二进制文件 读写 第一次写长度为 20 第二次 写长度 18 读出来还是 长度20 的 为什么不是覆盖的写

第一次用"ad,sad,as,dasf,我我,"
第二次用"ad,sad,as,dasf,我,"
结果:
  第一次:
    "ad,sad,as,dasf,我我,"
    20
    "ad,sad,as,dasf,我我,"
    20
  第二次:
    "ad,sad,as,dasf,我,"
    18
    "ad,sad,as,dasf,我,?"
    19
这个怎么回事???    我想不出来 ...麻烦大虾帮帮我!!1
CODE:
string SSConfig = Application.StartupPath + "\\s.data";
string str = "ad,sad,as,dasf,我我,";//改成"ad,sad,as,dasf,我," 出错
MessageBox.Show(str);
          byte[] bywr = System.Text.Encoding.Default.GetBytes(str);
          MessageBox.Show(bywr.Length.ToString());
          BinaryWriter bw;
          FileStream fs;
          try
          {
              fs = File.OpenWrite(SSConfig);
              bw = new BinaryWriter(fs);
              bw.Write(bywr);
              bw.Flush();
              bw.Close();
              fs.Close();
          }
          catch (Exception ee)
          {
              bw = null;
              fs = null;
              //FindException(ee.Message);
          }          //read
          byte[] byrr;          BinaryReader br1,br2;
          FileStream fs1, fs2;
          try
          {
              string s = "";
              StreamReader fs11 = new StreamReader(SSConfig, Encoding.Default);
              MessageBox.Show(s=fs11.ReadToEnd());
              fs11.Close();
              MessageBox.Show(System.Text.Encoding.Default.GetByteCount(s).ToString());
          }
          catch (Exception ee)
          {
          }