扩展WebBrowser控件，使其支持拖放文件

    public partial class UserControl1 : WebBrowser
    {
        private const int WmDropfiles = 0x233;

        [DllImport("shell32.dll")]
        private static extern uint DragQueryFile(IntPtr hDrop, uint iFile, StringBuilder lpszFile, uint cch);

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

        protected override void OnHandleCreated(EventArgs e)
        {
            base.OnHandleCreated(e);

            //指示接受文件拖放操作
            if (!DesignMode)
            {
                DragAcceptFiles(Handle, true);
            }
        }

        protected override void WndProc(ref Message m)
        {
            if (m.Msg == WmDropfiles)
            {
                #region 解析消息
                //获取拖放的文件数量
                var count = DragQueryFile(m.WParam, 0xffffffff, null, 0);
                //解析所有播放的文件路径
                var filePaths = new string[count];
                for (uint i = 0; i < count; i++)
                {
                    var sb = new StringBuilder();
                    DragQueryFile(m.WParam, i, sb, 1024);
                    filePaths[i] = sb.ToString();
                }
                #endregion

                //是否继续触发之后的消息处理
                var isCancel = false;

                #region 触发自定义文件拖放事件
                if (DragFile != null)
                {
                    var args = new DragFileEventArgs(filePaths);
                    DragFile(this, args);
                    isCancel = args.IsCancel;
                }
                #endregion

                if (isCancel) return;
            }
            base.WndProc(ref m);
        }

        public event EventHandler<DragFileEventArgs> DragFile;

        public class DragFileEventArgs : EventArgs
        {
            private readonly string[] _filePaths;
            public bool IsCancel { get; set; }

            public DragFileEventArgs(string[] filePaths)
            {
                _filePaths = filePaths;
            }

            public string[] FilePaths
            {
                get { return _filePaths; }
            }
        }
    }