Override default window/system menu

Question

I want to add an "Always-On-Top"-menuentry to the system menu of all windows (the menu which opens when you right click the titlebar or click the icon). I'd prefer C# or C++, but if worst comes to worst I'll also use VB...

I know there are some applications like Dexpot which do this, but I was unable to find useful source code or free applications which do this for all windows and not just their own. I also know that there are other ways to achieve this functionality (AutoHotkeys or small programs which live in the system tray and let you select windows which should stay on top), but I'm looking for a more fluent and intuitive way. Ideally I'd add a small pin button to the titlebar, but my guess is that that's much more involved, so I'll stick with the menu variant for now.

Ideas? Thanks!

Loathing · Accepted Answer

Use the AddMenuItems() method and test using MS-Paint. One thing I noticed is that after the program is closed, the modified system menu becomes wonky. Possibly this is because the events are not coming from the process' UI thread. A possible work-around is in the ApplicationExit event to call GetMenu(hMainWindowHandle, true), where true means revert the menu.

public static class AlwaysOnTop {

    static AlwaysOnTop() {
        Application.ApplicationExit += delegate {
            try {
                foreach (DictionaryEntry de in htThreads) {
                    Hook h = (Hook) de.Value;
                    RemoveMenu(h.hMenu, h.uniqueId, 0);
                    //DeleteMenu(h.hMenu, h.uniqueId, 0);
                    UnhookWinEvent(h.hWinEventHook);
                }
            } catch {
            }
        };
    }

    private const int EVENT_OBJECT_INVOKED = 0x8013;
    private const int OBJID_SYSMENU = -1;
    private const int WINEVENT_OUTOFCONTEXT = 0;
    private const int MF_STRING = 0x00000000;
    private const int HWND_TOPMOST = -1;
    private const int HWND_NOTOPMOST = -2;
    private const int SWP_NOMOVE = 0x0002;
    private const int SWP_NOSIZE = 0x0001;
    private const uint MF_UNCHECKED = 0x00000000;
    private const uint MF_CHECKED = 0x00000008;

    [DllImport("user32.dll")]
    private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);

    [DllImport("user32.dll")]
    private static extern bool AppendMenu(IntPtr hMenu, uint uFlags, uint uIDNewItem, String lpNewItem);

    [DllImport("user32.dll", SetLastError = true)]
    static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

    [DllImport("user32.dll",SetLastError=true)]
    private static extern IntPtr SetWinEventHook(uint eventMin, uint eventMax, IntPtr hmodWinEventProc, WinEventProc lpfnWinEventProc, uint idProcess, uint idThread, uint dwFlags);

    [DllImport("user32.dll", SetLastError = true)]
    internal static extern int UnhookWinEvent(IntPtr hWinEventHook);

    [DllImport("user32.dll", SetLastError=true)]
    private static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

    [DllImport("user32.dll")]
    private static extern bool CheckMenuItem(IntPtr hMenu, uint uIDCheckItem, uint uCheck);

    [DllImport("user32.dll")]
    private static extern bool RemoveMenu(IntPtr hMenu, uint uPosition, uint uFlags);

    //[DllImport("user32.dll")]
    //private static extern bool DeleteMenu(IntPtr hMenu, uint uPosition, uint uFlags);

    private static Hashtable htThreads = new Hashtable();
    private static WinEventProc CallWinEventProc = new WinEventProc(EventCallback);
    private delegate void WinEventProc(IntPtr hWinEventHook, int iEvent, IntPtr hWnd, int idObject, int idChild, int dwEventThread, int dwmsEventTime);
    private static void EventCallback(IntPtr hWinEventHook, int iEvent, IntPtr hWnd, int idObject, int idChild, int dwEventThread, int dwmsEventTime) {
        //callback function, called when message is intercepted
        if (iEvent == EVENT_OBJECT_INVOKED) {
            if (idObject == OBJID_SYSMENU) {
                Hook h = (Hook) htThreads[(uint) dwEventThread];
                if (h != null && h.uniqueId == idChild) {
                    bool b = !h.Checked;
                    if (b)
                        SetWindowPos(h.hMainWindowHandle, (IntPtr) HWND_TOPMOST, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE);
                    else
                        SetWindowPos(h.hMainWindowHandle, (IntPtr) HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE);

                    CheckMenuItem(h.hMenu, h.uniqueId, (b ? MF_CHECKED : MF_UNCHECKED));
                    h.Checked = b;
                }
            }
        }
    }

    private class Hook {
        public uint uniqueId = 1001;
        public IntPtr hWinEventHook;
        public IntPtr hMenu;
        public IntPtr hMainWindowHandle;
        public bool Checked;
    }

    public static void AddMenuItems() {
        Process[] arr = Process.GetProcesses();
        foreach (Process p in arr) {
            if (p.MainWindowHandle == IntPtr.Zero)
                continue;

            if (p.ProcessName != "mspaint") // <-- remove or change this line
                continue;

            IntPtr hMenu = GetSystemMenu(p.MainWindowHandle, false);
            if (hMenu == IntPtr.Zero)
                continue;

            bool b = AppendMenu(hMenu, MF_STRING, 1001, "Always On Top");
            uint pid = 0;
            uint tid = GetWindowThreadProcessId(p.MainWindowHandle, out pid);

            Hook h = (Hook) htThreads[tid];
            if (h == null) {
                h = new Hook();
                h.hMenu = hMenu;
                h.hWinEventHook = SetWinEventHook(EVENT_OBJECT_INVOKED, EVENT_OBJECT_INVOKED, IntPtr.Zero, CallWinEventProc, pid, tid, WINEVENT_OUTOFCONTEXT);
                h.hMainWindowHandle = p.MainWindowHandle;
                htThreads[tid] = h;
            }
        }
    }
}

Override default window/system menu

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