Marshall char** to string problem calling unmanaged code from managed code

I've this C++ function,

    bool MyClass::my_function(int num, TCHAR** filepath)

I've expose the function as

    extern "C"
    {
      __declspec(dllexport) bool MyFunction(int num, char* filepath[])
      {
          OutputDebugStringA("--MyFunction--");
          TCHAR **w_filepath = (TCHAR **)calloc(num, 2* sizeof(TCHAR **));
          for(int i = 0; i < num; i++) 
          {
            OutputDebugStringA(filepath[i]);
 开发者_Go百科           int len = strlen(filepath[i]) + 1;
            w_filepath[i] = (TCHAR *)calloc (1, len);
            ctow(w_filepath[i], filepath[i]); // converts char to WCHAR
          }

          bool ret = MyClass.my_function(num, w_filepath);
          OutputDebugStringA("End -- MyFunction --");
          free(w_filepath);
          return ret;
      }
    }

I've C# wrapper as

    [DllImport("MyDll.dll")]
    public static extern bool MyFunction(int num, [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPTStr)] string[] filepath);

In C#, i call Myfunction as

    string [] filepath = { "D:\\samplefile\\abc.txt", "D:\\samplefile\\def.txt"}
    MyFunction(2, filepath)

In C++ function, it gets only first character of filepath. For example, from above call if i print in C++ code using

    OutputDebugStringA

it prints only D for both first and second.

If i remove

    [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPTStr)]

from c# wrapper I'll get Access violation error in

    w_filepath[i] = (TCHAR *)calloc (1, len) 

for second file.

Please help me.

1) w_filepath[i] = (TCHAR *)calloc (1, len); - calloc requires size in bytes, so it should be w_filepath[i] = (wchar_t *)calloc (1, len*sizeof(wchar_t));

2) data from c# comes as wchar_t*, so you don't need converting routines at all , and should change function declaration to

__declspec(dllexport) bool MyFunction(int num, wchar_t* filepath[])