Task詳解

轉：https://www.cnblogs.com/zhaoshujie/p/11082753.html

1、Task的優勢

　　ThreadPool相比Thread來說具備了很多優勢，但是ThreadPool卻又存在一些使用上的不方便。比如：
　　◆ ThreadPool不支持線程的取消、完成、失敗通知等交互性操作；
　　◆ ThreadPool不支持線程執行的先後次序；
　　以往，如果開發者要實現上述功能，需要完成很多額外的工作，現在，FCL中提供了一個功能更強大的概念：Task。Task在線程池的基礎上進行了優化，並提供了更多的API。在FCL4.0中，如果我們要編寫多線程程序，Task顯然已經優於傳統的方式。

　　還有：public Task<string> test1() { ...... } 表示的是“未來完成時”，而public string test2() { ...... }是正常執行的方法。

　　以下是一個簡單的任務示例：

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Task t = new Task(() =>
            {
                Console.WriteLine("任務開始工作……");
                //模擬工作過程
                Thread.Sleep(5000);
            });
            t.Start();
            t.ContinueWith((task) =>
            {
                Console.WriteLine("任務完成，完成時候的狀態爲：");
                Console.WriteLine("IsCanceled={0}\tIsCompleted={1}\tIsFaulted={2}", task.IsCanceled, task.IsCompleted, task.IsFaulted);
            });
            Console.ReadKey();
        }
    }
}

2、Task的用法

　　2.1、創建任務

　　（一）無返回值的方式
　　方式1:

　　var t1 = new Task(() => TaskMethod("Task 1"));
　　t1.Start();
　　Task.WaitAll(t1);//等待所有任務結束 
　　注:任務的狀態:
　　Start之前爲:Created
　　Start之後爲:WaitingToRun

　　方式2:

　　Task.Run(() => TaskMethod("Task 2"));

　　方式3:

　　Task.Factory.StartNew(() => TaskMethod("Task 3")); 直接異步的方法 
　　//或者
　　var t3=Task.Factory.StartNew(() => TaskMethod("Task 3"));
　　Task.WaitAll(t3);//等待所有任務結束
　　//任務的狀態:
　　Start之前爲:Running
　　Start之後爲:Running

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            var t1 = new Task(() => TaskMethod("Task 1"));
            var t2 = new Task(() => TaskMethod("Task 2"));
            t2.Start();
            t1.Start();
            Task.WaitAll(t1, t2);
            Task.Run(() => TaskMethod("Task 3"));
            Task.Factory.StartNew(() => TaskMethod("Task 4"));
            //標記爲長時間運行任務,則任務不會使用線程池,而在單獨的線程中運行。
            Task.Factory.StartNew(() => TaskMethod("Task 5"), TaskCreationOptions.LongRunning);

            #region 常規的使用方式
            Console.WriteLine("主線程執行業務處理.");
            //創建任務
            Task task = new Task(() =>
            {
                Console.WriteLine("使用System.Threading.Tasks.Task執行異步操作.");
                for (int i = 0; i < 10; i++)
                {
                    Console.WriteLine(i);
                }
            });
            //啓動任務,並安排到當前任務隊列線程中執行任務(System.Threading.Tasks.TaskScheduler)
            task.Start();
            Console.WriteLine("主線程執行其他處理");
            task.Wait();
            #endregion

            Thread.Sleep(TimeSpan.FromSeconds(1));
            Console.ReadLine();
        }

        static void TaskMethod(string name)
        {
            Console.WriteLine("Task {0} is running on a thread id {1}. Is thread pool thread: {2}",
                name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
        }
    }
}

　　async/await的實現方式:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        async static void AsyncFunction()
        {
            await Task.Delay(1);
            Console.WriteLine("使用System.Threading.Tasks.Task執行異步操作.");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine(string.Format("AsyncFunction:i={0}", i));
            }
        }

        public static void Main()
        {
            Console.WriteLine("主線程執行業務處理.");
            AsyncFunction();
            Console.WriteLine("主線程執行其他處理");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine(string.Format("Main:i={0}", i));
            }
            Console.ReadLine();
        }
    }
}

　　（二）帶返回值的方式
　　方式4:

　　Task<int> task = CreateTask("Task 1");
　　task.Start(); 
　　int result = task.Result;

