<!--web.config下的部分配置-->
<appSettings>
  <!--下载的文件路径-->
  <add key="fileFullPath" value="D:\TDDOWNLOAD\mx3.1.3.2000cn.exe"/>
  <!--每次发送的包的大小(单位:kb)-->
  <add key="packSize" value="50"/>
  <!--休眠时间(单位:毫秒[注:10000毫秒 = 1秒])-->
  <add key="sleep" value="500"/>
</appSettings>

// DownloadFile.ashx

<%@ WebHandler Language="C#" CodeBehind="DownloadFile.ashx.cs" Class="WebDownload.DownloadFile" %>

//-----------------------------------------------
// DownloadFile.ashx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Text;
using System.IO;
using System.Configuration;

namespace WebDownload
{
    /// <summary>
    /// Summary description for DownloadFile
    /// 参考:http://www.codedigest.com/CodeDigest/39-File-Download-in-ASP-Net-with-C-.aspx
    /// </summary>
    public class DownloadFile : IHttpHandler
    {

        public void ProcessRequest(HttpContext context)
        {
            //context.Response.ContentType = "text/plain";
            //context.Response.Write("Hello World");

            string fileFullPath = ConfigurationManager.AppSettings["fileFullPath"];
            int packSize = int.Parse(ConfigurationManager.AppSettings["packSize"]);

            packSize *= 1024; //1024你懂的

            Download(context, fileFullPath, packSize);
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }

        public bool Download(HttpContext context,
                             string fileFullPath,
                             int packSize)
        {
            try
            {
                HttpResponse response = context.Response;
                HttpRequest request = context.Request;

                string fileName = Path.GetFileName(fileFullPath);
                fileName = HttpUtility.UrlEncode(fileName, Encoding.UTF8);

                using (Stream stream = new FileStream(fileFullPath,
                                               FileMode.Open,
                                               FileAccess.Read,
                                               FileShare.Read))
                {
                    #region 断点续传判断
                    string requestHeadRange = request.Headers["Range"];
                    if (requestHeadRange != null)
                    {
                        response.StatusCode = 206;

                        string[] rang = requestHeadRange.Split(new char[] { '=', '-' });
                        long offset = long.Parse(rang[1]);
                        stream.Seek(offset, SeekOrigin.Current);
                    } 
                    #endregion

                    long bytesToRead = stream.Length;
                    response.ContentType = "application/octet-stream";
                    response.AddHeader("Content-Disposition", "attachment; filename=" + fileName);
                    response.AddHeader("Content-Length", bytesToRead.ToString());

                    int sleep = int.Parse(ConfigurationManager.AppSettings["sleep"]);

                    while (bytesToRead > 0)
                    {
                        if (response.IsClientConnected)
                        {
                            byte[] buffer = new Byte[packSize];
                            int length = stream.Read(buffer, 0, packSize);

                            response.OutputStream.Write(buffer, 0, packSize);
                            response.Flush();

                            System.Threading.Thread.Sleep(sleep);

                            bytesToRead = bytesToRead - length;
                        }
                    }

                    return true;
                }
            }
            catch (Exception ex)
            {
                //这里自己记录异常信息了

                return false;
            }

        }
    }
}