Make AsyncLocal changes propagate to the calling function

Question

DotNet Fiddle link https://dotnetfiddle.net/GqA32R

I have the following sample code to demonstrate the async local functionality

static AsyncLocal<string> _asyncLocalString = new AsyncLocal<string>();
    public static async Task Main()
    {
        // _asyncLocalString.Value = "Value 1";
        var t1 = AsyncMethodA();
        var t2 = AsyncMethodB();
        await Task.WhenAny(t1, t2).ConfigureAwait(false);
        Console.WriteLine("Finished");
        Console.WriteLine("Async local inside Main: " + _asyncLocalString.Value); // null
    }

    static async Task AsyncMethodA()
    {
        // _asyncLocalString.Value = "Value 2";
        Console.WriteLine("Method A");
        Console.WriteLine("Async local inside A: " + _asyncLocalString.Value); // null
        await Task.Delay(200);
    }

static async Task AsyncMethodB()
    {
        _asyncLocalString.Value = "Value 3";
        Console.WriteLine("Method B");
        await AsyncMethodC().ConfigureAwait(false);
    }

    static async Task AsyncMethodC()
    {
        await Task.Delay(100);
        Console.WriteLine("Method C");
        Console.WriteLine("Async local inside C: " + _asyncLocalString.Value); // Value 3 is printed
    }

Output:

Method B
Method C
Async local inside C: Value 3
Method A
Async local inside A: 
Finished
Async local inside Main: 

Is there a way to propogate these changes up the call stack so that async local changes in B are visible in A and main?

My real world scenario is similar to what I have above - the asyncLocal is set only in methodB and we have a few statements up the call stack which logs that value differently.

Answer 1

Is there a way to propogate these changes up the call stack so that async local changes in B are visible in A and main?

No, not with AsyncLocal<T>. async methods set their value context to "copy-on-write", so if it's written to, a copy will be created. And the copies never "flow" upwards.

You will need to use some other mechanism to achieve whatever it is you're trying to do.