AsnycLocal与ThreadLocal

AsnycLocal与ThreadLocal

AsnyncLocal与ThreadLocal都是存储线程上下文的变量，可是，在实际使用过程当中二者又有区别主要的表如今：

• AsyncLocal变量能够在父子线程中传递，建立子线程时父线程会将本身的AsyncLocal类型的上下文变量赋值到子线程中，可是，当子线程改变线程上下文中AsnycLocal变量值后，父线程不会同步改变。也就是说AsnycLocal变量只会影响他的子线程，不会影响他的父级线程。
• TreadLocal只是当前线程的上下文变量，不能在父子线程间同步。

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

namespace await_aysnc
{
    class Program
    {
        static ThreadLocal<int> ThreadObj = new ThreadLocal<int>();
        static AsyncLocal<int> AsyncObj = new AsyncLocal<int>();

        static void Main(string[] args)
        {
            AsyncObj.Value = 1;
            ThreadObj.Value = 1;
            Console.WriteLine($"Task执行前：AsyncObj= {AsyncObj.Value} ThreadObj= {ThreadObj.Value} ThreeadId = {Thread.CurrentThread.ManagedThreadId}");
            Task.Run(async() =>
            {
                Console.WriteLine($"RunAsync异步执行前：AsyncObj= {AsyncObj.Value} ThreadObj= {ThreadObj.Value} ThreeadId = {Thread.CurrentThread.ManagedThreadId}");
                await RunAsync();
                Console.WriteLine($"RunAsync异步执行后：AsyncObj = {AsyncObj.Value} ThreadObj= {ThreadObj.Value} ThreeadId = {Thread.CurrentThread.ManagedThreadId}");
            });
            Console.WriteLine($"Task执行后：AsyncObj= {AsyncObj.Value} ThreadObj= {ThreadObj.Value} ThreeadId = {Thread.CurrentThread.ManagedThreadId}");
            Console.Read();
        }

        static async Task RunAsync()
        {           
            Console.WriteLine($"Delay异步执行前：AsyncObj = {AsyncObj.Value} ThreadObj= {ThreadObj.Value} ThreeadId = {Thread.CurrentThread.ManagedThreadId} ");
            AsyncObj.Value = 2;
            ThreadObj.Value = 2;
            await Task.Delay(100);
            Console.WriteLine($"Delay异步执行后：AsyncObj = {AsyncObj.Value} ThreadObj= {ThreadObj.Value} ThreeadId = {Thread.CurrentThread.ManagedThreadId}");
        }
    }

}

Task执行前：AsyncObj= 1 ThreadObj= 1 ThreeadId = 1
Task执行后：AsyncObj= 1 ThreadObj= 1 ThreeadId = 1
RunAsync异步执行前：AsyncObj= 1 ThreadObj= 0 ThreeadId = 3
Delay异步执行前：AsyncObj = 1 ThreadObj= 0 ThreeadId = 3
Delay异步执行后：AsyncObj = 2 ThreadObj= 0 ThreeadId = 4
RunAsync异步执行后：AsyncObj = 1 ThreadObj= 0 ThreeadId = 4

从结果上能够看出一下结论：

• RunAsync执行前ThreadLocal值被切成0即ThreadLocal变量在新线程里面没有继承主线程的上下文变量，可是AsyncLocal继承了。
• Delay返回后，线程id变了，同时，TheadLocal中的变量为0。
• RunAsync结束后AyncLocal又切回主线程的上下文值，，一样Threadlocal中的值丢失。
• 因为await执行完返回以后，.net会在线程池中随机选取一个线程来执行await以后的逻辑，因此，await以后同时也会有必定概率获取到以前的线程，若是出现这种情形，TreadLocal会获取到以前的上下文，此时会出现意料以外的问题，从现象上看报错可能会是一个几率性问题。