#region 异步返回文件流(提高应用程序的吞吐能力)
        public async Task<ActionResult> AwaitFile()
        {
            var responseHtml = await GetResponseStream("http://static.xuexiba.com/uploadfile/UserInfo/Avatar/201311/1302844759237319155175.jpg");
            return File(responseHtml, "image/jpeg");
        }

        private async Task<Stream> GetResponseStream(string url)
        {
            return await GetResponseContentAsync(url);
        }

        private async Task<Stream> GetResponseContentAsync(string url)
        {
            var httpClient = new System.Net.Http.HttpClient();
            var response = await httpClient.GetAsync(url);
            if (response.StatusCode == System.Net.HttpStatusCode.OK)
            {
                return await response.Content.ReadAsStreamAsync();
            }
            else
            {
                return null;//error
            }
        }

        #endregion

#region 异步返回文本(提高应用程序的吞吐能力)

        public async Task<ActionResult> AwaitHtml()
        {
            var responseHtml = await GetResponseHtml("http://www.sina.com");
            return Content(responseHtml);
        }

        private async Task<string> GetResponseHtml(string url)
        {
            return await GetResponseStringAsync(url);
        }

        private async Task<string> GetResponseStringAsync(string url)
        {
            var httpClient = new System.Net.Http.HttpClient();
            var response = await httpClient.GetAsync(url);
            if (response.StatusCode == System.Net.HttpStatusCode.OK)
            {
                return await response.Content.ReadAsStringAsync();//加await的意思是说，主ＵＩ等待它执行完成后，再继续执行，这种就叫作并行！
            }
            else
            {
                return null;//error
            }
        }
        #endregion