static void Main(string[] args)
    {
      Console.WriteLine("开始进行计算");
      // ThreadPool.QueueUserWorkItem(Sum, 10);
      Task<int> task = new Task<int>(Sum, 100);
      task.Start();
      //显示等待获取结果
      task.Wait();
      //调用Result时，等待返回结果
      Console.WriteLine("程序结果为 Sum = {0}",task.Result);
      Console.WriteLine("程序结束");
      Console.ReadLine();
    }
    public static int Sum(object i)
    {
      var sum = 0;
      for (var j = 0; j <= (int) i; j++)
      {
        Console.Write("{0} + ",sum);
        sum += j;
      }
      Console.WriteLine( " = {0}",sum);
      return sum;
    }

static void Main(string[] args)
    {
      Console.WriteLine("开始进行计算");
      // ThreadPool.QueueUserWorkItem(Sum, 10);
      var ctx = new CancellationTokenSource();
      var task = new Task<int>(() => Sum(ctx.Token, 100000));
      task.Start();
      //显示等待获取结果
      //task.Wait(ctx.Token);
      Thread.Sleep(1000);
      ctx.Cancel();
      //调用Result时，等待返回结果
      Console.WriteLine("程序结果为 Sum = {0}", task.Result);
      Console.WriteLine("程序结束");
      Console.ReadLine();
    }
    public static int Sum(CancellationToken cts, object i)
    {
      var sum = 0;    
      for (var j = 0; j <= (int)i; j++)
      {
        if (cts.IsCancellationRequested) return sum;
        Thread.Sleep(50);
        Console.Write("{0} + ", sum);
        sum += j;
      }
      Console.WriteLine(" = {0}", sum);
      return sum;
    }

static void Main(string[] args)
    {
      Console.WriteLine("开始进行计算");
      // ThreadPool.QueueUserWorkItem(Sum, 10);
      var ctx = new CancellationTokenSource();
      var task = new Task<int>(() => Sum(ctx.Token, 100000));
      task.Start();
      var cwt = task.ContinueWith(p =>
      {
        Console.WriteLine("task result ={0} ",task.Result);
      });
      //显示等待获取结果
      //task.Wait(ctx.Token);
      Thread.Sleep(1000);
      ctx.Cancel();
      //调用Result时，等待返回结果
      Console.WriteLine("程序结果为 Sum = {0}", task.Result);
      Console.WriteLine("程序结束");
      Console.ReadLine();
    }
    public static int Sum(CancellationToken cts, object i)
    {
      var sum = 0;    
      for (var j = 0; j <= (int)i; j++)
      {
        if (cts.IsCancellationRequested) return sum;
        Thread.Sleep(50);
        Console.Write("{0} + ", sum);
        sum += j;
      }
      Console.WriteLine(" = {0}", sum);
      return sum;
    }

private static async Task<string> DownloadStringWithRetries(string uri)
    {
      using (var client = new HttpClient())
      {
        // 第1 次重试前等1 秒，第2 次等2 秒，第3 次等4 秒。
        var nextDelay = TimeSpan.FromSeconds(1);
        for (int i = 0; i != 3; ++i)
        {
          try
          {
            return await client.GetStringAsync(uri);
          }
          catch
          {
          }
          await Task.Delay(nextDelay);
          nextDelay = nextDelay + nextDelay;
        }
        // 最后重试一次，以便让调用者知道出错信息。
        return await client.GetStringAsync(uri);
      }
    }

static void Main(string[] args)
    {
      Console.WriteLine("获取百度数据");
      ExecuteAsync();
      Console.WriteLine("线程结束");
      Console.ReadLine();
    }
    public static async void ExecuteAsync()
    {
      string text = await DownloadStringWithRetries("http://wwww.baidu.com");
      Console.WriteLine(text);
    }