gistfile1.cs

private List<Func<Task>> ShutdownCallbacks = new List<Func<Task>>();

public void RegisterShutdownCallback(Func<Task> callback)
{
    this.ShutdownCallbacks.Add(callback);
}

public async Task Shutdown()
{
    var callbackTasks = new List<Task>();
    foreach(var callback in this.ShutdownCallbacks)
    {
        callbackTasks.Add(callback());
    }

    await Task.WhenAll(callbackTasks);
}