Why isn't the object collected by GC in C# when assigned null and GC.Collect() is called, despite a background task referencing it?

Question

In the code below, the Starter object is set to null, and GC.Collect() is called, but the background task still runs and the object isn't collected:

namespace ConsoleApp5
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Starter s = new();
            s.Start();
            s = null;
            GC.Collect(0, GCCollectionMode.Forced);
            GC.Collect(1, GCCollectionMode.Forced);
            GC.Collect(2, GCCollectionMode.Forced);
            Console.ReadLine();
        }
    }
    public class Starter
    {
        int a = 0;
        public void Start()
        {
            Task.Run(() =>
            {
              
                while (true)
                {
                    a++;
                    Console.WriteLine(a);
                }
            });
        }
    }
}

Answer 1

Thread invocation methods (and thus their targets) count as a GC root, so the moment Start() is invoked, the object is rooted by the thread machinery. That thread is short-lived and only serves to create a thread-pool item - and almost certainly hasn't even started by the time GC.Collect() executes (and completes), but even if it had finished: the thread-pool queue and active workers are also rooted (via the thread-pool machinery). Since a is a field on the object, and the thread-pool callback touches that field: the thread-pool task can see the object, thus it is not collectable.

However, it is very likely that at the time GC.Collect() executes, the ThreadStart delegate is still in a pending queue waiting for the thread to be fully created and activated - but since the ThreadStart has a target of the Starter object you created: it is still reachable and thus not collectable.