Get A String From An IntPtr From A WPARAM?

i am using SendMessage to send a message with a LPTSTR in WPARAM from a dll to my vb.net form and i get it by overriding WndProc but the problem is when using Marshal.PtrToStringAuto on the WPARAM it doesn't work, the strange thing is that if i monitor WM_SETTEXT of the form and use PtrToStringAuto to get the string from LPARAM it works, i guess mine doesn't work that's because the dll is loaded by other application.

C++ DLL Proc
Code:
LRESULT CALLBACK CallWndProc(int nCode,WPARAM wParam,LPARAM lParam)
{

[Code]...