C#异步编程（一）：异步基础

class Program
    {
        static void Main(string[] args)
        {
            var result = DownloadStringWithTimeout("http://www.yueyangdujia.com");
            Console.WriteLine("异步方法执行中");
            for (int i = 0; i < 3; i++)
            {
                System.Threading.Thread.Sleep(1000);
                Console.WriteLine("休息了"+(i+1)+"秒");
            }     
            var str= result.Result;   //这里使用到异步方法的返回值，需要等待异步方法执行完毕。         
            Console.WriteLine("异步方法执行后,同步执行以下代码");
            Console.WriteLine(str);
            Console.Read();
        }

        static async Task<string> DownloadStringWithTimeout(string uri)
        {
            using (var client=new HttpClient())
            {
                var downloadTask = client.GetStringAsync(uri);
                var timeoutTask = Task.Delay(3000);

                var completedTask = await Task.WhenAny(downloadTask, timeoutTask);
                if (completedTask == timeoutTask)
                {
                    System.Threading.Thread.Sleep(2000);
                    Console.WriteLine("异步方法执行完成");
                    return null;
                }
                System.Threading.Thread.Sleep(2000);
                Console.WriteLine("异步方法执行完成");
                return await downloadTask;
            }
            
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var result = DownloadStringWithTimeout("http://www.yueyangdujia.com");         
            var str= result.Result;   //这里使用到异步方法的返回值，需要等待异步方法执行完毕。                   
            Console.WriteLine(str);
            Console.Read();
        }

        static async Task<string> DownloadStringWithTimeout(string uri)
        {
            using (var client=new HttpClient())
            {
                var downloadTask = client.GetStringAsync(uri);
                var timeoutTask = Task.Delay(3000);
                var completedTask = await Task.WhenAny(downloadTask, timeoutTask);
                if (completedTask == timeoutTask)
                {
                
                    return null;
                }
                return await downloadTask;
            }
            
        }
    }