C#richtextbox控件插入行问题

C#的richtextbox插入行，总是不换行，插入行代码如下：

        public int AppText(string FilePath,string Content)
        {
            int res = -1;
            try
            {
                StreamWriter sw =File.AppendText(FilePath);
               
                sw.Write(Content);
                res = 0;
                sw.Close();<