转自 https://www.cnblogs.com/lgx5/p/10235567.html

// 方法一：TransmitFile实现下载
  protected void Button1_Click(object sender, EventArgs e)
  {
      Response.ContentType = "application/x-zip-compressed";
      Response.AddHeader("Content-Disposition", "attachment;filename=z.zip");
      string filename = Server.MapPath("DownLoad/z.zip");
      Response.TransmitFile(filename);
  } 

 ContentType 属性指定响应的 HTTP 内容类型。如果未指定 ContentType，默认为 text/HTML。

语法 Response.ContentType [= ContentType ] 参数 ContentType 描述内容类型的字符串。该字符串通常被格式化为类型/子类型，其中类型是常规内容范畴而子类为特定内容类型。

// 方法二：WriteFile实现下载
 protected void Button2_Click(object sender, EventArgs e)
 {
     string fileName = "asd.txt";//客户端保存的文件名
     string filePath = Server.MapPath("DownLoad/aaa.txt");//路径
 
     FileInfo fileInfo = new FileInfo(filePath);
     Response.Clear();
     Response.ClearContent();
     Response.ClearHeaders();
     Response.AddHeader("Content-Disposition", "attachment;filename=" + fileName);
     Response.AddHeader("Content-Length", fileInfo.Length.ToString());
     Response.AddHeader("Content-Transfer-Encoding", "binary");
     Response.ContentType = "application/octet-stream";
     Response.ContentEncoding = System.Text.Encoding.GetEncoding("gb2312");
     Response.WriteFile(fileInfo.FullName);
     Response.Flush();
     Response.End();
 }

 response.write的意思是返回一个消息给其它页面也可以，当前页面也行的。     

// 方法三：WriteFile分块下载
protected void Button3_Click(object sender, EventArgs e)
{
    string fileName = "aaa.txt";//客户端保存的文件名
    string filePath = Server.MapPath("DownLoad/aaa.txt");//路径
 
    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();
    }
}

// 方法四：流方式下载
 protected void Button4_Click(object sender, EventArgs e)
 {
     string fileName = "aaa.txt";//客户端保存的文件名
     string filePath = Server.MapPath("DownLoad/aaa.txt");//路径
 
     //以字符流的形式下载文件
     FileStream fs = new FileStream(filePath, FileMode.Open);
     byte[] bytes = new byte[(int)fs.Length];
     fs.Read(bytes, 0, bytes.Length);
     fs.Close();
     Response.ContentType = "application/octet-stream";
     //通知浏览器下载文件而不是打开
     Response.AddHeader("Content-Disposition", "attachment;  filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));
     Response.BinaryWrite(bytes);
     Response.Flush();
     Response.End();
 
 }