public frmMain()
{

   this. richTextBox1.DragDrop += new System.Windows.Forms.DragEventHandler(arg);
}

private void richTextBox1_DragEnter(object sender, 
System.Windows.Forms.DragEventArgs e)
{
   if (e.Data.GetDataPresent(DataFormats.Text)) 
      e.Effect = DragDropEffects.Copy;
   else
      e.Effect = DragDropEffects.None;
}