namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            richTextBox1.AllowDrop = true;
            richTextBox1.DragEnter += new DragEventHandler(richTextBox1_DragEnter);
            richTextBox1.DragDrop += new DragEventHandler(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);

            string strFileName = arrayFileName.GetValue(0).ToString();

            StreamReader sr = new StreamReader(strFileName,System.Text.Encoding.Default);
            richTextBox1.Text = sr.ReadToEnd();
            sr.Close();
        }
    }
}