下載網絡資源

代碼：

#region 下載文件
    /// <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
            }
        }
        else
        {
            writeStream = new FileStream(localfile, FileMode.Create);// 文件不保存建立一個文件
            startPosition = 0;
        }


        try
        {
            HttpWebRequest myRequest = (HttpWebRequest)HttpWebRequest.Create(url);// 打開網絡鏈接

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


            Stream readStream = myRequest.GetResponse().GetResponseStream();// 向服務器請求,得到服務器的迴應數據流


            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)// 若是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;
    }

    // 從文件頭獲得遠程文件的長度
    private static long GetHttpLength(string url)
    {
        long length = 0;

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

            if (rsp.StatusCode == HttpStatusCode.OK)
            {
                length = rsp.ContentLength;// 從文件頭獲得遠程文件的長度
            }

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

    }
    #endregion