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static Task<int> CreateTask(string name)
        {
            return new Task<int>(() => TaskMethod(name));
        }

        static void Main(string[] args)
        {
            TaskMethod("Main Thread Task");
            Task<int> task = CreateTask("Task 1");
            task.Start();
            int result = task.Result;
            Console.WriteLine("Task 1 Result is: {0}", result);

            task = CreateTask("Task 2");
            //該任務會運行在主線程中
            task.RunSynchronously();
            result = task.Result;
            Console.WriteLine("Task 2 Result is: {0}", result);

            task = CreateTask("Task 3");
            Console.WriteLine(task.Status);
            task.Start();

            while (!task.IsCompleted)
            {
                Console.WriteLine(task.Status);
                Thread.Sleep(TimeSpan.FromSeconds(0.5));
            }

            Console.WriteLine(task.Status);
            result = task.Result;
            Console.WriteLine("Task 3 Result is: {0}", result);

            #region 常規使用方式
            //創建任務
            Task<int> getsumtask = new Task<int>(() => Getsum());
            //啓動任務,並安排到當前任務隊列線程中執行任務(System.Threading.Tasks.TaskScheduler)
            getsumtask.Start();
            Console.WriteLine("主線程執行其他處理");
            //等待任務的完成執行過程。
            getsumtask.Wait();
            //獲得任務的執行結果
            Console.WriteLine("任務執行結果：{0}", getsumtask.Result.ToString());
            #endregion
        }

        static int TaskMethod(string name)
        {
            Console.WriteLine("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(2));
            return 42;
        }

        static int Getsum()
        {
            int sum = 0;
            Console.WriteLine("使用Task執行異步操作.");
            for (int i = 0; i < 100; i++)
            {
                sum += i;
            }
            return sum;
        }
    }
}

async/await的實現:

using System;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        public static void Main()
        {
            var ret1 = AsyncGetsum();
            Console.WriteLine("主線程執行其他處理");
            for (int i = 1; i <= 3; i++)
                Console.WriteLine("Call Main()");
            int result = ret1.Result;                  //阻塞主線程
            Console.WriteLine("任務執行結果：{0}", result);
        }

        async static Task<int> AsyncGetsum()
        {
            await Task.Delay(1);
            int sum = 0;
            Console.WriteLine("使用Task執行異步操作.");
            for (int i = 0; i < 100; i++)
            {
                sum += i;
            }
            return sum;
        }
    }
}

　　2.2、組合任務.ContinueWith

　　 簡單Demo:

using System;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        public static void Main()
        {
            //創建一個任務
            Task<int> task = new Task<int>(() =>
            {
                int sum = 0;
                Console.WriteLine("使用Task執行異步操作.");
                for (int i = 0; i < 100; i++)
                {
                    sum += i;
                }
                return sum;
            });
            //啓動任務,並安排到當前任務隊列線程中執行任務(System.Threading.Tasks.TaskScheduler)
            task.Start();
            Console.WriteLine("主線程執行其他處理");
            //任務完成時執行處理。
            Task cwt = task.ContinueWith(t =>
            {
                Console.WriteLine("任務完成後的執行結果：{0}", t.Result.ToString());
            });
            task.Wait();
            cwt.Wait();
        }
    }
}

　　 任務的串行:

using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            ConcurrentStack<int> stack = new ConcurrentStack<int>();

            //t1先串行
            var t1 = Task.Factory.StartNew(() =>
            {
                stack.Push(1);
                stack.Push(2);
            });

            //t2,t3並行執行
            var t2 = t1.ContinueWith(t =>
            {
                int result;
                stack.TryPop(out result);
                Console.WriteLine("Task t2 result={0},Thread id {1}", result, Thread.CurrentThread.ManagedThreadId);
            });

            //t2,t3並行執行
            var t3 = t1.ContinueWith(t =>
            {
                int result;
                stack.TryPop(out result);
                Console.WriteLine("Task t3 result={0},Thread id {1}", result, Thread.CurrentThread.ManagedThreadId);
            });

            //等待t2和t3執行完
            Task.WaitAll(t2, t3);

            //t7串行執行
            var t4 = Task.Factory.StartNew(() =>
            {
                Console.WriteLine("當前集合元素個數：{0},Thread id {1}", stack.Count, Thread.CurrentThread.ManagedThreadId);
            });
            t4.Wait();
        }
    }
}

