下载网络资源

代码：

#region 下载文件
    /// <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 Download(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

        long remoteFileLength = GetHttpLength(url);// 取得远程文件长度
        System.Console.WriteLine("remoteFileLength=" + remoteFileLength);
        if (remoteFileLength == 745)
        {
            System.Console.WriteLine("远程文件不存在.");
            return false;
        }

        // 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

            if (startPosition >= remoteFileLength)
            {
                System.Console.WriteLine("本地文件长度" + startPosition + "已经大于等于远程文件长度" + remoteFileLength);
                writeStream.Close();

                return false;
            }
            else
            {
                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

            long currPostion = startPosition;

            while (contentSize > 0)// 如果读取长度大于零则继续读
            {
                currPostion += contentSize;
                int percent = (int)(currPostion * 100 / remoteFileLength);
                System.Console.WriteLine("percent=" + percent + "%");

                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;
    }

    // 从文件头得到远程文件的长度
    private static long GetHttpLength(string url)
    {
        long length = 0;

        try
        {
            HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(url);// 打开网络连接
            HttpWebResponse rsp = (HttpWebResponse)req.GetResponse();

            if (rsp.StatusCode == HttpStatusCode.OK)
            {
                length = rsp.ContentLength;// 从文件头得到远程文件的长度
            }

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

    }
    #endregion