代碼：

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;

namespace WebSiteTest.Common
{
    public 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 Download(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

            long remoteFileLength = GetHttpLength(url);// 取得遠程文件長度
            System.Console.WriteLine("remoteFileLength=" + remoteFileLength);
            if (remoteFileLength == 745)
            {
                //System.Console.WriteLine("遠程文件不存在.");
                return false;
            }

            
 
// 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;                  // 獲取已經下載的長度

                //if (startPosition >= remoteFileLength)
                //{
                //    //System.Console.WriteLine("本地文件長度" + startPosition + "已經大於等於遠程文件長度" + remoteFileLength);
                //    writeStream.Close();
                //    return false;
                //}
                //else
                //{
                //    writeStream.Seek(startPosition, SeekOrigin.Current); // 本地文件寫入位置定位
                //}

                return false;
            }
            else
            {
                writeStream = new FileStream(localfile, FileMode.Create);// 文件不保存創建一個文件
                startPosition = 0;
            }


            try
            {
                HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create(url);// 打開網絡連接

                if (startPosition > 0)
                {
                    myRequest.AddRange((int)startPosition);// 設置Range值,與上面的writeStream.Seek用意相同,是為了定義遠程文件讀取位置
                }


                Stream readStream = myRequest.GetResponse().GetResponseStream();// 向服務器請求,獲得服務器的回應數據流


                byte[] btArray = new byte[512];// 定義一個字節數據,用來向readStream讀取內容和向writeStream寫入內容
                int contentSize = readStream.Read(btArray, 0, btArray.Length);// 向遠程文件讀第一次

                long currPostion = startPosition;

                while (contentSize > 0)// 如果讀取長度大於零則繼續讀
                {
                    currPostion += contentSize;
                    int percent = (int)(currPostion * 100 / remoteFileLength);
                    //System.Console.WriteLine("percent=" + percent + "%");

                    writeStream.Write(btArray, 0, contentSize);// 寫入本地文件
                    contentSize = readStream.Read(btArray, 0, btArray.Length);// 繼續向遠程文件讀取
                }

                //關閉流
                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;
        }

        // 從文件頭得到遠程文件的長度
        private static long GetHttpLength(string url)
        {
            long length = 0;

            try
            {
                HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(url);// 打開網絡連接
                HttpWebResponse rsp = (HttpWebResponse)req.GetResponse();

                if (rsp.StatusCode == HttpStatusCode.OK)
                {
                    length = rsp.ContentLength;// 從文件頭得到遠程文件的長度
                }

                rsp.Close();
                return length;
            }
            catch (Exception e)
            {
                return length;
            }

        }
    }
}

Http方式下載文件