string URLAddress = @"http://xiazai.jb51.net";

string receivePath=@"C:\";

client.DownloadFile(URLAddress, receivePath + System.IO.Path.GetFileName(URLAddress));

　Stream str = client.OpenRead(URLAddress);
 StreamReader reader = new StreamReader(str);
 byte[] mbyte = new byte[1000000];
 int allmybyte = (int)mbyte.Length;
 int startmbyte = 0;

 while (allmybyte > 0)
 {

 int m = str.Read(mbyte, startmbyte, allmybyte);
 if (m == 0)
  break;

 startmbyte += m;
 allmybyte -= m;
 }

 reader.Dispose();
 str.Dispose();

 string path = receivePath + System.IO.Path.GetFileName(URLAddress);
 FileStream fstr = new FileStream(path, FileMode.OpenOrCreate, FileAccess.Write);
 fstr.Write(mbyte, 0, startmbyte);
 fstr.Flush();
 fstr.Close();