//把txt清空
FileStream stream = File.Open(Adr,FileMode.OpenOrCreate,FileAccess.Write);
stream.Seek(0, SeekOrigin.Begin);
stream.SetLength(0);
stream.Close();
//向txt里面追加信息
StreamWriter sw = new StreamWriter(Adr,true,Encoding.GetEncoding("gb2312"));
sw.WriteLine(str);
sw.Flush();
sw.Close();