把memerystream中的内容取出，付给一个字符串

Stream stream=new MemoryStream();
XmlSerializer ser = new XmlSerializer(t1.GetType());
ser.Serialize(stream, t1);

stream.Position = 0;
//Test t2 = (Test)ser.Deserialize(stream);
StreamReader sr = new StreamReader(stream);
string str = sr.ReadToEnd();