private string demoText = "---[白天非诚勿扰，晚上让子弹飞，结果有了赵氏孤儿！]"; 
private void GetContent(int line)
        {
            //StringBuilder sb = new StringBuilder();
            string s = string.Empty;
            for (int i = 1; i <= line; i++)
            {
                s = DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss") + "-" + "第" + i.ToString() + "次日记" + demoText;
                rtbMain.AppendText(s + System.Environment.NewLine);
            }

        }

 private void rtbMain_TextChanged(object sender, EventArgs e)
        {
            if (rtbMain.Lines.Length > 20)
            {
                rtbMain.Text = rtbMain.Text.Substring(rtbMain.Lines[0].Length + 1);
            }
           
        }