RegisterClassEx

ATOM MyRegisterClass(HINSTANCE hInstance)
{
	WNDCLASSEX wcex;
 
	wcex.cbSize = sizeof(WNDCLASSEX);
 
	wcex.style			= CS_HREDRAW | CS_VREDRAW;
	wcex.lpfnWndProc	= WndProc;
	wcex.cbClsExtra		= 0;
	wcex.cbWndExtra		= 0;
	wcex.hInstance		= hInstance;
	wcex.hIcon			= LoadIcon(hInstance, MAKEINTRESOURCE(IDI_FINDWINDOWSAMPLE));
	wcex.hCursor		= LoadCursor(NULL, IDC_ARROW);
	wcex.hbrBackground	= (HBRUSH)(COLOR_WINDOW+1);
	wcex.lpszMenuName	= MAKEINTRESOURCE(IDC_FINDWINDOWSAMPLE);
	wcex.lpszClassName	= szWindowClass;
	wcex.hIconSm		= LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));
 
	return RegisterClassEx(&wcex);
}

BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   HWND hWnd;
 
   hInst = hInstance; // Store instance handle in our global variable
 
   hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);
 
   if (!hWnd)
   {
      return FALSE;
   }
 
   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);
 
   return TRUE;
}

WM_MY_MESSAGE = RegisterWindowMessage(TEXT("WM_MY_MESSAGE"));

if(message == WM_MY_MESSAGE) {
		MessageBox(hWnd,TEXT("Messsage From Command Line App"),TEXT("Message"),MB_OK);
	}

FindWindow

int _tmain(int argc, _TCHAR* argv[])
{
	HWND hWnd = NULL ;
 
	hWnd = FindWindow(szWindowClass,szWindowClass);
	if(!hWnd) 
	{
		printf("FindWindow failed %d", GetLastError());
	}
	WM_MY_MESSAGE = RegisterWindowMessage(TEXT("WM_MY_MESSAGE"));
	SendMessage(hWnd,WM_MY_MESSAGE,0,0);
 
	return 0;
}