程序代码：

if (richTextBox1.Text != "")
{
    string mb = MessageBox.Show("文件 无标题 的文字已经发生改变。\r\n想保存文件吗?", "记事", MessageBoxButtons.YesNoCancel, MessageBoxIcon.Exclamation).ToString();
    if (mb == "No")
    {
        richTextBox1.Clear();
    }
    else if (mb == "Yes")
    {
        if (saveFileDialog1.ShowDialog() == DialogResult.OK)
        {
            int len = richTextBox1.Text.Length;
            StreamWriter sw = new StreamWriter(saveFileDialog1.FileName, true, Encoding.UTF8, len);
            //...
        }
        richTextBox1.Clear();
    }
}