Send Message(string) c# exe to vc++ exe(unmanaged) using WindowsSendMessage

I tried followinf but working fine fo int but how to do it for string using
user custome message

in c#
[DllImport("User32.dll")]

public static extern int FindWindow(string strClassName,string
strWindowName);

[DllImport("User32.dll")]

public static extern Int32 SendMessage(

int hWnd, // handle to destination window

int Msg, // message

int wParam, // first message parameter

[MarshalAs(UnmanagedType.LPStr)] string lParam);


public const int WM_APP = 0x8000;

public const int WM_DELETEALL =WM_APP + 0x100;

int wnd=FindWindow(null,"TestMfcSendMsgExe");

Form1.SendMessage(wnd,WM_DELETEALL,0,55);


and MFC Exe is like this

#define WM_DELETEALL WM_APP + 0x100

BEGIN_MESSAGE_MAP(CTestMfcSendMsgExeDlg, CDialog)

ON_MESSAGE (WM_DELETEALL, OnDeleteAll)


END_MESSAGE_MAP()

LRESULT CTestMfcSendMsgExeDlg::OnDeleteAll(WPARAM wParam, LPARAM lParam)

{

MessageBox("Hello","Mayur",MB_OK);

}

I am getting the message box after using SendMessage() in c# exe but i want
to send text(string ) information.

I need help..

Regards,

Mayur.