C#连续任务Task.ContinueWith方法

一、简介

通过任务，可以指定在任务完成之后，应开始运行之后另一个特定任务。ContinueWith是Task根据其自身状况，决定后续应该作何操作。也就是说，在运行完task后，会执行task.continuewith(XX)中的XX语句，但是是否执行、如何执行等需要看task的运行情况。例如：一个使用前一个任务的结果的新任务，如果前一个任务失败了，这个任务就应执行一些清理工作。任务处理程序都不带参数或者带一个对象参数，而任务的连续处理方法都有一个Task类型的参数。

二、案例

案例一：

代码：

        static int TaskMethod(string name, int seconds)
        {
            Console.WriteLine("Frist Task Method : Task {0} is running on a thread id: {1}. Is thread pool thread: {2}",
            name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            return 60 * seconds;
        }
        static void Main(string[] args)
        {

            var FirstTask = new Task<int>(() => TaskMethod("Frist Task", 3));
            FirstTask.ContinueWith(t => Console.WriteLine("Frist Task Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion);//線程池線程
            FirstTask.Start();

            Console.ReadKey();
        }

结果：

Start()和ContinueWith()的先后顺序没有关系，ContinueWith()会等待直到firstTask运行状态达到 IsCompleted，因为TaskContinuationOptions中的OnlyOnRanToCompletion.必须指出的是，ContinueWith()中的参数是需要以Task为参数的，也就是firstTask作为参数被传入，而且ContinueWith()运行在线程池中的线程中。我觉得比较重要的一点是：把ContinueWith()中的语句当做一块新的语句块，他们独立于主线程。无论如何，他们都要被判断，如果状态（status）不满足，那么他们不执行；当指定了多个状态，则使用合理的对应状态。

案例二：

代码：

    class Program
    {
        static int TaskMethod(string name, int seconds)
        {
            Console.WriteLine("Task Method : Task {0} is running on a thread id: {1}. Is thread pool thread: {2}",
            name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            return 60 * seconds;
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Main Thread id {0}, Is in Thred Pool: {1}",
            Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);

            var firstTask = new Task<int>(() => TaskMethod("frist task", 3));
            var secondTask = new Task<int>(() => TaskMethod("second task", 2));

            firstTask.ContinueWith(t => Console.WriteLine("Result：Frist Thread Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion);

            firstTask.Start();
            secondTask.Start();

            Thread.Sleep(TimeSpan.FromSeconds(4)); //给予足够时间，让firstTask、secondTask及其后续操作执行完毕。

            Task continuation = secondTask.ContinueWith(t => Console.WriteLine("Result：Second Thread Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion | TaskContinuationOptions.ExecuteSynchronously);
            Console.ReadLine();
            Console.ReadKey();
        }
    }

结果：

这里主线程休眠了足足4秒钟，足以让firstTask和secondTask两个任务完成运行，而后，由于secondTask的后续除了接受OnlyOnRanToCompletion外，还接受ExecuteSynchronously。因此，后续运行中，由于主线程还没有结束，因此 ExecuteSynchronously得到认可，故secondTask的后续是在主线程上运行。

案例三：

代码：

    class Program
    {
        static int TaskMethod(string name, int seconds)
        {
            Console.WriteLine("Task Method : Task {0} is running on a thread id: {1}. Is thread pool thread: {2}",
            name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            return 60 * seconds;
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Main Thread id {0}, Is in Thred Pool: {1}",
            Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);

            var firstTask = new Task<int>(() => TaskMethod("frist task", 3));
            var secondTask = new Task<int>(() => TaskMethod("second task", 2));

            firstTask.ContinueWith(t => Console.WriteLine("Result：Frist Thread Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion);

            firstTask.Start();
            secondTask.Start();

           //Thread.Sleep(TimeSpan.FromSeconds(4)); //给予足够时间，让firstTask、secondTask及其后续操作执行完毕。

            Task continuation = secondTask.ContinueWith(t => Console.WriteLine("Result：Second Thread Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion | TaskContinuationOptions.ExecuteSynchronously);
            Console.ReadLine();
            Console.ReadKey();
        }
    }

结果：

然而，如果把4秒钟的休眠注释掉，那么由于主线程很早就结束了，因此secondTask只能接受到OnlyOnRanToCompletion，因此还是运行在线程池中。

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