asyncmain.cs

using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;

static class AsyncMainHelper
{
    class Ctx : SynchronizationContext
    {
        BlockingCollection<Action> _callbacks = new BlockingCollection<Action>();
        Thread _mainThread = Thread.CurrentThread;
        public override void Send(SendOrPostCallback d, object state)
        {
            if (Thread.CurrentThread == _mainThread)
                d(state);
            else
            {
                var tcs = new TaskCompletionSource<int>();
                _callbacks.Add(() =>
                {
                    try
                    {
                        d(state);
                        tcs.SetResult(0);
                    }
                    catch (Exception e)
                    {
                        tcs.SetException(e);
                    }
                });
                tcs.Task.Wait();
            }
            
        }

        public override void Post(SendOrPostCallback d, object state)
        {
            _callbacks.Add(() => d(state));
        }

        public void DoJobs() => _callbacks.Take()();
    }

    public static void Run(Func<Task> asyncMain)
    {
        var ctx = new Ctx();
        SynchronizationContext.SetSynchronizationContext(ctx);
        var task = asyncMain();
        while (!task.IsCompleted)
            ctx.DoJobs();
    }
}


public static class Program
{
    public static void Main()
    {
        Console.WriteLine("Actual main thread id " + Thread.CurrentThread.ManagedThreadId);
        AsyncMainHelper.Run(AsyncMain);
    }

    private static async Task AsyncMain()
    {
        Console.WriteLine("Initial thread id " + Thread.CurrentThread.ManagedThreadId);
        await Task.Delay(100);
        Console.WriteLine("Thread id after await " + Thread.CurrentThread.ManagedThreadId);
    }
}