【C#】线程之Task

Task开启线程

　　有两种启动方式:

　　　　1.构造创建线程，然后启动

　　　　　　

            var taskForAction = new Task(() =>
            {
                //do something
            });
　　　　　　　　taskForAction.Start();

　　　　　　注：构造所有的重载并没有传入Func函数的，而且我们这个时候看线程池中活动线程数会发现改变

            //打印线程池中线程活动数
            PrintAvailabeWorkThreadNum();

            var taskForAction = new Task(() =>
            {
                //do something
            });
　　　　　　　 taskForAction.Start();
            PrintAvailabeWorkThreadNum();

　　

　　输出结果：

　　　　　　

　　

　　　　2.直接使用静态方法　　　

            //打印线程池中线程活动数
            PrintAvailableWorkThreadNum();

            var taskForAction = Task.Run(() => Console.WriteLine("print string for Action"));

            var taskForFunc = Task.Run(() => "return string for Func<string>");

            PrintAvailableWorkThreadNum();

            //Result内部会调用Wait,所以这里不需要调

            Console.WriteLine(taskForFunc.Result);

　　　同样的，直接调用静态方法来创建一个线程,并返回当前正在执行的线程副本以供我们调用，Result只有传递进去的是Func函数才会在返回的Task中存在，如果传入的是Action函数,Result是不存在的， 这个时候线程活动数量也会改变。

　　　   　

取消任务

　　 已经在 【C#】线程协作式取消 这章里面好好讨论过如何去取消了，如何注册回调函数等技术了.

任务完成时重新开启一个任务（ConintueWith）

我们有时候想在执行完一个任务以后，再开始做一个其他的任务,这个时候如果我们用Wait就会堵塞线程，如果我们用线程嵌套的方式去做，会浪费资源并损害的伸缩性。

　　

           //这样会堵塞我们的线程
            Task.Run(() =>
            {
                //do something
            }).Wait();

            Task.Run(() =>
            {
                //do another thing
            });

            //虽然不会堵塞线程了，但这样会浪费资源
            Task.Run(() =>
            {
                Task.Run(() =>
                {
                    //do something
                }).Wait();

                Task.Run(() =>
                {
                    //do another thing
                });
            });

　　  CLR给我们提供了另一个方法:ContinueWith.

　　　　这个方法会不会堵塞当前的线程,并且会等第一个任务做好了以后再做第二个任务(当然可以开启多个）

　　　　　　 var t = Task.Run(() =>
            {
                int index = ;
                int count = ;
                while (index != )
                {
                    count += index;
                    Console.WriteLine("Task:" + index++);
                    Thread.Sleep( * );
                }
                return count;
            });

            t.ContinueWith(task =>
            {
                //这里的参数Task，就是我们上面那个线程对象(t)，可以用于获取结果集，状态等数据
                Console.WriteLine("First continue task:" + task.Status);
                Console.WriteLine("First continue task:" + (task.Result + )+"\n");
            });

            t.ContinueWith(task =>
            {
                Console.WriteLine("Second continue task:" + task.Status);
                Console.WriteLine("Second continue task:" + (task.Result - ));
            });

            t.ContinueWith(task =>
            {
                //Do another thing
            });

　　　ContinueWith方法延伸

　　　　需求肯定是很复杂的，比如我们希望在各种状态（取消，完成，失败等）情况下执行各种ContinueWith的方法，这个时候我们需要关注一个枚举类型：TaskContinuationOptions， 以下给出官方的定义:

　　

