using System;

using System.Threading;

using System.Threading.Tasks;

​

public class Program

{

    private static async Task Main()

    {

        SynchronizationContext.SetSynchronizationContext(new CustomSyncContext());

        var t1 = FunAsync();

        var t2 = FunAsync();

        await Task.WhenAll(t1, t2);

        Console.WriteLine("end");       

    }

    static async Task FunAsync()

    {

        SynchronizationContext old = SynchronizationContext.Current;

        SynchronizationContext.SetSynchronizationContext(null);

        try

        {

            await Task.Delay(300);

        }

        finally

        {

            SynchronizationContext.SetSynchronizationContext(old);

        }

    }

}

​

internal sealed class CustomSyncContext : SynchronizationContext

{

    public override void Post(SendOrPostCallback d, object state)

    {

        Console.WriteLine("in Post");

        base.Post(d, state);

    }

}