c#多线程与async/await的结合

本文关键字：await 结合 async 多线程 | 更新日期: 2023-09-27 17:54:02

这又是一个与c#中多线程相关的问题。

然而，当多线程与async/await相结合时，似乎很有趣。

我有一个方法生成线程异步调用对象的方法。

但是，上下文不能被不同类的对象共享。

例如，考虑代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ThreadDataSharing
{
    class Data
    {
        public static ThreadLocal<string> Name = new ThreadLocal<string>();
    }
    class Locker { public static object lobj = new object { };}
    class Program2
    {
        static void Main(string[] args)
        {
            for (int i = 0; i < 3; i++)
            {
                var pObj = new Program();
                new Thread(new ThreadStart(pObj.MyThreadMethod)).Start();
            }
        }
    }
    class Program
    {
        ///my async method
        ///
        public async Task MyAsyncMethod()
        {
            lock (Locker.lobj)
            {
                Console.WriteLine("From async method");
                Console.WriteLine("thread: " + Thread.CurrentThread.ManagedThreadId);
                Console.WriteLine("id: " + Data.Name.Value);
            }
            await MyAsyncMethod2();
            await new AsyncClass().AsyncMethod();
        }
        public async Task MyAsyncMethod2()
        {
            lock (Locker.lobj)
            {
                Console.WriteLine("From async 2 method");
                Console.WriteLine("thread: " + Thread.CurrentThread.ManagedThreadId);
                Console.WriteLine("id: " + Data.Name.Value);
            }
            await Task.Run(() =>
            {
                lock (Locker.lobj)
                {
                    Console.WriteLine("From task run method");
                    Console.WriteLine("thread: " + Thread.CurrentThread.ManagedThreadId);
                    Console.WriteLine("id: " + Data.Name.Value);
                }
            });
        }
        public void MyThreadMethod()
        {
            lock (Locker.lobj)
            {
                Console.WriteLine("From thread method");
                Console.WriteLine("thread: " + Thread.CurrentThread.ManagedThreadId);
                var id = Guid.NewGuid();
                Data.Name.Value = id.ToString();
                Console.WriteLine("id: " + Data.Name.Value);
            }
            MyAsyncMethod().Wait();
        }
    }

    public class AsyncClass
    {
        public async Task AsyncMethod()
        {
            lock (Locker.lobj)
            {
                Console.WriteLine("From class async method");
                Console.WriteLine("thread: " + Thread.CurrentThread.ManagedThreadId);
                Console.WriteLine("id: " + Data.Name.Value);
            }
            var newId = Guid.NewGuid();
            Data.Name.Value = newId.ToString();
            await AsyncMethod2();
        }
        public async Task AsyncMethod2()
        {
            lock (Locker.lobj)
            {
                Console.WriteLine("From class async 2 method");
                Console.WriteLine("thread: " + Thread.CurrentThread.ManagedThreadId);
                Console.WriteLine("id: " + Data.Name.Value);
            }
            await Task.Run(() => { });
        }
    }
}

在我的机器上运行这段代码会产生以下输出

From thread method
thread: 3
id: 3a7ef8e3-ebe1-49e1-94bb-798358ac1567
From thread method
thread: 4
id: 341d3371-f905-4e1c-aac3-47bda16c8d88
From thread method
thread: 5
id: b7e79901-81b7-430f-b158-59f091e43a0c
From async method
thread: 3
id: 3a7ef8e3-ebe1-49e1-94bb-798358ac1567
From async method
thread: 5
id: b7e79901-81b7-430f-b158-59f091e43a0c
From async method
thread: 4
id: 341d3371-f905-4e1c-aac3-47bda16c8d88
From async 2 method
thread: 3
id: 3a7ef8e3-ebe1-49e1-94bb-798358ac1567
From async 2 method
thread: 4
id: 341d3371-f905-4e1c-aac3-47bda16c8d88
From async 2 method
thread: 5
id: b7e79901-81b7-430f-b158-59f091e43a0c
From task run method
thread: 6
id:
From task run method
thread: 8
id:
From task run method
thread: 7
id:
From class async method
thread: 7
id:
From class async method
thread: 6
id:
From class async method
thread: 8
id:
From class async 2 method
thread: 8
id: f52ed654-0e55-4906-bfc1-65c6b25a7785
From class async 2 method
thread: 7
id: 1e53e03b-a3a0-4296-8622-7716b45d1462
From class async 2 method
thread: 6
id: 1adca81d-b11a-4860-b37d-a017afe877b8

问题是，为什么它不与第二个类(AsyncClass)的实例共享ThreadLocal<T>对象?

c#多线程与async/await的结合

在一个带有await/async的控制台应用程序中，没有共享的同步上下文，可能这就是为什么它没有被共享的原因。

上下文不能被不同类的对象共享。

实际上，你看到的行为是(thread-local)上下文没有在线程之间共享。

我有一篇异步介绍博客文章，可能会澄清这种行为。基本概念是async方法不会在同一个线程上恢复;相反，它们在捕获的上下文上恢复(默认情况下)。在这种情况下，线程池上下文用于恢复async方法，这意味着这些延续只是被调度到线程池中，并且可以由任何线程池线程运行。

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