namespace System.Threading.Tasks
{
    // Summary:
    //     Specifies the behavior for a task that is created by using the System.Threading.Tasks.Task.ContinueWith(System.Action<System.Threading.Tasks.Task>,System.Threading.CancellationToken,System.Threading.Tasks.TaskContinuationOptions,System.Threading.Tasks.TaskScheduler)
    //     or System.Threading.Tasks.Task<TResult>.ContinueWith(System.Action<System.Threading.Tasks.Task<TResult>>,System.Threading.Tasks.TaskContinuationOptions)
    //     method.
    [Serializable]
    [Flags]
    public enum TaskContinuationOptions
    {
        // Summary:
        //     Default = "Continue on any, no task options, run asynchronously" Specifies
        //     that the default behavior should be used. Continuations, by default, will
        //     be scheduled when the antecedent task completes, regardless of the task's
        //     final System.Threading.Tasks.TaskStatus.
        None = ,
        //
        // Summary:
        //     A hint to a System.Threading.Tasks.TaskScheduler to schedule a task in as
        //     fair a manner as possible, meaning that tasks scheduled sooner will be more
        //     likely to be run sooner, and tasks scheduled later will be more likely to
        //     be run later.
        PreferFairness = ,
        //
        // Summary:
        //     Specifies that a task will be a long-running, course-grained operation. It
        //     provides a hint to the System.Threading.Tasks.TaskScheduler that oversubscription
        //     may be warranted.
        LongRunning = ,
        //
        // Summary:
        //     Specifies that a task is attached to a parent in the task hierarchy.
        AttachedToParent = ,
        //
        // Summary:
        //     Specifies that an System.InvalidOperationException will be thrown if an attempt
        //     is made to attach a child task to the created task.
        DenyChildAttach = ,
        //
        // Summary:
        //     Prevents the ambient scheduler from being seen as the current scheduler in
        //     the created task. This means that operations like StartNew or ContinueWith
        //     that are performed in the created task will see System.Threading.Tasks.TaskScheduler.Default
        //     as the current scheduler.
        HideScheduler = ,
        //
        // Summary:
        //     In the case of continuation cancellation, prevents completion of the continuation
        //     until the antecedent has completed.
        LazyCancellation = ,
        //
        // Summary:
        //     Specifies that the continuation task should not be scheduled if its antecedent
        //     ran to completion. This option is not valid for multi-task continuations.
        NotOnRanToCompletion = ,
        //
        // Summary:
        //     Specifies that the continuation task should not be scheduled if its antecedent
        //     threw an unhandled exception. This option is not valid for multi-task continuations.
        NotOnFaulted = ,
        //
        // Summary:
        //     Specifies that the continuation task should be scheduled only if its antecedent
        //     was canceled. This option is not valid for multi-task continuations.
        OnlyOnCanceled = ,
        //
        // Summary:
        //     Specifies that the continuation task should not be scheduled if its antecedent
        //     was canceled. This option is not valid for multi-task continuations.
        NotOnCanceled = ,
        //
        // Summary:
        //     Specifies that the continuation task should be scheduled only if its antecedent
        //     threw an unhandled exception. This option is not valid for multi-task continuations.
        OnlyOnFaulted = ,
        //
        // Summary:
        //     Specifies that the continuation task should be scheduled only if its antecedent
        //     ran to completion. This option is not valid for multi-task continuations.
        OnlyOnRanToCompletion = ,
        //
        // Summary:
        //     Specifies that the continuation task should be executed synchronously. With
        //     this option specified, the continuation will be run on the same thread that
        //     causes the antecedent task to transition into its final state. If the antecedent
        //     is already complete when the continuation is created, the continuation will
        //     run on the thread creating the continuation. Only very short-running continuations
        //     should be executed synchronously.
        ExecuteSynchronously = ,
    }
}

这里就不一一解释了，这里面有一些参数只是建议，会不会执行两说，这里我只介绍几个常用的，直接附上代码：

　　

    var t = Task.Run(() =>
            {

                int index = ;
                int count = ;
                while (index != )
                {
                    count += index;
                    Console.WriteLine("Task:" + index++);
                    Thread.Sleep( * );
                }
                return count;
            });

            t.ContinueWith(task =>
            {
                //只有执行成功以后才会继续做
            }, TaskContinuationOptions.OnlyOnRanToCompletion);

            t.ContinueWith(task =>
            {
               //只有取消的时候才做操作
            }, TaskContinuationOptions.OnlyOnCanceled);

            t.ContinueWith(task =>
            {
                //只有失败的时候才会运行，抛出未知异常什么的.
                AggregateException ex = task.Exception;
                Console.WriteLine(ex.Message);
            }, TaskContinuationOptions.OnlyOnFaulted);

　     个人对这个机制是十分喜欢的，即不堵塞我们的线程，又可以按照状态来分别做操作。

任务启动子任务

var t = new Task<Int32[]>(() =>
            {
                var results = new int[];
                new Task(() =>
                {
                    Thread.Sleep( * );
                    results[] = ;

                }, TaskCreationOptions.AttachedToParent).Start();

                new Task(() =>
                {
                    results[] = ;

                }, TaskCreationOptions.AttachedToParent).Start();

                new Task(() =>
                {
                    results[] = ;

                }, TaskCreationOptions.AttachedToParent).Start();

                return results;
            });
            t.ContinueWith(task =>
                Array.ForEach(task.Result, Console.WriteLine),
                TaskContinuationOptions.AttachedToParent);

            t.Start();

　　

　　　　这里主要是用到了TaskCreationOptions.AttachedToParent枚举标志，用到了这个标志，父线程会等待子线程所有线程都执行完毕以后才会继续往下走（注：这里也不能当前主线程）.这里我尝试过用Task.Run这个去建立这样的机制，可惜的是这个没有办法完成(Task.Run没有参数包含TaskCreationOptions的重载），具体的原因还在探索中，如果有朋友知道，请告知，谢谢:)!

