打混中

using System.IO;string file_name = "sav.txt";
FileStream fs = File.Open(file_name ,FileMode.Open, FileAccess.Write, FileShare.None);
BinaryWriter bw =new BinaryWriter(fs);
bw.BaseStream.Seek(5,SeekOrigin.Begin);
bw.Write("aaa\r\n".ToCharArray());
bw.Close();