class StaCollection<T> : Engine
{
    List<T> payload = new List<T>();
    public async Task Add(T t)
    {
        await RedirectToSelf(); // ru.stackoverflow.com/a/458425/10105
        payload.Add(t);
        FireAdded(t);
    }
}

public async IReadOnlyCollection<T> GetSnapshot()
{
    await RedirectToSelf();
    return payload.Clone().AsReadOnly();
}

private async void View_FormClosing(object sender, FormClosingEventArgs e)
{
	// Сохраняем данные
	await appContext.SaveChangesAsync();
	appContext.Dispose();
}

if (_closeReason == CloseReason.NativeClose)
{
    e.Cancel = true;
    await Task.Yield(); // дожидаемся следующей итерации
    _vm.Cancel();
}

public async Task<IReadOnlyCollection<T>> GetInitialContentAndSubscribe(ItemAddedHandler h)
{
    await RedirectToSelf();
    added += h;
    return payload.ToList().AsEnumerable();
}

public async Task<IReadOnlyCollection<T>> GetInitialContentAndSubscribe(ItemAddedHandler hadded = null, ItemRemovedHandler hremoved = null, ItemChangedHandler hchanged = null)
{
    await RedirectToSelf();
    if (hadded != null)
        added += hadded;
    if (hremoved != null)
        removed += hremoved;
      if (hchanged != null)
        changed += hchanged;
    return payload.ToList().AsEnumerable();
}