任务工厂

　 关于这个方法，我在网上查到的也都是CLR那本书上的东西，但是关于这个，如果用起来不注意的话，会出现很多的问题，先给出代码：

　　

       private static Int32 Sum(CancellationToken ct, Int32 n)
        {
            Int32 sum = 0;
            for (; n > 0; n--)
            {
                　　　　　　　  　ct.ThrowIfCancellationRequested();
                checked { sum += n; }
                           }
            return sum;
        }

        public static void TaskFactory()
        {
            var parent = new Task(() =>
            {
                var cts = new CancellationTokenSource();
                var tf = new TaskFactory<Int32>(cts.Token,
 　　　　　　　　　　　　TaskCreationOptions.AttachedToParent, 
　　　　　　　　　　　　 TaskContinuationOptions.ExecuteSynchronously, 
　　　　　　　　　　    TaskScheduler.Default);

                // This tasks creates and starts 3 child tasks
                var childTasks = new[] {
                 tf.StartNew(() => Sum(cts.Token, 10000)),
                 tf.StartNew(() => Sum(cts.Token, 20000)),
                 tf.StartNew(() => Sum(cts.Token, Int32.MaxValue)),  // Too big, throws OverflowException
                };

                //如果有一个线程错误了就暂停所有的任务
                Array.ForEach(childTasks,
                    task => task.ContinueWith(t => cts.Cancel(), TaskContinuationOptions.OnlyOnFaulted));

                tf.ContinueWhenAll(
                   childTasks,
                   completedTasks => completedTasks.Where(t => t.Status == TaskStatus.RanToCompletion).Max(t => t.Result),
                   CancellationToken.None)
                   .ContinueWith(t => Console.WriteLine("The maximum is: " + t.Result),
                      TaskContinuationOptions.ExecuteSynchronously).Wait(); // Wait is for testing only
            });

            parent.ContinueWith(p =>
            {
                var sb = new StringBuilder("The following exception(s) occurred:" + Environment.NewLine);
                foreach (var e in p.Exception.Flatten().InnerExceptions)
                    sb.AppendLine("   " + e.GetType().ToString());
                Console.WriteLine(sb.ToString());
            }, TaskContinuationOptions.OnlyOnFaulted);

            parent.Start();

            try
            {
                parent.Wait(); // For testing purposes
            }
            catch (AggregateException)
            {
            }
        }

　　首先我们看一下

　　　　　var childTasks = new[] {
                 tf.StartNew(() => Sum(cts.Token, 10000)),
                 tf.StartNew(() => Sum(cts.Token, 20000)),
                 tf.StartNew(() => Sum(cts.Token, Int32.MaxValue)),  // Too big, throws OverflowException
                };

　　　　　　//如果有一个线程错误了就暂停所有的任务
                Array.ForEach(childTasks,
                    task => task.ContinueWith(t => cts.Cancel(), TaskContinuationOptions.OnlyOnFaulted));

　　　　  这段代码是创建了三个线程放入工厂中,并建立一个会抛出异常的线程,下面那段代码会取消线程的操作（3个线程都取消操作，因为注册了同一个TOKEN),但是这里需要注意的是：如果我其他线程跑的比抛出异常的线程块，这会导致取消不了，因为结束了（这个的确很难做，因为无法控制线程的执行速度和优先级）。

　　　　tf.ContinueWhenAll(
                   childTasks,
                   completedTasks => completedTasks.Where(t => t.Status == TaskStatus.RanToCompletion).Max(t => t.Result),
                   CancellationToken.None)
                   .ContinueWith(t => Console.WriteLine("The maximum is: " + t.Result),
                      TaskContinuationOptions.ExecuteSynchronously).Wait(); // Wait is for testing only

　　　　这段代码才是用TaskFactory的核心，这个会等待所有工厂中的线程执行完毕（包括被取消）才会执行，还有一个方法叫ContinueWhenAny：当有一个线程结束操作就会执行。这里要注意的是：

两个方法都有带TaskContinuationOptions参数的重载，但是有那么几个是不能用的:

1. 　　NotOnRanToCompletion
2. 　　NotOnFaulted
3. 　　OnlyOnCanceled
4. 　　NotOnCanceled
5. 　　OnlyOnFaulted
6. 　　OnlyOnRanToCompletion

也就是说无论前面任务是什么状态，这个方法都会执行，所以我们必须要自己去判断：Where(t => t.Status == TaskStatus.RanToCompletion).