using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Net;
namespace ConsoleTest
{
    class HttpDldFile
    {
       /// <summary>
       /// Http方式下载文件
       /// </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; // 上次下载的文件起始位置
            FileStream writeStream; // 写入本地文件流对象
            // 判断要下载的文件夹是否存在
            if (File.Exists(localfile))
            {
                writeStream = File.OpenWrite(localfile);             // 存在则打开要下载的文件
                startPosition = writeStream.Length;                  // 获取已经下载的长度
                writeStream.Seek(startPosition, SeekOrigin.Current); // 本地文件写入位置定位
            }
            else
            {
                writeStream = new FileStream(localfile, FileMode.Create);// 文件不保存创建一个文件
                startPosition = 0;
            }
            try
            {
                HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create(url);// 打开网络连接
                if (startPosition > 0)
                {
                    myRequest.AddRange((int)startPosition);// 设置Range值,与上面的writeStream.Seek用意相同,是为了定义远程文件读取位置
                }
                Stream readStream = myRequest.GetResponse().GetResponseStream();// 向服务器请求,获得服务器的回应数据流
                byte[] btArray = new byte[512];// 定义一个字节数据,用来向readStream读取内容和向writeStream写入内容
                int contentSize = readStream.Read(btArray, 0, btArray.Length);// 向远程文件读第一次
                while (contentSize > 0)// 如果读取长度大于零则继续读
                {
                    writeStream.Write(btArray, 0, contentSize);// 写入本地文件
                    contentSize = readStream.Read(btArray, 0, btArray.Length);// 继续向远程文件读取
                }
                //关闭流
                writeStream.Close();
                readStream.Close();
                flag = true;        //返回true下载成功
            }
            catch (Exception)
            {
                writeStream.Close();
                flag = false;       //返回false下载失败
            }
            return flag;
        }
    }
}

测试代码：
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");
        }
    }
}