//c#使用线程下载文件的控制技巧和缺陷

//系统引用//定义线程公共变量//开始线程下载文件//中止线程下载

//系统引用

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Net;
using System.IO;
using System.Text;
using System.Threading;
using System.Data;

//定义线程公共变量
  public System.Threading.Thread thread001;
  public System.Threading.Thread thread002;

   //开始线程下载文件

private void button7_Click(object sender, System.EventArgs e)
  {
   //开始线程下载文件
   DownloadClass a=new DownloadClass(); 
   thread001= new Thread(new ThreadStart(a.DownloadFile));
   a.StrUrl=textBox1.Text;
   a.StrFileName=textBox2.Text;   
   thread001.Start();
   DownloadClass b=new DownloadClass(); 
   thread002= new Thread(new ThreadStart(b.DownloadFile));
   b.StrUrl=textBox3.Text;
   b.StrFileName=textBox4.Text;   
   thread002.Start();
  }

   //中止线程下载

private void button5_Click(object sender, System.EventArgs e)
  {
   //中止线程下载
   thread001.Abort();  
   thread002.Abort();
   MessageBox.Show("线程已经中止！","警告！");
   
  }

//定义下载文件类.线程传参数用

  public class DownloadClass
  {  
   //打开上次下载的文件或新建文件
   public string StrUrl;//文件下载网址
   public string StrFileName;//下载文件保存地址
   public string strError;//返回结果
   public long lStartPos =0; //返回上次下载字节
   public long lCurrentPos=0;//返回当前下载字节
   public long lDownloadFile;//返回当前下载文件长度

   public void DownloadFile()
　　  {    
    System.IO.FileStream fs;
    if (System.IO.File.Exists(StrFileName))
    {
     fs= System.IO.File.OpenWrite(StrFileName);
     lStartPos=fs.Length;
     fs.Seek(lStartPos,System.IO.SeekOrigin.Current);
     //移动文件流中的当前指针
    }
    else
    {
     fs = new System.IO.FileStream(StrFileName,System.IO.FileMode.Create);
     lStartPos =0;
    }

    //打开网络连接
    try
    {
     System.Net.HttpWebRequest request =(System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(StrUrl);
     long length=request.GetResponse().ContentLength;
     lDownloadFile=length;
     if (lStartPos>0)
      request.AddRange((int)lStartPos); //设置Range值
    
     //向服务器请求，获得服务器回应数据流
     System.IO.Stream ns= request.GetResponse().GetResponseStream();
    
     byte[] nbytes = new byte[512];
     int nReadSize=0;    
     nReadSize=ns.Read(nbytes,0,512);
     while( nReadSize >0)
     {
      fs.Write(nbytes,0,nReadSize);