string fileName = “a.zip”;//最终下载到客户端保存的文件名
string filePath = Server.MapPath(“DownLoad/a.zip”);//服务器上的文件路径
WriteFile分块下载

 public void xzcz(string fileName, string filePath)
{
    
    
System.IO.FileInfo fileInfo = new System.IO.FileInfo(filePath);
 if (fileInfo.Exists == true)
   {
    
    
    const long ChunkSize = 102400;//100K 每次读取文件，只读取100K，这样可以缓解服务器的压力
    byte[] buffer = new byte[ChunkSize];
    Response.Clear();
    System.IO.FileStream iStream = System.IO.File.OpenRead(filePath);
   long dataLengthToRead = iStream.Length;//获取下载的文件总大小
    Response.ContentType = "application/octet-stream";
    Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName));
    while (dataLengthToRead > 0 && Response.IsClientConnected)
          {
    
    
          int lengthRead = iStream.Read(buffer, 0, Convert.ToInt32(ChunkSize));//读取的大小
          Response.OutputStream.Write(buffer, 0, lengthRead);
          Response.Flush();
         dataLengthToRead = dataLengthToRead - lengthRead;
              }
                Response.Close();
                }
                }