//Собственно сам обработчик 
        private void keydown(object sender, KeyEventArgs e)
        {
            var type = sender.GetType();
            if (e.Control && e.KeyCode == Keys.C)
            {
                if (type == typeof(TextBox))
                {
                    textBox2.Text = (sender as TextBox).SelectedText;
                }
                if (type == typeof(RichTextBox))
                {
                    textBox2.Text = (sender as RichTextBox).SelectedText;
                }
            }
        }