如何使用richtextbox将文本垂直居中？

richTextBox1.SelectionAlignment = HorizontalAlignment.Center;
richTextBox1.SelectionStart = 0;
richTextBox1.SelectionLength = richTextBox1.Text.Length;
richTextBox1.SelectionFont = new Font(richTextBox1.Font, FontStyle.Regular);
richTextBox1.SelectionCharOffset = 0;
richTextBox1.SelectionAlignment = HorizontalAlignment.Center;
richTextBox1.SelectionIndent = 0;
richTextBox1.SelectionRightIndent = 0;
richTextBox1.SelectionLength = 0;