using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace clipboard_event_handler_win32 {
    internal static class NativeMethods {
        //Reference https://docs.microsoft.com/en-us/windows/desktop/dataxchg/wm-clipboardupdate
        public const int WM_CLIPBOARDUPDATE = 0x031D;
        //Reference https://www.pinvoke.net/default.aspx/Constants.HWND
        public static IntPtr HWND_MESSAGE = new IntPtr(-3);
        //Reference https://www.pinvoke.net/default.aspx/user32/AddClipboardFormatListener.html
        [DllImport("user32.dll", SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool AddClipboardFormatListener(IntPtr hwnd);
        //Reference https://www.pinvoke.net/default.aspx/user32.setparent
        [DllImport("user32.dll", SetLastError = true)]
        public static extern IntPtr SetParent(IntPtr hWndChild, IntPtr hWndNewParent);
    }

    public sealed class ClipboardNotification {
        private class NotificationForm : Form {
            public NotificationForm() {
                //Turn the child window into a message-only window (refer to Microsoft docs)
                NativeMethods.SetParent(Handle, NativeMethods.HWND_MESSAGE);
                //Place window in the system-maintained clipboard format listener list
                NativeMethods.AddClipboardFormatListener(Handle);
            }

            protected override void WndProc(ref Message m) {
                //Listen for operating system messages
                if (m.Msg == NativeMethods.WM_CLIPBOARDUPDATE)
                {
                    Console.WriteLine("CLIPBOARD_CHANGE");
                }
                //Called for any unhandled messages
                base.WndProc(ref m);
            }
        }

        private static void Main(string[] args) {
            Application.Run(new NotificationForm());
        }
    }
}