C# Task 与 SynchronizationContext
示例代码
using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;namespace Test
{internal class Program{static void Main(string[] args){_mainThreadSynchronizationContext = new ThreadSynchronizationContext("main");_threadSynchronizationContext = new ThreadSynchronizationContext("thread");if (SynchronizationContext.Current == null){Console.WriteLine("Main SynchronizationContext.Current is null");}SynchronizationContext.SetSynchronizationContext(_mainThreadSynchronizationContext);if (SynchronizationContext.Current != null){Console.WriteLine("Main SynchronizationContext.Current is not null!");}Thread thread = new Thread(ThreadLoop);thread.Start();MainLoop();}private static ThreadSynchronizationContext _mainThreadSynchronizationContext;private static ThreadSynchronizationContext _threadSynchronizationContext;private static void MainLoop(){MainTest();while (true){Thread.Sleep(1);_mainThreadSynchronizationContext.Update();}}private static void ThreadLoop(){if (SynchronizationContext.Current == null){Console.WriteLine("Thread SynchronizationContext.Current is null");}SynchronizationContext.SetSynchronizationContext(_threadSynchronizationContext);if (SynchronizationContext.Current != null){Console.WriteLine("Thread SynchronizationContext.Current is not null");}Thread.Sleep(10000);ThreadTest();while (true){Thread.Sleep(1);_threadSynchronizationContext.Update();}}private static async Task MainTest(){Console.WriteLine($"MainTest {Thread.CurrentThread.ManagedThreadId}");await TaskNew();Console.WriteLine($"MainTest End {Thread.CurrentThread.ManagedThreadId}");}private static async Task ThreadTest(){Console.WriteLine($"ThreadTest {Thread.CurrentThread.ManagedThreadId}");await TaskNew();Console.WriteLine($"ThreadTest End {Thread.CurrentThread.ManagedThreadId}");}private static Task TaskNew(){return Task.Factory.StartNew(() =>{Console.WriteLine($"TaskNew Begin {Thread.CurrentThread.ManagedThreadId}");Thread.Sleep(5000);Console.WriteLine($"TaskNew End {Thread.CurrentThread.ManagedThreadId}");});}public class ThreadSynchronizationContext : SynchronizationContext{// 线程同步队列,发送接收socket回调都放到该队列,由poll线程统一执行private readonly ConcurrentQueue<Action> queue = new();private Action a;private string name;public ThreadSynchronizationContext(string name){this.name = name;}public void Update(){while (true){if (!this.queue.TryDequeue(out a)){return;}try{a();}catch (Exception e){Console.WriteLine(e);}}}public override void Post(SendOrPostCallback callback, object state){this.Post(() => callback(state));}public void Post(Action action){Console.WriteLine($"Post: {name}");this.queue.Enqueue(action);}}}
}
输出结果
Thread SynchronizationContext.Current is null
Thread SynchronizationContext.Current is not null
MainTest 1
TaskNew Begin 4
TaskNew End 4
Post: main
MainTest End 1
ThreadTest 3
TaskNew Begin 5
TaskNew End 5
Post: thread
ThreadTest End 3SynchronizationContext
这个类的作用是将代码从一个线程转移到另外一个线程执行。
Send 表示同步执行代码快，会阻塞当前线程，直到Send执行完成。
Post 表示异步执行代码快，不阻塞当前线程。
SynchronizationContext.Current 与 SynchronizationContext.SetSynchronizationContext
这个静态属性与方法，是线程隔离的，也就是线程1 调用SynchronizationContext.SetSynchronizationContext。线程2 通过 SynchronizationContext.Current 是获取不到值的，从上面例子中也可以看出。
可以看到 SynchronizationContext 的源码，Send 方法是当前线程同步调用。Post 是从线程池中找一个线程进行调用。
SynchronizationContext.Current 这个值在不同环境中的主线程结果是不一样的。在控制台情况下默认是NULL,在WindowForm的情况下是 WindowsFormsSynchronizationContext。在Unity中是 UnitySynchronizationContext。
当通过await Task开启一个异步任务，当任务完成后会调用SynchronizationContext.Current.Post 方法。在例子中可以看出。如果SynchronizationContext.Current 是空，则会使用默认值SynchronizationContext对象。通过这个特性，重写SynchronizationContext 就可以让线程执行完成Task后，回调到原来线程进行执行，这个经常需要使用到。