C# 5.0引入了两个关键字 --async和await

private async void button1_Click(object sender, EventArgs e)
        {
            var length = AccessWebAsync();

            // 这里可以做一些不依赖回复的操作
            OtherWork();

            this.textBox1.Text += String.Format("\n 回复的字节长度为:  {0}.\r\n", await length);
            this.textBox2.Text = Thread.CurrentThread.ManagedThreadId.ToString();
        }

        private async Task<long> AccessWebAsync()
        {
            MemoryStream content = new MemoryStream();

            // 对MSDN发起一个Web请求
            HttpWebRequest webRequest = WebRequest.Create("http://msdn.microsoft.com/zh-cn/") as HttpWebRequest;
            if (webRequest != null)
            {
                // 返回回复结果
                using (WebResponse response = await webRequest.GetResponseAsync())
                {
                    using (Stream responseStream = response.GetResponseStream())
                    {
                        await responseStream.CopyToAsync(content);
                    }
                }
            }

            this.textBox3.Text = Thread.CurrentThread.ManagedThreadId.ToString();
            return content.Length;
        }

        private void OtherWork()
        {
            this.textBox1.Text += "\r\n等待服务器回复中.................\n";
        }