c#richtextBox 字体加粗

private void Button2_Click(System.Object sender, System.EventArgs e)
{

    if (this.BackColor != SystemColors.ControlDark)
    {
        this.BackColor = SystemColors.ControlDark;
    }
    if (!(this.Font.Bold))
    {
        this.Font = new Font(this.Font, FontStyle.Bold);
    }
}