Webclient在下载请求时无法设置请求超时时间和请求读写超时时间。WebClient在异步下载时遇到网络不通等问题时没有响应超时造成app挂起。

1.Webclient请求超时设置
      重写Webclient的GetWebRequest方法，为HttpWebRequest添加请求超时及读写超时

        protected override WebRequest GetWebRequest(Uri address)
        {
            HttpWebRequest request = (HttpWebRequest)base.GetWebRequest(address);
            request.Timeout = 1000 * Timeout;
            request.ReadWriteTimeout = 1000 * Timeout;
            return request;
        }

2.WebClient在异步下载

       创建计时器监视响应情况，过期则取消下载

    public class Calculagraph
    {
        /// <summary>
        /// 时间到事件
        /// </summary>
        public event TimeoutCaller TimeOver;

        /// <summary>
        /// 开始时间
        /// </summary>
        private DateTime _startTime;
        private TimeSpan _timeout = new TimeSpan(0, 0, 10);
        private bool _hasStarted = false;
        object _userdata;

        /// <summary>
        /// 计时器构造方法
        /// </summary>
        /// <param name="userdata">计时结束时回调的用户数据</param>
        public Calculagraph(object userdata)
        {
            TimeOver += new TimeoutCaller(OnTimeOver);
            _userdata = userdata;
        }

        /// <summary>
        /// 超时退出
        /// </summary>
        /// <param name="userdata"></param>
        public virtual void OnTimeOver(object userdata)
        {
            Stop();
        }

        /// <summary>
        /// 过期时间(秒)
        /// </summary>
        public int Timeout
        {
            get
            {
                return _timeout.Seconds;
            }
            set
            {
                if (value <= 0)
                    return;
                _timeout = new TimeSpan(0, 0, value);
            }
        }

        /// <summary>
        /// 是否已经开始计时
        /// </summary>
        public bool HasStarted
        {
            get
            {
                return _hasStarted;
            }
        }

        /// <summary>
        /// 开始计时
        /// </summary>
        public void Start()
        {
            Reset();
            _hasStarted = true;
            Thread th = new Thread(WaitCall);
            th.IsBackground = true;
            th.Start();
        }

        /// <summary>
        /// 重置
        /// </summary>
        public void Reset()
        {
            _startTime = DateTime.Now;
        }

        /// <summary>
        /// 停止计时
        /// </summary>
        public void Stop()
        {
            _hasStarted = false;
        }

        /// <summary>
        /// 检查是否过期
        /// </summary>
        /// <returns></returns>
        private bool checkTimeout()
        {
            return (DateTime.Now - _startTime).Seconds >= Timeout;
        }

        private void WaitCall()
        {
            try
            {
                //循环检测是否过期
                while (_hasStarted && !checkTimeout())
                {
                    Thread.Sleep(1000);
                }
                if (TimeOver != null)
                    TimeOver(_userdata);
            }
            catch (Exception)
            {
                Stop();
            }
        }
    }

    /// <summary>
    /// 过期时回调委托
    /// </summary>
    /// <param name="userdata"></param>
    public delegate void TimeoutCaller(object userdata);

    public class CNNWebClient : WebClient
    {

        private Calculagraph _timer;
        private int _timeOut = 10;

        /// <summary>
        /// 过期时间
        /// </summary>
        public int Timeout
        {
            get
            {
                return _timeOut;
            }
            set
            {
                if (value <= 0)
                    _timeOut = 10;
                _timeOut = value;
            }
        }

        /// <summary>
        /// 重写GetWebRequest,添加WebRequest对象超时时间
        /// </summary>
        /// <param name="address"></param>
        /// <returns></returns>
        protected override WebRequest GetWebRequest(Uri address)
        {
            HttpWebRequest request = (HttpWebRequest)base.GetWebRequest(address);
            request.Timeout = 1000 * Timeout;
            request.ReadWriteTimeout = 1000 * Timeout;
            return request;
        }

        /// <summary>
        /// 带过期计时的下载
        /// </summary>
        public void DownloadFileAsyncWithTimeout(Uri address, string fileName, object userToken)
        {
            if (_timer == null)
            {
                _timer = new Calculagraph(this);
                _timer.Timeout = Timeout;
                _timer.TimeOver += new TimeoutCaller(_timer_TimeOver);
                this.DownloadProgressChanged += new DownloadProgressChangedEventHandler(CNNWebClient_DownloadProgressChanged);
            }

            DownloadFileAsync(address, fileName, userToken);
            _timer.Start();
        }

        /// <summary>
        /// WebClient下载过程事件，接收到数据时引发
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        void CNNWebClient_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
        {
            _timer.Reset();//重置计时器
        }

        /// <summary>
        /// 计时器过期
        /// </summary>
        /// <param name="userdata"></param>
        void _timer_TimeOver(object userdata)
        {
            this.CancelAsync();//取消下载
        }
    }