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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Net;
namespace ConsoleTest
{
    class HttpDldFile
    {
       /// <summary>
       /// Http方式下載文件
       /// </summary>
       /// <param name="url">http地址</param>
       /// <param name="localfile">本地文件</param>
       /// <returns></returns>
        public bool httpDownload(string url,string localfile)
        {
            bool flag = false;
            long startPosition = 0; // 上次下載的文件起始位置
            FileStream writeStream; // 寫入本地文件流對象
            // 判斷要下載的文件夾是否存在
            if (File.Exists(localfile))
            {
                writeStream = File.OpenWrite(localfile);             // 存在則打開要下載的文件
                startPosition = writeStream.Length;                  // 獲取已經(jīng)下載的長度
                writeStream.Seek(startPosition, SeekOrigin.Current); // 本地文件寫入位置定位
            }
            else
            {
                writeStream = new FileStream(localfile, FileMode.Create);// 文件不保存創(chuàng)建一個(gè)文件
                startPosition = 0;
            }
            try
            {
                HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create(url);// 打開網(wǎng)絡(luò)連接
                if (startPosition > 0)
                {
                    myRequest.AddRange((int)startPosition);// 設(shè)置Range值,與上面的writeStream.Seek用意相同,是為了定義遠(yuǎn)程文件讀取位置
                }
                Stream readStream = myRequest.GetResponse().GetResponseStream();// 向服務(wù)器請求,獲得服務(wù)器的回應(yīng)數(shù)據(jù)流
                byte[] btArray = new byte[512];// 定義一個(gè)字節(jié)數(shù)據(jù),用來向readStream讀取內(nèi)容和向writeStream寫入內(nèi)容
                int contentSize = readStream.Read(btArray, 0, btArray.Length);// 向遠(yuǎn)程文件讀第一次
                while (contentSize > 0)// 如果讀取長度大于零則繼續(xù)讀
                {
                    writeStream.Write(btArray, 0, contentSize);// 寫入本地文件
                    contentSize = readStream.Read(btArray, 0, btArray.Length);// 繼續(xù)向遠(yuǎn)程文件讀取
                }
                //關(guān)閉流
                writeStream.Close();
                readStream.Close();
                flag = true;        //返回true下載成功
            }
            catch (Exception)
            {
                writeStream.Close();
                flag = false;       //返回false下載失敗
            }
            return flag;
        }
    }
}

測試代碼：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleTest
{
    class Program
    {
        static void Main(string[] args)
        {
            HttpDldFile df = new HttpDldFile();
            df.httpDownload("http://files.cnblogs.com/files/xiandedanteng/Convertor20170624.zip","C:\\123\\Convertor20170624.zip");
        }
    }
}