Asp.Net任务Task和线程Thread

Task是.NET4.0加入的，跟线程池ThreadPool的功能类似，用Task开启新任务时，会从线程池中调用线程，而Thread每次实例化都会创建一个新的线程。任务（Task）是架构在线程之上的，也就是说任务最终还是要抛给线程（Thread）去执行。

1、首次构造一个Task对象时，他的状态是Created。

2、当任务启动时，他的状态变成WaitingToRun。

3、Task在一个线程上运行时，他的状态变成Running。

4、任务停止运行，等待他的任何子任务时，状态变成WaitingForChildrenToComplete。

5、任务完全结束时，它进入以下三个状态之一：RanToCompletion，Canceled或者Faulted。

6、一个Task<TResult>运行完成时，可通过Task<TResult>的Result属性来查询任务的结果，

7、一个Task或者Task<TResult>出错时，可以查询Task的Exception属性来获得任务抛出的未处理的异常，该属性总是返回一个AggregateException对象，他包含所有未处理的异常。

8、为简化代码，Task提供了几个只读的Boolean属性，IsCanceled，IsFaulted，IsCompleted。

9、注意，当Task处于RanToCompleted，Canceled或者Faulted状态时，IsCompleted返回True。

10、为了判断一个Task是否成功完成，最简单的方法是：if(task.Status == TaskStatus.RanToCompletion)。

11、task的ContinueWith方法用于在一个任务完成后发起一个新任务。

任务Task和线程Thread的区别：

static void Main(string[] args)
{
    for (int i = 0; i < 5; i++)
    {
        new Thread(Run1).Start();
    }
    for (int i = 0; i < 5; i++)
    {
        Task.Run(() => { Run2(); });
    }
}
static void Run1()
{
    Console.WriteLine("Thread Id =" + Thread.CurrentThread.ManagedThreadId);
}
static void Run2()
{
    Console.WriteLine("Task调用的Thread Id =" + Thread.CurrentThread.ManagedThreadId);
}

Task使用方法

创建Task有两种方式，一种是使用构造函数创建，另一种是使用 Task.Factory.StartNew 进行创建。如下代码所示

1.使用构造函数创建Task

　　Task t1 = new Task(MyMethod);

2.使用Task.Factory.StartNew 进行创建Task

　　Task t1 = Task.Factory.StartNew(MyMethod);

多任务

public void test()
    {
        Task[] tasks = new Task[10];
        for (int i = 0; i < 100; i++)
        {
            if (i % 2 == 0)
            {
                tasks[i] = new Task(test2, (object)i);//传参，必须是obj
            }
            else
            {
                tasks[i] = new Task(test1);
            }
            tasks[i].Start();
        }
        Task.WaitAll(tasks);//等待所有线程执行完成后才会继续往下执行
　　　　　//Task.WaitAll(tasks,5000);//最多等待5秒

        Response.Write("执行完成");
    }
    private void test1()
    { 
        //do
    }
    private void test2(object ourl)
    {
        //do
    }

连续任务

static void DownLoad(object str)
    {
       //下载文件
    }
    static void ReadNews(Task obj)
    {
        //读取文件
    }
    static void Main(string[] args)
    {
        Task task = new Task(DownLoad, "人民日报");
        Task task2 = task.ContinueWith(ReadNews);
        task.Start();
        //DownLoad执行完才执行ReadNews
    }

Task返回值

Task<string> t1 = Task.Factory.StartNew(() => "测试");
t1.Wait();
Console.WriteLine(t1.Result);
Console.ReadLine();
//返回值可以是任意的类型

Task线程池最大数量

这个TaskScheduler是微软开源的一个任务调度器

LimitedConcurrencyLevelTaskScheduler.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Threading.Tasks;
using System.Threading;

/// <summary>
///LimitedConcurrencyLevelTaskScheduler 的摘要说明
/// </summary>
public class LimitedConcurrencyLevelTaskScheduler : TaskScheduler
{
    // Indicates whether the current thread is processing work items.
    [ThreadStatic]
    private static bool _currentThreadIsProcessingItems;

    // The list of tasks to be executed 
    private readonly LinkedList<Task> _tasks = new LinkedList<Task>(); // protected by lock(_tasks)

    // The maximum concurrency level allowed by this scheduler. 
    private readonly int _maxDegreeOfParallelism;

    // Indicates whether the scheduler is currently processing work items. 
    private int _delegatesQueuedOrRunning = 0;

