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C#連續(xù)任務(wù)Task.ContinueWith方法

更新時間：2022年04月20日 16:39:17 作者：農(nóng)碼一生

這篇文章介紹了C#中的連續(xù)任務(wù)Task.ContinueWith方法，文中通過示例代碼介紹的非常詳細(xì)。對大家的學(xué)習(xí)或工作具有一定的參考借鑒價值，需要的朋友可以參考下

一、簡介

通過任務(wù)，可以指定在任務(wù)完成之后，應(yīng)開始運行之后另一個特定任務(wù)。ContinueWith是Task根據(jù)其自身狀況，決定后續(xù)應(yīng)該作何操作。也就是說，在運行完task后，會執(zhí)行task.continuewith(XX)中的XX語句，但是是否執(zhí)行、如何執(zhí)行等需要看task的運行情況。例如：一個使用前一個任務(wù)的結(jié)果的新任務(wù)，如果前一個任務(wù)失敗了，這個任務(wù)就應(yīng)執(zhí)行一些清理工作。任務(wù)處理程序都不帶參數(shù)或者帶一個對象參數(shù)，而任務(wù)的連續(xù)處理方法都有一個Task類型的參數(shù)。

二、案例

案例一：

代碼：

        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();
        }

結(jié)果：

Start()和ContinueWith()的先后順序沒有關(guān)系，ContinueWith()會等待直到firstTask運行狀態(tài)達(dá)到 IsCompleted，因為TaskContinuationOptions中的OnlyOnRanToCompletion.必須指出的是，ContinueWith()中的參數(shù)是需要以Task為參數(shù)的，也就是firstTask作為參數(shù)被傳入，而且ContinueWith()運行在線程池中的線程中。我覺得比較重要的一點是：把ContinueWith()中的語句當(dāng)做一塊新的語句塊，他們獨立于主線程。無論如何，他們都要被判斷，如果狀態(tài)（status）不滿足，那么他們不執(zhí)行；當(dāng)指定了多個狀態(tài)，則使用合理的對應(yīng)狀態(tài)。

案例二：

代碼：

    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)); //給予足夠時間，讓firstTask、secondTask及其后續(xù)操作執(zhí)行完畢。

            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();
        }
    }

結(jié)果：

這里主線程休眠了足足4秒鐘，足以讓firstTask和secondTask兩個任務(wù)完成運行，而后，由于secondTask的后續(xù)除了接受OnlyOnRanToCompletion外，還接受ExecuteSynchronously。因此，后續(xù)運行中，由于主線程還沒有結(jié)束，因此 ExecuteSynchronously得到認(rèn)可，故secondTask的后續(xù)是在主線程上運行。

案例三：

代碼：

    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)); //給予足夠時間，讓firstTask、secondTask及其后續(xù)操作執(zhí)行完畢。

            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();
        }
    }

結(jié)果：