Restricting the maximum allowed length in a RichTextBox in WPF

private void richTexKeyDown(object sender, KeyEventArgs e)
       {
           TextRange tr = new TextRange(richTextBox1.Document.ContentStart ,richTextBox1.Document.ContentEnd);
           if (tr.Text.Length >= 300 || e.Key == Key.Space || e.Key == Key.Enter)
           {
               e.Handled = true;
               return;
           }
       }