private void rtfContent_KeyPress(Object sender, KeyPressEventArgs e)
    {
        if (e.KeyChar == 'C')
        {
            MessageBox.Show("C");
            e.Handled = true;
        }
    }

this.KeyPress += new System.Windows.Forms.KeyPressEventHandler(this.rtfContent_KeyPress);

this.KeyPress +=
    new System.Windows.Forms.KeyPressEventHandler(this.rtfContent_KeyPress)

rtfContent.KeyPress +=
    new System.Windows.Forms.KeyPressEventHandler(this.rtfContent_KeyPress)

private void richTextBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == 'C')
    {
        MessageBox.Show("C");
        e.Handled = true;
    }
}