C#

[DllImport("user32.dll")]
private static extern IntPtr GetForegroundWindow();

public Form1()
{
   InitializeComponent();
   richTextBox1.DragEnter += new DragEventHandler(richTextBox1_DragEnter);
   richTextBox1.DragDrop += new DragEventHandler(richTextBox1_DragDrop);
}

void richTextBox1_DragDrop(object sender, DragEventArgs e)
{
   string[] files = (string[])e.Data.GetData(DataFormats.FileDrop); //array of the full paths of all files that were dropped
   if (files.Length > 0)
   {
       try
       {
           System.IO.StreamReader r = new System.IO.StreamReader(files[0]);
           richTextBox1.Text = r.ReadToEnd();
       }
       catch (Exception ex)
       {
           System.Diagnostics.Debug.WriteLine("Error opening file " + files[0] + ":\n"+ex.Message);
       }
   }
}

void richTextBox1_DragEnter(object sender, DragEventArgs e)
{
   if (e.Data.GetDataPresent(DataFormats.FileDrop, false))
   {
       e.Effect = DragDropEffects.All;
   }
}