C# - Task的各种用法和详解

1、Task简介【*所有的线程任务都会随着主线程的退出而退出】

ThreadPool相比Thread来说具备了很多优势，但是ThreadPool却又存在一些使用上的不方便。比如：
- ThreadPool不支持线程的取消、完成、失败通知等交互性操作；
- ThreadPool不支持线程执行的先后次序；
以往，如果开发者要实现上述功能，需要完成很多额外的工作，现在，FCL中提供了一个功能更强大的概念：Task。Task在线程池的基础上进行了优化，并提供了更多的API。在FCL4.0中，如果我们要编写多线程程序，Task显然已经优于传统的方式。
以下是一个简单的任务示例：

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    class Task1
    {
        public void TaskMethod1()
        {
            Task t = new Task(() =>
            {
                Console.WriteLine("任务开始...");
                Thread.Sleep(5000);
            });
            t.Start();
            t.ContinueWith((task) =>
            {
                Console.WriteLine("任务完成，完成时的状态为：");
                Console.WriteLine("IsCanceled={0}\tIsCompleted={1}\tIsFaulted={2}", task.IsCanceled, task.IsCompleted, task.IsFaulted);
            });
        }
    }
}

2、Task用法【new只是创建了一个任务，需要Start才会执行，Task.Run是直接开始执行】

2.1创建任务

不带返回参数的

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    class Task2
    {
        public void TaskMethod1(string param)
        {
            Console.WriteLine($"输入参数：{param}");
        }
 
        public void TaskMethod2()
        {
            //方式1
            var t1 = new Task(() => TaskMethod1("无返回值方式1.1"));
            var t2 = new Task(() => TaskMethod1("无返回值方式1.2"));
            t1.Start();
            t2.Start();
            Task.WaitAll(t1, t2);//会等待所有任务结束，主线程才会退出
 
            //方式2
            Task.Run(() => TaskMethod1("无返回值方式2"));
 
            //方式3
            Task.Factory.StartNew(() => TaskMethod1("无返回值方式3"));//异步方法
            //or
            Task t3 = Task.Factory.StartNew(() => TaskMethod1("无返回值方式3"));
            t3.Wait();
        }
    }
}

async/await的实现方式

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    class Task2
    {
        /// <summary>
        /// async/await的实现方式:
        /// </summary>
        public async void TaskMethod3()
        {
            //Task.Delay方法只会延缓异步方法中后续部分执行时间，当程序执行到await表达时，一方面会立即返回调用方法，执行调用方法中的剩余部分，这一部分程序的执行不会延长。另一方面根据Delay()方法中的参数，延时对异步方法中后续部分的执行。
            await Task.Delay(1000);
            Console.WriteLine("执行异步方法");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine(i);
            }
        }
    }
}

using System;
 
namespace ThreadStudy
{
    class Program
    {
        static void Main(string[] args)
        {
            //Task1 task1 = new Task1();
            //task1.TaskMethod1();
 
            Task2 task2 = new Task2();
            Console.WriteLine("主线程执行其他任务...");
            task2.TaskMethod3();
            Console.WriteLine("主线程执行其他处理...");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine("主线程{i}");
            }
 
            Console.ReadKey();
        }
    }
}

带返回值得方式

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    public class Task3
    {
        int TestMethod1()
        {
            Console.WriteLine("测试方法");
            Thread.Sleep(2000);
            return 12;
        }
 
        public Task<int> TaskMethod1()
        {
            return Task.Run(() => TestMethod1());//Task<int>.Run(() => TestMethod1()); 简化了<int>
        }
 
        public int TestMethod2()
        {
            int sum = 0;
            Console.WriteLine("执行异步操作");
            for (int i = 0; i < 100; i++)
            {
                sum += i;
            }
            Thread.Sleep(1000);
            return sum;
        }
    }
}

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    class Program
    {
        static void Main(string[] args)
        {
            //方式1
            Task3 task3 = new Task3();
            Task<int> task = task3.TaskMethod1();
            int result = task.Result;//方法执行完，主线程才会结束
            Console.WriteLine($"1 = {result}");
 
            //方式2
            task = task3.TaskMethod1();
            Console.WriteLine(task.Status);
            while (!task.IsCompleted)
            {
                Console.WriteLine(task.Status);
                Thread.Sleep(200);
            }
            Console.WriteLine(task.Status); 
            result = task.Result;
            Console.WriteLine($"2 = {result}");
 
            //方式3
            Task<int> task2 = Task.Run(() => task3.TestMethod2());
            Console.WriteLine("主线程执行其他操作");
            //task2.Wait();Result为必须执行完，主线程才结束，所以这里不写Wait不影响
            Console.WriteLine($"3 = {task2.Result}");
        }
    }
}

async/await方式

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    class Program
    {
        static void Main(string[] args)
        {
            var task = AsyncMethod1();
            Console.WriteLine("主线程执行其他处理");
            for (int i = 0; i < 3; i++)
            {
                Console.WriteLine($"Main{i}");
            }
            int result = task.Result;
            Console.WriteLine(result);
        }
 
        async static Task<int> AsyncMethod1()
        {
            await Task.Delay(1000);//立刻返回执行调用方法的后续部分，延迟1秒之后后面的部分
            int sum = 0;
            Console.WriteLine("使用Task执行异步操作.");
            for (int i = 0; i < 1000; i++)
            {
                sum += i;
            }
            return sum;
        }
    }
}

