//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;
}
}
}