　　子任務:

using System;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        public static void Main()
        {
            Task<string[]> parent = new Task<string[]>(state =>
            {
                Console.WriteLine(state);
                string[] result = new string[2];
                //創建並啓動子任務
                new Task(() => { result[0] = "我是子任務1。"; }, TaskCreationOptions.AttachedToParent).Start();
                new Task(() => { result[1] = "我是子任務2。"; }, TaskCreationOptions.AttachedToParent).Start();
                return result;
            }, "我是父任務，並在我的處理過程中創建多個子任務，所有子任務完成以後我纔會結束執行。");
            //任務處理完成後執行的操作
            parent.ContinueWith(t =>
            {
                Array.ForEach(t.Result, r => Console.WriteLine(r));
            });
            //啓動父任務
            parent.Start();
            //等待任務結束 Wait只能等待父線程結束,沒辦法等到父線程的ContinueWith結束
            //parent.Wait();
            Console.ReadLine();

        }
    }
}

　　動態並行(TaskCreationOptions.AttachedToParent) 父任務等待所有子任務完成後整個任務纔算完成

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Node
    {
        public Node Left { get; set; }
        public Node Right { get; set; }
        public string Text { get; set; }
    }


    class Program
    {
        static Node GetNode()
        {
            Node root = new Node
            {
                Left = new Node
                {
                    Left = new Node
                    {
                        Text = "L-L"
                    },
                    Right = new Node
                    {
                        Text = "L-R"
                    },
                    Text = "L"
                },
                Right = new Node
                {
                    Left = new Node
                    {
                        Text = "R-L"
                    },
                    Right = new Node
                    {
                        Text = "R-R"
                    },
                    Text = "R"
                },
                Text = "Root"
            };
            return root;
        }

        static void Main(string[] args)
        {
            Node root = GetNode();
            DisplayTree(root);
        }

        static void DisplayTree(Node root)
        {
            var task = Task.Factory.StartNew(() => DisplayNode(root),
                                            CancellationToken.None,
                                            TaskCreationOptions.None,
                                            TaskScheduler.Default);
            task.Wait();
        }

        static void DisplayNode(Node current)
        {

            if (current.Left != null)
                Task.Factory.StartNew(() => DisplayNode(current.Left),
                                            CancellationToken.None,
                                            TaskCreationOptions.AttachedToParent,
                                            TaskScheduler.Default);
            if (current.Right != null)
                Task.Factory.StartNew(() => DisplayNode(current.Right),
                                            CancellationToken.None,
                                            TaskCreationOptions.AttachedToParent,
                                            TaskScheduler.Default);
            Console.WriteLine("當前節點的值爲{0};處理的ThreadId={1}", current.Text, Thread.CurrentThread.ManagedThreadId);
        }
    }
}

　　2.3、取消任務 CancellationTokenSource

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        private static int TaskMethod(string name, int seconds, CancellationToken token)
        {
            Console.WriteLine("Task {0} is running on a thread id {1}. Is thread pool thread: {2}",
                name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
            for (int i = 0; i < seconds; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(1));
                if (token.IsCancellationRequested) return -1;
            }
            return 42 * seconds;
        }

        private static void Main(string[] args)
        {
            var cts = new CancellationTokenSource();
            var longTask = new Task<int>(() => TaskMethod("Task 1", 10, cts.Token), cts.Token);
            Console.WriteLine(longTask.Status);
            cts.Cancel();
            Console.WriteLine(longTask.Status);
            Console.WriteLine("First task has been cancelled before execution");
            cts = new CancellationTokenSource();
            longTask = new Task<int>(() => TaskMethod("Task 2", 10, cts.Token), cts.Token);
            longTask.Start();
            for (int i = 0; i < 5; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(0.5));
                Console.WriteLine(longTask.Status);
            }
            cts.Cancel();
            for (int i = 0; i < 5; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(0.5));
                Console.WriteLine(longTask.Status);
            }

            Console.WriteLine("A task has been completed with result {0}.", longTask.Result);
        }
    }
}

　　2.4、處理任務中的異常

　　單個任務:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static int TaskMethod(string name, int seconds)
        {
            Console.WriteLine("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));
            throw new Exception("Boom!");
            return 42 * seconds;
        }

        static void Main(string[] args)
        {
            try
            {
                Task<int> task = Task.Run(() => TaskMethod("Task 2", 2));
                int result = task.GetAwaiter().GetResult();
                Console.WriteLine("Result: {0}", result);
            }
            catch (Exception ex)
            {
                Console.WriteLine("Task 2 Exception caught: {0}", ex.Message);
            }
            Console.WriteLine("----------------------------------------------");
            Console.WriteLine();
        }
    }
}