    // Creates a new instance with the specified degree of parallelism. 
    public LimitedConcurrencyLevelTaskScheduler(int maxDegreeOfParallelism)
    {
        if (maxDegreeOfParallelism < 1) throw new ArgumentOutOfRangeException("maxDegreeOfParallelism");
        _maxDegreeOfParallelism = maxDegreeOfParallelism;
    }

    // Queues a task to the scheduler. 
    protected sealed override void QueueTask(Task task)
    {
        // Add the task to the list of tasks to be processed.  If there aren't enough 
        // delegates currently queued or running to process tasks, schedule another. 
        lock (_tasks)
        {
            _tasks.AddLast(task);
            if (_delegatesQueuedOrRunning < _maxDegreeOfParallelism)
            {
                ++_delegatesQueuedOrRunning;
                NotifyThreadPoolOfPendingWork();
            }
        }
    }

    // Inform the ThreadPool that there's work to be executed for this scheduler. 
    private void NotifyThreadPoolOfPendingWork()
    {
        ThreadPool.UnsafeQueueUserWorkItem(_ =>
        {
            // Note that the current thread is now processing work items.
            // This is necessary to enable inlining of tasks into this thread.
            _currentThreadIsProcessingItems = true;
            try
            {
                // Process all available items in the queue.
                while (true)
                {
                    Task item;
                    lock (_tasks)
                    {
                        // When there are no more items to be processed,
                        // note that we're done processing, and get out.
                        if (_tasks.Count == 0)
                        {
                            --_delegatesQueuedOrRunning;
                            break;
                        }

                        // Get the next item from the queue
                        item = _tasks.First.Value;
                        _tasks.RemoveFirst();
                    }

                    // Execute the task we pulled out of the queue
                    base.TryExecuteTask(item);
                }
            }
            // We're done processing items on the current thread
            finally { _currentThreadIsProcessingItems = false; }
        }, null);
    }

    // Attempts to execute the specified task on the current thread. 
    protected sealed override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
    {
        // If this thread isn't already processing a task, we don't support inlining
        if (!_currentThreadIsProcessingItems) return false;

        // If the task was previously queued, remove it from the queue
        if (taskWasPreviouslyQueued)
            // Try to run the task. 
            if (TryDequeue(task))
                return base.TryExecuteTask(task);
            else
                return false;
        else
            return base.TryExecuteTask(task);
    }

    // Attempt to remove a previously scheduled task from the scheduler. 
    protected sealed override bool TryDequeue(Task task)
    {
        lock (_tasks) return _tasks.Remove(task);
    }

    // Gets the maximum concurrency level supported by this scheduler. 
    public sealed override int MaximumConcurrencyLevel { get { return _maxDegreeOfParallelism; } }

    // Gets an enumerable of the tasks currently scheduled on this scheduler. 
    protected sealed override IEnumerable<Task> GetScheduledTasks()
    {
        bool lockTaken = false;
        try
        {
            Monitor.TryEnter(_tasks, ref lockTaken);
            if (lockTaken) return _tasks;
            else throw new NotSupportedException();
        }
        finally
        {
            if (lockTaken) Monitor.Exit(_tasks);
        }
    }
}

使用方法

控制多线程数量，并实时输出已完成任务数量

protected void Page_Load(object sender, EventArgs e)
    {
        Response.Write("<script type="text/javascript">function load(i) {document.getElementById("now").innerHTML = i;}</script>");
        Response.Write("<span id='now'>0</span>/<span id='sum'>" + arr.Length + "</span>");
        Response.Flush();

        //设置最大线程数为5
        var scheduler = new LimitedConcurrencyLevelTaskScheduler(5);
        var Factory = new TaskFactory(scheduler);
        Task[] tasks = new Task[200];
        int res = 0;
        for (int i = 0; i < 200; i++)
        {
            Task tk = Factory.StartNew(() => downFile((object)i));
            tk.ContinueWith(t =>
            {
                res++;
                Response.Write("<script>load(" + res.ToString() + ")</script>");
                Response.Flush();
            });
            tasks[i] = tk;
            
        }
        Task.WaitAll(tasks);
        Response.Write("完成");
    }
    private void downFile(object oid)
    {
        string iid= (string)id;
        //do...
        
    }

Task使用过程中遇到的坑

使用过程中发现有的task任务不执行，经过长时间调试发现在task调用的方法里很多C#方法不可用，

例如HttpContext.Current.Request.Url、HttpContext.Current.Server.MapPath等方法不可用，真是坑

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