如何设置字体

private void btnBold_Click(object sender, EventArgs e)
        {
            Font oldfont, newfont;
            oldfont = this.richTextBox1.SelectionFont;//获取选定字的字体格式
            if (oldfont.Bold)
            {
                newfont = new Font(oldfont, oldfont.Style &~FontStyle.Bold);//清除粗体字格式
            }
            else
                newfont = new Font(oldfont, oldfont.Style | FontStyle.Bold); //设置为粗体字格式
            richTextBox1.SelectionFont = newfont;
            richTextBox1.Focus();
        }

        private void btnUnderline_Click(object sender, EventArgs e)
        {
            Font oldfont, newfont;
            oldfont = this.richTextBox1.SelectionFont;//获取选定字的字体格式
            if (oldfont.Underline)
            {
                newfont = new Font(oldfont, oldfont.Style & ~FontStyle.Underline);//清除下划线字格式
            }
            else
                newfont = new Font(oldfont, oldfont.Style | FontStyle.Underline); //设置为下划线字格式
            richTextBox1.SelectionFont = newfont;
            richTextBox1.Focus();
        }

        private void btnItalic_Click(object sender, EventArgs e)
        {
            Font oldfont, newfont;
            oldfont = this.richTextBox1.SelectionFont;//获取选定字的字体格式
            if (oldfont.Italic)
            {
                newfont = new Font(oldfont, oldfont.Style & ~FontStyle.Italic);//清除斜体字格式
            }
            else
                newfont = new Font(oldfont, oldfont.Style | FontStyle.Italic); //设置为斜体字字格式
            richTextBox1.SelectionFont = newfont;
            richTextBox1.Focus();
        }

        private void btnCenter_Click(object sender, EventArgs e)
        {
            if (richTextBox1.SelectionAlignment == HorizontalAlignment.Left)
                richTextBox1.SelectionAlignment = HorizontalAlignment.Center;
            else
                richTextBox1.SelectionAlignment = HorizontalAlignment.Left;
            richTextBox1.Focus();
        }