Very often, I want to disable the X button on a windows form so I can control what happens when a user closes the form. The following code has been working great for dialogs.

        public static extern int ExitWindowsEx(int uFlags, int dwReason);

        [DllImport("user32.dll", EntryPoint = "GetSystemMenu")]
        private static extern IntPtr GetSystemMenu(IntPtr hwnd, int revert);

        [DllImport("user32.dll", EntryPoint = "GetMenuItemCount")]
        private static extern int GetMenuItemCount(IntPtr hmenu);

        [DllImport("user32.dll", EntryPoint = "RemoveMenu")]
        private static extern int RemoveMenu(IntPtr hmenu, int npos, int wflags);

        [DllImport("user32.dll", EntryPoint = "DrawMenuBar")]
        private static extern int DrawMenuBar(IntPtr hwnd);

        private const int MF_BYPOSITION = 0x0400;
        private const int MF_DISABLED = 0x0002;

        public MainForm()

            IntPtr hmenu = GetSystemMenu(this.Handle, 0);
            int cnt = GetMenuItemCount(hmenu);

            // remove 'close' action
            RemoveMenu(hmenu, cnt-1, MF_DISABLED | MF_BYPOSITION);

            // remove extra menu line
            RemoveMenu(hmenu, cnt - 2, MF_DISABLED | MF_BYPOSITION);


However, the above code doesn’t work when the form is the main window of the program. So, I find the following method more robust.

        protected override CreateParams CreateParams
                CreateParams cp = base.CreateParams;
                const int CS_NOCLOSE = 0x200;
                cp.ClassStyle = cp.ClassStyle | CS_NOCLOSE;
                return cp;