputClass input);
 
        private static InputClass input;
 
        static void Main(string[] args)
        {
            //int ans = addnum(5, 6);
            input = new InputClass();
            int iRes = testBSTR(input);
 
            Console.WriteLine("success");
        }
 

2. The defination for the Input class is follows:

 
public class InputClass
        {
            [MarshalAs(UnmanagedType.LPStr)]
            private string fuserid;
 
            internal InputClass()
            {
                fuserid = "";
            }
       }
 

3. I create the object of input class which is passed to a function named
testBSTR which is defined in my VC++ dll.
The function defination in VC++ dll is as follows:

extern "C" int __declspec(dllexport) __stdcall testBSTR(t_sys_lgn_in *in)
{
    char* source = "sujeet";
 
    psz2BStr(&in->fuserid, source);
 
    return 0;
}
 

4. t_sys_lgn_in is structure defined in one of the header file in my VC++ dll
as follows:

#include <comdef.h>
#include <iostream>
 
typedef struct tag_sys_lgn_in
{
    BSTR          fuserid;
} t_sys_lgn_in;
 

5. the defination of psz2BStr is as follows:

void psz2BStr(BSTR* to, char* from)
{
     BSTR temp = *to;
     BSTR temp = SysAllocString(*to);
     *to = SysAllocStringByteLen(from, strlen(from)+1);
 
     if(!*to)
     {
          *to = temp;
     }
     else 
     {
          SysFreeString( temp  );
     }
}
 

6. When I run my .NET client normally, program returns with no error.

7. Now, if I turn on the "Enable unmanaged code debugging" option in
my .NET project and step into the C++ function into the DLL, I got the
following error:
"Invalid Address specified to RtlSizeHeap" at the SysFreeString( temp );
statement.

please help me on this error...

some observations:
1. If I turn of the "Enable unmanaged code debugging" option of my .NET project, program runs normally without any error.
2. The same client implementation in VB 6.0 for the same VC++ DLL
doesn't give any error.