this.richTextBox1.AllowDrop = true;//设置允许拖拽
            this.richTextBox1.DragEnter += new DragEventHandler(this.richTextBox1_DragEnter);//注册事件
            this.richTextBox1.DragDrop += new DragEventHandler(this.richTextBox1_DragDrop);

        private void richTextBox1_DragEnter(object sender, DragEventArgs e)
        {
            if (e.Data.GetDataPresent(DataFormats.FileDrop))
            {
                e.Effect = DragDropEffects.Link;
            }
            else
            {
                e.Effect = DragDropEffects.None;
            }
        }

        private void richTextBox1_DragDrop(object sender, DragEventArgs e)
        {
            Array arrayfilename = (Array)e.Data.GetData(DataFormats.FileDrop);//有可能直接拖拽很多文件 这里生成一个所有文件全路径的列表

            string filename = arrayfilename.GetValue(0).ToString();//选择第一个文件 当然循环可实现加载所有文件

            using (StreamReader sr = new StreamReader(filename, System.Text.Encoding.Default))
            {
                string strred = sr.ReadToEnd().ToString();
                richTextBox1.Text = strred;
            }
        }