C#笔记23：多线程之Task（并行编程）

C#笔记23：多线程之Task（并行编程）

 

1：Task是什么

2：任务取消

3：TPL 和传统 .NET 异步编程

 

    任务并行库 (TPL) 是 .NET Framework 4 版的 System.Threading 和 System.Threading.Tasks 命名空间中的一组公共类型和 API。TPL 的目的在于简化向应用程序中添加并行性和并发性的过程，从而提高开发人员的工作效率。 TPL 会动态地按比例调节并发程度，以便最有效地使用所有可用的处理器。此外，TPL 还处理工作分区、ThreadPool 上的线程调度、取消支持、状态管理以及其他低级别的细节操作。通过使用 TPL，您可以在将精力集中于程序要完成的工作，同时最大程度地提高代码的性能。

 

1：Task是什么

    表示一个异步操作。它是异步操作的首选方式。Task是FRAMEWORK4中的新特性，封装了以前的Thread，并管理Thread。

    任务由 System.Threading.Tasks..::.Task 类表示。返回值的任务由 System.Threading.Tasks..::.Task<(Of <(TResult>)>) 类表示，该类从 Task 继承。任务对象处理基础结构详细信息，并提供可在任务的整个生存期内从调用线程访问的方法和属性。例如，可以随时访问任务的 Status 属性，以确定它是已开始运行、已完成运行、已取消还是引发了异常。状态由 TaskStatus 枚举表示。

 

2：任务取消

namespace CancellationWithOCE{    using System;    using System.Threading;    using System.Threading.Tasks;    class Program    {        static void Main(string[] args)        {            Console.WriteLine("Press any key to start. Press 'c' to cancel.");            Console.ReadKey();            var tokenSource = new CancellationTokenSource();            var token = tokenSource.Token;            // Store references to the tasks so that we can wait on them and            // observe their status after cancellation.            Task[] tasks = new Task[10];            // Request cancellation of a single task when the token source is canceled.            // Pass the token to the user delegate, and also to the task so it can             // handle the exception correctly.            tasks[0] = Task.Factory.StartNew(() => DoSomeWork(1, token), token);            // Request cancellation of a task and its children. Note the token is passed            // to (1) the user delegate and (2) as the second argument to StartNew, so             // that the task instance can correctly handle the OperationCanceledException.            tasks[1] = Task.Factory.StartNew(() =>            {                // Create some cancelable child tasks.                for (int i = 2; i < 10; i++)                {                    // For each child task, pass the same token                    // to each user delegate and to StartNew.                    tasks[i] = Task.Factory.StartNew(iteration =>                                DoSomeWork((int)iteration, token), i, token);                }                // Passing the same token again to do work on the parent task.                 // All will be signaled by the call to tokenSource.Cancel below.                DoSomeWork(2, token);            }, token);            // Give the tasks a second to start.            Thread.Sleep(1000);            // Request cancellation from the UI thread.            if (Console.ReadKey().KeyChar == 'c')            {                tokenSource.Cancel();                Console.WriteLine("\nTask cancellation requested.");                // Optional: Observe the change in the Status property on the task.                // It is not necessary to wait on tasks that have canceled. However,                // if you do wait, you must enclose the call in a try-catch block to                // catch the OperationCanceledExceptions that are thrown. If you do                 // not wait, no OCE is thrown if the token that was passed to the                 // StartNew method is the same token that requested the cancellation.                #region Optional_WaitOnTasksToComplete                try                {                    Task.WaitAll(tasks);                }                catch (AggregateException e)                {                    // For demonstration purposes, show the OCE message.                    foreach (var v in e.InnerExceptions)                        Console.WriteLine("msg: " + v.Message);                }                // Prove that the tasks are now all in a canceled state.                for (int i = 0; i < tasks.Length; i++)                    Console.WriteLine("task[{0}] status is now {1}", i, tasks[i].Status);                #endregion            }            // Keep the console window open while the            // task completes its output.            Console.ReadLine();        }        static void DoSomeWork(int taskNum, CancellationToken ct)        {            // Was cancellation already requested?            if (ct.IsCancellationRequested)            {                Console.WriteLine("We were cancelled before we got started.");                Console.WriteLine("Press Enter to quit.");                ct.ThrowIfCancellationRequested();            }            int maxIterations = 1000;            // NOTE!!! A benign "OperationCanceledException was unhandled            // by user code" error might be raised here. Press F5 to continue. Or,            //  to avoid the error, uncheck the "Enable Just My Code"            // option under Tools > Options > Debugging.            for (int i = 0; i < maxIterations; i++)            {                // Do a bit of work. Not too much.                var sw = new SpinWait();                for (int j = 0; j < 3000; j++) sw.SpinOnce();                Console.WriteLine("...{0} ", taskNum);                if (ct.IsCancellationRequested)                {                    Console.WriteLine("bye from {0}.", taskNum);                    Console.WriteLine("\nPress Enter to quit.");                    ct.ThrowIfCancellationRequested();                }            }        }    }}

3：TPL 和传统 .NET 异步编程

    见http://msdn.microsoft.com/zh-cn/library/dd997423.aspx。