　　多個任務:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static int TaskMethod(string name, int seconds)
        {
            Console.WriteLine("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));
            throw new Exception(string.Format("Task {0} Boom!", name));
            return 42 * seconds;
        }


        public static void Main(string[] args)
        {
            try
            {
                var t1 = new Task<int>(() => TaskMethod("Task 3", 3));
                var t2 = new Task<int>(() => TaskMethod("Task 4", 2));
                var complexTask = Task.WhenAll(t1, t2);
                var exceptionHandler = complexTask.ContinueWith(t =>
                        Console.WriteLine("Result: {0}", t.Result),
                        TaskContinuationOptions.OnlyOnFaulted
                    );
                t1.Start();
                t2.Start();
                Task.WaitAll(t1, t2);
            }
            catch (AggregateException ex)
            {
                ex.Handle(exception =>
                {
                    Console.WriteLine(exception.Message);
                    return true;
                });
            }
        }
    }
}

async/await的方式:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static async Task ThrowNotImplementedExceptionAsync()
        {
            throw new NotImplementedException();
        }

        static async Task ThrowInvalidOperationExceptionAsync()
        {
            throw new InvalidOperationException();
        }

        static async Task Normal()
        {
            await Fun();
        }

        static Task Fun()
        {
            return Task.Run(() =>
            {
                for (int i = 1; i <= 10; i++)
                {
                    Console.WriteLine("i={0}", i);
                    Thread.Sleep(200);
                }
            });
        }

        static async Task ObserveOneExceptionAsync()
        {
            var task1 = ThrowNotImplementedExceptionAsync();
            var task2 = ThrowInvalidOperationExceptionAsync();
            var task3 = Normal();


            try
            {
                //異步的方式
                Task allTasks = Task.WhenAll(task1, task2, task3);
                await allTasks;
                //同步的方式
                //Task.WaitAll(task1, task2, task3);
            }
            catch (NotImplementedException ex)
            {
                Console.WriteLine("task1 任務報錯!");
            }
            catch (InvalidOperationException ex)
            {
                Console.WriteLine("task2 任務報錯!");
            }
            catch (Exception ex)
            {
                Console.WriteLine("任務報錯!");
            }

        }

        public static void Main()
        {
            Task task = ObserveOneExceptionAsync();
            Console.WriteLine("主線程繼續運行........");
            task.Wait();
        }
    }
}

　　2.5、Task.FromResult的應用

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

namespace ConsoleApp1
{
    class Program
    {
        static IDictionary<string, string> cache = new Dictionary<string, string>()
        {
            {"0001","A"},
            {"0002","B"},
            {"0003","C"},
            {"0004","D"},
            {"0005","E"},
            {"0006","F"},
        };

        public static void Main()
        {
            Task<string> task = GetValueFromCache("0006");
            Console.WriteLine("主程序繼續執行。。。。");
            string result = task.Result;
            Console.WriteLine("result={0}", result);

        }

        private static Task<string> GetValueFromCache(string key)
        {
            Console.WriteLine("GetValueFromCache開始執行。。。。");
            string result = string.Empty;
            //Task.Delay(5000);
            Thread.Sleep(5000);
            Console.WriteLine("GetValueFromCache繼續執行。。。。");
            if (cache.TryGetValue(key, out result))
            {
                return Task.FromResult(result);
            }
            return Task.FromResult("");
        }

    }
}

　　2.6、使用IProgress實現異步編程的進程通知

　　IProgress<in T>只提供了一個方法void Report(T value)，通過Report方法把一個T類型的值報告給IProgress,然後IProgress<in T>的實現類Progress<in T>的構造函數接收類型爲Action<T>的形參，通過這個委託讓進度顯示在UI界面中。

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void DoProcessing(IProgress<int> progress)
        {
            for (int i = 0; i <= 100; ++i)
            {
                Thread.Sleep(100);
                if (progress != null)
                {
                    progress.Report(i);
                }
            }
        }

        static async Task Display()
        {
            //當前線程
            var progress = new Progress<int>(percent =>
            {
                Console.Clear();
                Console.Write("{0}%", percent);
            });
            //線程池線程
            await Task.Run(() => DoProcessing(progress));
            Console.WriteLine("");
            Console.WriteLine("結束");
        }

        public static void Main()
        {
            Task task = Display();
            task.Wait();
        }
    }
}

1、Task的優勢

2、Task的用法

2.1、創建任務

2.2、組合任務.ContinueWith

2.3、取消任務 CancellationTokenSource

2.4、處理任務中的異常

2.5、Task.FromResult的應用

2.6、使用IProgress實現異步編程的進程通知

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