C#修改RichTextBox选中文本样式

Font f=new Font("Verdana", 20F, FontStyle.Italic, GraphicsUnit.Point);
            richTextBox1.Focus();
            richTextBox1.Select(1, 100);
            richTextBox1.SelectionFont = f;
            richTextBox1.SelectionColor = Color.Red;
            richTextBox1.SelectionBackColor = Color.Orange;
            richTextBox1.Select(0, 0);