爲什麼AsyncLocal <T>傳播到ThreadLocal <T>沒有的子線程？

Question

我不得不在我的代碼中用AsyncLocal替換ThreadLocal的使用，以便在等待異步操作時維持'環境狀態'。

但是，AsyncLocal令人討厭的行爲是它'流'到子線程。這與ThreadLocal不同。無論如何要防止這一點？

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存儲在執行上下文中的數據，它是由大多數API（包括Task.Run）自動飛行。防止

一種方法是在需要時明確抑制流量：

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