2.2组合任务ContinueWith

简单demo

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    class Program
    {
        static void Main(string[] args)
        {
            Task<int> task = new Task<int>(() =>
            {
                int sum = 0;
                Console.WriteLine("任务1");
                for (int i = 0; i < 100; i++)
                {
                    sum += i;
                }
                return sum;
            });
            task.Start();
            Console.WriteLine("主线程执行其他任务");
            Task task1 = task.ContinueWith(t =>
            {
                Thread.Sleep(1000);
                Console.WriteLine($"task = {t.Result}");
            });
            task1.Wait();//task.Wait();对t.Result不起作用，主线程会直接结束
        }
    }
}

任务的串行【没有研究】

using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    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并行执行
            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并行执行
            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执行完
            Task.WaitAll(t2, t3);
 
            //t7串行执行
            var t4 = Task.Factory.StartNew(() =>
            {
                Console.WriteLine("当前集合元素个数：{0},Thread id {1}", stack.Count, Thread.CurrentThread.ManagedThreadId);
            });
            t4.Wait();
        }
    }
}

子任务

using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    class Program
    {
        static void Main(string[] args)
        {
            Task<string[]> task = new Task<string[]>(state =>
            {
                Console.WriteLine("父任务开始");
                string[] result = new string[2];
                new Task(() => { result[0] = "子任务1"; }, TaskCreationOptions.AttachedToParent).Start();
                new Task(() => { result[1] = "子任务2"; }, TaskCreationOptions.AttachedToParent).Start();
                Thread.Sleep(1000);
                return result;
            }, "我是父任务，创建了子任务，等子任务执行完才会执行结束");
 
            task.ContinueWith(t =>
            {
                Array.ForEach(t.Result, r => Console.WriteLine(r));
            });
 
            task.Start();
            task.Wait();
 
        }
    }
}

动态并行(TaskCreationOptions.AttachedToParent) 父任务等待所有子任务完成后整个任务才算完成【没有研究过】

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    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("当前节点的值为{0};处理的ThreadId={1}", current.Text, Thread.CurrentThread.ManagedThreadId);
        }
    }
}

2.3取消任务 CancellationTokenSource【没研究明白】

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    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处理任务中的异常

单个任务

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    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();
        }
    }
}

多个任务

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    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方式

using System;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    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
            {
                //异步的方式
                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 任务报错!");
            }
            catch (InvalidOperationException ex)
            {
                Console.WriteLine("task2 任务报错!");
            }
            catch (Exception ex)
            {
                Console.WriteLine("任务报错!");
            }
 
        }
 
        public static void Main()
        {
            Task task = ObserveOneExceptionAsync();
            Console.WriteLine("主线程继续运行........");
            task.Wait();
        }
 
    }
}

2.5Task.FromResult的应用

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    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("主程序继续执行。。。。");
            string result = task.Result;
            Console.WriteLine("result={0}", result);
 
        }
 
        private static Task<string> GetValueFromCache(string key)
        {
            Console.WriteLine("GetValueFromCache开始执行。。。。");
            string result = string.Empty;
            //Task.Delay(5000);
            Thread.Sleep(5000);
            Console.WriteLine("GetValueFromCache继续执行。。。。");
            if (cache.TryGetValue(key, out result))
            {
                return Task.FromResult(result);
            }
            return Task.FromResult("");
        }
    }
}

2.6使用IProgress实现异步编程的进程通知【没有研究】

IProgress<in T>只提供了一个方法void Report(T value)，通过Report方法把一个T类型的值报告给IProgress,然后IProgress<in T>的实现类Progress<in T>的构造函数接收类型为Action<T>的形参，通过这个委托让进度显示在UI界面中。

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    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()
        {
            //当前线程
            var progress = new Progress<int>(percent =>
            {
                Console.Clear();
                Console.Write("{0}%", percent);
            });
            //线程池线程
            await Task.Run(() => DoProcessing(progress));
            Console.WriteLine("");
            Console.WriteLine("结束");
        }
 
        public static void Main()
        {
            Task task = Display();
            task.Wait();
        }
    }
}

2.7 C#Task返回值

Task返回值，目前有2种情况，一种是异步async返回值，一种是同步返回值

第一种：异步返回值
- Task方法如果加了async关键字，那么就是异步返回方法，如果是异步返回方法，需要返回一个值时，直接return value，就可以了。
第二种：同步返回值
- Task方法如果没有加async关键字，需要返回一个值时，使用Task.FromResult方法，Task.FromResult(value)就可以了。

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
 
namespace ThreadStudy
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(TaskMethod().Result);
            Console.WriteLine(TaskMethod2().Result);
        }
 
        async static Task<int> TaskMethod()
        {
            return 1;
        }
 
        static Task<int> TaskMethod2()
        {
            return Task.FromResult(2);
        }
    }
}