string值转换XML格式

 /// <summary>
        /// xml格式呈现文本
        /// </summary>
        /// <param name="xmlString"></param>
        private void GetXMLDocument(string xmlString)
        {
            if(string.IsNullOrEmpty(xmlString))
            {
                return;
            }
            try
            {
                XmlDocument doc = new XmlDocument();
                doc.LoadXml(xmlString);
                MemoryStream mstream = new MemoryStream(1024);
                XmlTextWriter writer = new XmlTextWriter(mstream, null);
                XmlDocument xmldoc = new XmlDocument();
                writer.Formatting = Formatting.Indented;

                doc.WriteTo(writer);
                writer.Flush();
                writer.Close();

                Encoding encoding = Encoding.GetEncoding("utf-8");
                string aaaa = encoding.GetString(mstream.ToArray());
                this.TextBox1.Text = encoding.GetString(mstream.ToArray());
                mstream.Close();
            }
            catch
            {
                this.TextBox1.Text = xmlString;
            }
        }