读写流我把byte数组长度设置成1000或2000时得到的文件总出错
private string down(string inUrl,string title)
  {
  //以字符流的形式下载文件  
 
  WebRequest objRequest = System.Net.HttpWebRequest.Create(inUrl);
  try
  {
  BufferedStream buff = new BufferedStream(objRequest.GetResponse().GetResponseStream());

byte[] bytes = new byte[1024];//这里如果是1000或2000时有时得到的文件（大小5-6M左右）出错有时不出错

  FileStream fs2 = new FileStream(title, FileMode.Create, FileAccess.Write);
  BufferedStream bs2 = new BufferedStream(fs2);

  while (buff.Read(bytes, 0, bytes.Length) > 0)
  {
  // buff.Read(bytes, 0, bytes.Length);
  int aa = bytes.Length;

  bs2.Write(bytes, 0, bytes.Length);
  bs2.Flush();
  }

  buff.Close();
  bs2.Close();
  fs2.Close();
  }
  catch
  {
  title = "123";
  }
  return title;
  }

------解决方案--------------------
int aa = bytes.Length;

bs2.Write(bytes, 0, bytes.Length);

就这两句出错，bytes.Length永远是你上面设定的固定值。而实际并不一定读取恰好等于数组长度的字节数。应该设定个表示实际读取的字节数的变量
int readByteNum;
while ((readByteNum = buff.Read(bytes, 0, bytes.Length)) > 0)
{

bs2.Write(bytes, 0, readByteNum);
bs2.Flush();
}