using System.Runtime.InteropServices;

  this.AllowDrop = true;

        [DllImport("shell32.dll")]

        public static extern uint DragQueryFile(int hDrop, uint iFile, StringBuilder lpszFile, uint cch);

        [DllImport("shell32.dll")]

        private static extern void DragAcceptFiles(IntPtr hWnd, bool fAccept);

        [DllImport("shell32.dll")]

        private static extern void DragFinish(int hDrop);

        const int WM_DROPFILES        = 0x0233;

 protected override void WndProc(ref Message Msg)

        {

            if (Msg.Msg == WM_DROPFILES)

            {

                uint FilesCount = DragQueryFile((int)Msg.WParam, 0xFFFFFFFF, null, );

                richTextBox1.Clear();

                StringBuilder FileName = new StringBuilder();

                for (uint i = ; i < FilesCount; i++)

                {

                    DragQueryFile((int)Msg.WParam, i, FileName, );

                    richTextBox1.AppendText(FileName.ToString() + "\r\n");

                }

                DragFinish((int)Msg.WParam);

                return;

            }

            base.WndProc(ref Msg);

        }

        private void Form1_Load(object sender, EventArgs e)

        {

            DragAcceptFiles(this.Handle, true);

        }