为什么AsyncLocal <T>传播到ThreadLocal <T>没有的子线程？

Question

我不得不在我的代码中用AsyncLocal替换ThreadLocal的使用，以便在等待异步操作时维持'环境状态'。

但是，AsyncLocal令人讨厌的行为是它'流'到子线程。这与ThreadLocal不同。无论如何要防止这一点？

class Program 
{ 
    private static readonly ThreadLocal<string> ThreadState = new ThreadLocal<string>(); 
    private static readonly AsyncLocal<string> AsyncState = new AsyncLocal<string>(); 

    static async Task MainAsync() 
    { 
     ThreadState.Value = "thread"; 
     AsyncState.Value = "async"; 

     await Task.Yield(); 

     WriteLine("After await: " + ThreadState.Value); 
     WriteLine("After await: " + AsyncState.Value); // <- I want this behaviour 

     Task.Run(() => WriteLine("Inside child task: " + ThreadState.Value)).Wait(); // <- I want this behaviour 
     Task.Run(() => WriteLine("Inside child task: " + AsyncState.Value)).Wait(); 

Answer 1

AsyncLocal存储在执行上下文中的数据，它是由大多数API（包括Task.Run）自动飞行。防止

一种方法是在需要时明确抑制流量：

using (ExecutionContext.SuppressFlow()) 
{ 
    Task.Run(...); 
}