using System;

using System.Threading;

using System.Threading.Tasks;

using System.Runtime.CompilerServices;

public class Program

{

    public static async Task Main()

    {

        await Task.Run(async () => 

            {

                Console.WriteLine("Async flow entered...");             

                // Init async value

                if (Cache.Instance.Item.Value is {})

                    throw new InvalidOperationException("The async flow has just startet. A value should not be initialized.");

                var newValue = new object();

                Console.WriteLine($"Create: value = #{RuntimeHelpers.GetHashCode(newValue)}");

                Cache.Instance.Item.Value = newValue;

                await Foo();

                Console.WriteLine("Async flow exitted.");

            });

        Console.WriteLine("Main finished.\n\n");

    }

    private static async Task Foo()

    {

        Console.WriteLine($"Foo: entered...");

        await Task.Delay(5);

        Console.WriteLine($"Foo: getting value...");

        var knownValue = Cache.Instance.Item.Value;

        Console.WriteLine($"Foo: value = #{RuntimeHelpers.GetHashCode(knownValue)}");

        Console.WriteLine($"Foo: exitted.");

    }

}

​

public sealed class Cache

{

    public static Cache Instance = new Cache();

    public AsyncLocal<object> Item { get; } = new AsyncLocal<object>(OnValueChanged);

    private static void OnValueChanged(AsyncLocalValueChangedArgs<object> args)

    {

        Console.WriteLine($"OnValueChanged! Prev: #{RuntimeHelpers.GetHashCode(args.PreviousValue)} Current: #{RuntimeHelpers.GetHashCode(args.CurrentValue)}");

    }

}