a.cs

static void Main(string[] args)
{
    Thing current = new Thing(1);

    var first = Task.Run(()=>DoStuffTo(current));
    var second = Task.Delay(1000).ContinueWith(t => current = null);
    
    Task.WaitAll(first, second);
    
    Console.ReadLine();
}

static void DoStuffTo(in Thing thing)
{
    Console.WriteLine($"Press enter to process {thing.Id}");
    
    Console.ReadLine();
    
    Console.WriteLine(thing.SayHello());
}