C#的AsnycLocal与ThreadLocal 浅析C#中的AsnycLocal与ThreadLocal
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AsnyncLocal与ThreadLocal都是存储线程上下文的变量,但是,在实际使用过程中两者又有区别主要的表现在:
- AsyncLocal变量可以在父子线程中传递,创建子线程时父线程会将自己的AsyncLocal类型的上下文变量赋值到子线程中,但是,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
从结果上可以看出一下结论:
- 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,但是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,,同样Threadlocal中的值丢失。
- 由于await执行完返回之后,.net会在线程池中随机选取一个线程来执行await之后的逻辑,所以,await之后同时也会有一定几率获取到之前的线程,如果出现这种情形,TreadLocal会获取到之前的上下文,此时会出现意料之外的问题,从现象上看报错可能会是一个概率性问题。
总结
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