kev/Drawer/UCDraw/PcgDrawR/FileDropHandler.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace PcgDrawR
{
    /// <summary>
    /// The file drop handler.
    /// </summary>
    public class FileDropHandler : IMessageFilter, IDisposable
    {
        #region native members

        [DllImport("user32.dll", SetLastError = true, CallingConvention = CallingConvention.Winapi)]
        [return: MarshalAs(UnmanagedType.Bool)]
        private static extern bool ChangeWindowMessageFilterEx(IntPtr hWnd, uint message, ChangeFilterAction action, in ChangeFilterStruct pChangeFilterStruct);

        [DllImport("shell32.dll", SetLastError = false, CallingConvention = CallingConvention.Winapi)]
        private static extern void DragAcceptFiles(IntPtr hWnd, bool fAccept);

        [DllImport("shell32.dll", SetLastError = false, CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Winapi)]
        private static extern uint DragQueryFile(IntPtr hWnd, uint iFile, StringBuilder lpszFile, int cch);

        [DllImport("shell32.dll", SetLastError = false, CallingConvention = CallingConvention.Winapi)]
        private static extern void DragFinish(IntPtr hDrop);

        [StructLayout(LayoutKind.Sequential)]
        private struct ChangeFilterStruct
        {
            public uint CbSize;
            public ChangeFilterStatu ExtStatus;
        }

        private enum ChangeFilterAction : uint
        {
            MSGFLT_RESET,
            MSGFLT_ALLOW,
            MSGFLT_DISALLOW
        }

        private enum ChangeFilterStatu : uint
        {
            MSGFLTINFO_NONE,
            MSGFLTINFO_ALREADYALLOWED_FORWND,
            MSGFLTINFO_ALREADYDISALLOWED_FORWND,
            MSGFLTINFO_ALLOWED_HIGHER
        }

        private const uint WM_COPYGLOBALDATA = 0x0049;
        private const uint WM_COPYDATA = 0x004A;
        private const uint WM_DROPFILES = 0x0233;

        #endregion

        private const uint GetIndexCount = 0xFFFFFFFFU;

        private Control _ContainerControl;

        private readonly bool _DisposeControl;

        public Control ContainerControl { get; }

        /// <summary>
        /// Initializes a new instance of the <see cref="FileDropHandler"/> class.
        /// </summary>
        /// <param name="containerControl"></param>
        public FileDropHandler(Control containerControl) : this(containerControl, false) { }

        /// <summary>
        /// Initializes a new instance of the <see cref="FileDropHandler"/> class.
        /// </summary>
        /// <param name="containerControl">The container control.</param>
        /// <param name="releaseControl">If true, release control.</param>
        public FileDropHandler(Control containerControl, bool releaseControl)
        {
            _ContainerControl = containerControl ?? throw new ArgumentNullException("control", "control is null.");

            if (containerControl.IsDisposed) throw new ObjectDisposedException("control");

            _DisposeControl = releaseControl;

            var status = new ChangeFilterStruct() { CbSize = 8 };

            if (!ChangeWindowMessageFilterEx(containerControl.Handle, WM_DROPFILES, ChangeFilterAction.MSGFLT_ALLOW, in status)) throw new Win32Exception(Marshal.GetLastWin32Error());
            if (!ChangeWindowMessageFilterEx(containerControl.Handle, WM_COPYGLOBALDATA, ChangeFilterAction.MSGFLT_ALLOW, in status)) throw new Win32Exception(Marshal.GetLastWin32Error());
            if (!ChangeWindowMessageFilterEx(containerControl.Handle, WM_COPYDATA, ChangeFilterAction.MSGFLT_ALLOW, in status)) throw new Win32Exception(Marshal.GetLastWin32Error());
            DragAcceptFiles(containerControl.Handle, true);

            Application.AddMessageFilter(this);
        }

        /// <summary>
        /// Pres the filter message.
        /// </summary>
        /// <param name="m">The m.</param>
        /// <returns>A bool.</returns>
        public bool PreFilterMessage(ref Message m)
        {
            if (_ContainerControl == null || _ContainerControl.IsDisposed) return false;
            if (_ContainerControl.AllowDrop) return _ContainerControl.AllowDrop = false;
            if (m.Msg == WM_DROPFILES)
            {
                var handle = m.WParam;

                var fileCount = DragQueryFile(handle, GetIndexCount, null, 0);

                var fileNames = new string[fileCount];

                var sb = new StringBuilder(262);
                var charLength = sb.Capacity;
                for (uint i = 0; i < fileCount; i++)
                {
                    if (DragQueryFile(handle, i, sb, charLength) > 0) fileNames[i] = sb.ToString();
                }
                DragFinish(handle);
                _ContainerControl.AllowDrop = true;
                _ContainerControl.DoDragDrop(fileNames, DragDropEffects.All);
                _ContainerControl.AllowDrop = false;
                return true;
            }
            return false;
        }

        /// <summary>
        /// Disposes the.
        /// </summary>
        public void Dispose()
        {
            if (_ContainerControl == null)
            {
                if (_DisposeControl && !_ContainerControl.IsDisposed) _ContainerControl.Dispose();
                Application.RemoveMessageFilter(this);
                _ContainerControl = null;
            }
        }
    }
}