How do I detect the addition of a new monitor?

Question

I have a windowless program that handles some window management hotkeys. I'd like to provide features such as the ability to move a window between monitors. I've used EnumDisplayMonitors to enumerate all existing monitors in the system, and I've written code to handle WM_DEVICECHANGE, but I'm not actually receiving the message.

Here's my message loop:

// I've tried GetMessage(&msg, (HWND) NULL, 0, 0) here too
while (GetMessage(&msg, (HWND) -1, 0, 0) > 0)
{
    int key;
    int mod;

    MessageBox(NULL, (LPCWSTR) ((std::wostringstream&) (std::wostringstream() << L"You got a message: " << msg.message)).str().c_str(), L"Got Message", MB_OK);

    switch (msg.message)
    {
    case WM_HOTKEY:
        key = HIWORD(msg.lParam);
        mod = LOWORD(msg.lParam);

        if (mod != MOD_WIN) continue;
        ProcessHotkey(key);
        break;
    case WM_DEVICECHANGE:
        InitMonitorInfo();
    }
}

The program compiles and runs fine, and the hotkeys work. Upon adding or removing a monitor though, nothing happens. The message box to indicate a message has been received never appears.

I suppose I could just poll the monitor configuration every 5 seconds, but that's not the right way to solve the problem.

Do I need to actually create a window to receive WM_DEVICECHANGE? Because I don't. The hotkeys post their messages to NULL when they fire since they're not bound to a window, to be handled by the main thread.

Answer 1

You must create a window to get the WM_DEVICECHANGE message.

WM_DEVICECHANGE is a message that's broadcast, SendMessage(HWND_BROADCAST,...) style. Only top-level windows can receive it. The window doesn't need to be visible so there's little reason to look for an alternative.

RegisterDeviceNotification() is an alternative. But that still needs a window. Or a service handle, but you don't want to move windows around from a service. They run in an isolated session with their own desktop. So creating a window is a hard requirement.