using System;
using System.Runtime.InteropServices;
 
namespace SendWindowMessage
{
    class Program
    {
        const int WM_COPYDATA = 0x004A;
        [return: MarshalAs(UnmanagedType.Bool)]
        [DllImport("user32.dll")]
        static extern bool PostMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
        [DllImport("user32.dll")]
        static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
        [System.Runtime.InteropServices.DllImport("user32.dll",CharSet = CharSet.Auto)]
        public static extern IntPtr SendMessage(IntPtr hwnd, int msg,IntPtr wparam, IntPtr lparam);
        [DllImport("user32.dll", EntryPoint = "FindWindow")]
        static extern IntPtr FindWindowByCaption(IntPtr ZeroOnly, string lpWindowName);
        [StructLayout(LayoutKind.Sequential)]
        struct COPYDATASTRUCT
        {
            public uint dwData;
            public int cbData;
            public IntPtr lpData;
        }
        public static IntPtr IntPtrAlloc<T>(T param)
        {
            IntPtr retval = Marshal.AllocHGlobal(Marshal.SizeOf(param));
            Marshal.StructureToPtr(param, retval, false);
            return (retval);
        }
 
        public static void IntPtrFree(IntPtr preAllocated)
        {
            if (IntPtr.Zero == preAllocated) throw (new Exception("Go Home"));
            Marshal.FreeHGlobal(preAllocated); preAllocated = IntPtr.Zero;
        }
 
        static void Main(string[] args)
        {
 
            string message = "This is a test" ;
            IntPtr hWnd = FindWindow("Example", "" );
            if ( hWnd == IntPtr.Zero){
 
            }else{
 
                COPYDATASTRUCT cds ;
                cds.dwData = 1;
                cds.cbData = message.Length + 1;
                cds.lpData = Marshal.StringToHGlobalAnsi(message);
                IntPtr cdsBuffer = IntPtrAlloc ( cds ) ;
                PostMessage(hWnd, WM_COPYDATA, IntPtr.Zero, cdsBuffer);
                IntPtrFree(cds.lpData);
                IntPtrFree(cdsBuffer);
            }
        }
    }
}

#include <tchar.h>
#include <windows.h>
 
#define WND_CLASS_NAME _T ( "Example" )
 
LRESULT CALLBACK WndProc (  HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam ) ;
 
int APIENTRY WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd ){
    WNDCLASSEX ex ;
    MSG msg ;
    ex.cbClsExtra = 0 ;
    ex.cbSize = sizeof ( WNDCLASSEX ) ;
    ex.cbWndExtra = 0 ;
    ex.hbrBackground = ( HBRUSH ) GetStockObject ( BLACK_BRUSH ) ;
    ex.hCursor = LoadCursor ( NULL, _T ( "IDC_ARROW" ) ) ;
    ex.hIcon = LoadIcon ( NULL, _T ( "IDI_APPLICATION" ) ) ;
    ex.hIconSm = LoadIcon ( NULL, _T( "IDI_APPLICATION" ) ) ;
    ex.hInstance = hInstance ;
    ex.lpfnWndProc = (WNDPROC )WndProc ;
    ex.lpszClassName = WND_CLASS_NAME ;
    ex.lpszMenuName = NULL ;
    ex.style = 0 ;
    if ( !RegisterClassEx ( &ex ) ){
        return 0 ;
    }
    HWND hWnd = CreateWindow ( WND_CLASS_NAME, _T ( "" ), WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, hInstance, NULL ) ;
    if ( !hWnd ){
        UnregisterClass ( WND_CLASS_NAME, hInstance ) ;
        return 0 ;
    }
 
    ShowWindow(hWnd, SW_HIDE ) ;
    UpdateWindow ( hWnd ) ;
    if ( !ChangeWindowMessageFilter ( WM_COPYDATA, MSGFLT_ADD ) ){
        MessageBox ( HWND_DESKTOP, _T("Failed to change message filter"),_T("STATUS"), MB_OK ) ;
    }
    while ( TRUE ) {
        UINT res = GetMessage ( &msg, hWnd, 0, 0 ) ;
        if ( res == 0 ){
            break ;
        }else if ( res <0){
 
        }else{
            DispatchMessage ( &msg ) ;
        }
    }
 
    return 0 ;
}
 
LRESULT CALLBACK WndProc (  HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam ){
    PCOPYDATASTRUCT pcds;
    switch ( uMsg ){
        case WM_CREATE :
        break ;
        case WM_COPYDATA :
            pcds=(PCOPYDATASTRUCT)lParam;
            if ( pcds->cbData > 0 && pcds->cbData < 512 ){
                if ( pcds->dwData == 1 ){
                    ((CHAR*)pcds->lpData)[pcds->cbData-1] = '\0' ;
                    MessageBoxA ( HWND_DESKTOP, (LPCSTR)pcds->lpData, "STATUS", MB_OK ) ;
                }else if ( pcds->dwData == -1 ){
                    ((CHAR*)pcds->lpData)[pcds->cbData-1]= L'\0' ;
                    MessageBoxW ( HWND_DESKTOP, (LPCWSTR)pcds->lpData, L"STATUS", MB_OK ) ;
                }
            }
 
        break ;
        case WM_DESTROY :
            PostQuitMessage(0);
        default :
            return DefWindowProc ( hWnd, uMsg, wParam, lParam ) ;
    }
    return 0L ;
 
}