Old school Win32 Message Only Window cannot receive messages - c++

Question

I'm realize the console win32 app does not quit cleanly so I'm trying to switch to message only windows instead. I'm starting the app from another process and trying to kill it cleanly.
This is the win32 app, it spawns a calc.exe on startup and on clean shutdown, it should kill the calc.exe
LRESULT CALLBACK WindowProc (HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_USER: PostQuitMessage (0); break;
default: return DefWindowProc (hWnd, message, wParam, lParam);break;
}
return 0;
}
int CALLBACK WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int show)
{
WNDCLASSEX wc = { 0 };
wc.cbSize = sizeof (WNDCLASSEX);
wc.lpfnWndProc = WindowProc;
wc.hInstance = hInstance;
wc.lpszClassName = L"WindowClass1";
RegisterClassEx (&wc);
HWND hWnd = CreateWindowEx (NULL,
L"WindowClass1", // name of the window class
L"Our First Windowed Program", // title of the window
0,
//WS_OVERLAPPEDWINDOW, // window style
300,300,500,400,
HWND_MESSAGE,
//NULL, // parent window, NULL
NULL, hInstance, NULL);
//ShowWindow (hWnd, SW_HIDE);
PROCESS_INFORMATION pi;
CreateWindowProcess (L"calc.exe", pi); // helper class to createprocess
MSG msg = { 0 };
while (true)
{
if (PeekMessage (&msg, 0, 0, 0, PM_REMOVE))
{
if (WM_QUIT == msg.message)
break;
TranslateMessage (&msg);
DispatchMessage (&msg);
}
}
// terminate the spawn process, this is not called cleanly
TerminateProcess (pi.hProcess, 0);
return (int)msg.wParam;
}
I made a c# program to start/kill the app cleanly (the calc.exe gets destroyed) by sending a WM_QUIT/WM_CLOSE/WM_USER message . The Win32 App does not receive messages unless the window is visible (WS_OVERLAPPED and ShowWindow true). PostMessage WM_QUIT is received but the calc.exe does not get destroyed, meaning it is not a clean exit.
How should I kill it cleanly from C# app?
class Program
{
[DllImport ("user32.dll")]
public static extern bool PostMessage (IntPtr hwnd, uint msg, int wparam, int lparam);
[DllImport ("User32.dll")]
public static extern int SendMessage (IntPtr hWnd, uint uMsg, int wParam, int lParam);
static void Main (string [] args)
{
try
{
Process myProcess;
myProcess = Process.Start ("My.exe");
// Display physical memory usage 5 times at intervals of 2 seconds.
for (int i = 0; i < 3; i++)
{
if (myProcess.HasExited) break;
else
{
// Discard cached information about the process.
myProcess.Refresh ();
Thread.Sleep (4000);
Console.WriteLine ("Sending Message");
const int WM_USER = 0x0400;
const int WM_CLOSE = 0xF060; // Command code for close window
const int WM_QUIT = 0x0012;
// Received only when windows is visible
//int result = SendMessage (myProcess.MainWindowHandle, WM_USER, 0, 0);
// not clean exit
PostMessage (myProcess.MainWindowHandle, WM_QUIT, 0, 0);
// doesn't receive
SendMessage (myProcess.MainWindowHandle, WM_QUIT, 0, 0);
}
}
}
}
}

Answer 1

Processes on Windows do not really have a "MainWindow". Process.MainWindowHandle is C# making a guess, and it guesses by looking to see if the process in question has a window with focus - which will only find visible windows. Use FindWindowEx to find the window handle you want to close.
Next, when a window closes, it does not automatically try to exit the current threads message loop. You need to handle WM_DESTROY to call PostQuitMessage.
In your message loop use GetMessage rather than PeekMessage if you are not doing any other work as PeekMessage returns immediately if there are no messages meaning the application thread will never have an opportunity to sleep.
With these changes in place you should be fine to simply post a WM_CLOSE to the valid window handle as it will be destroyed, post itself a WM-QUIT message to exit the message loop, and terminate the calc process properly.