Async local gotchas

DisposableThing.cs

using (var thing = new DisposableThing())
{
    // Make the disposable object available ambiently
    DisposableThing.Current = thing;

    Dispatch();

    // We're about the dispose the object so make sure nobody else captures this instance
    DisposableThing.Current = null;
}

void Dispatch()
{
    // Task.Run will capture the current execution context (which means async locals are captured in the callback)
    _ = Task.Run(async () =>
    {
        // Delay for a second then log
        await Task.Delay(1000);

        Log();
    });
}

void Log()
{
    try
    {
        // Get the current value and make sure it's not null before reading the value
        var thing = DisposableThing.Current;
        if (thing is not null)
        {
            Console.WriteLine($"Logging ambient value {thing.Value}");
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex);
    }
}

Console.ReadLine();


class DisposableThing : IDisposable
{
    private static AsyncLocal<DisposableThing?> _current { get; } = new();

    private bool _disposed;

    public static DisposableThing? Current
    {
        get => _current.Value;
        set
        {
            _current.Value = value;
        }
    }

    public int Value
    {
        get
        {
            if (_disposed) throw new ObjectDisposedException(GetType().FullName);
            return 1;
        }
    }

    public void Dispose()
    {
        _disposed = true;
    }
}

NonThreadSafeThing.cs

AmbientValues.Current = new();

Parallel.For(0, 10, i =>
{
    AmbientValues.Current[i.ToString()] = "processing";
    Log();
    AmbientValues.Current[i.ToString()] = "done";
});

void Log()
{
    foreach (var pair in AmbientValues.Current)
    {
        Console.WriteLine(pair);
    }
}

class AmbientValues
{
    private static AsyncLocal<Dictionary<string, string>> _current { get; } = new();

    public static Dictionary<string, string> Current
    {
        get => _current.Value!;
        set => _current.Value = value;
    }
}