xUnit test to count list items in service

Question

i'm new in Unit testing. I have to create xUnit test for DataService, which is count _allEntity items after some seconds. I did something like this, but it's not works, because it returns 0. I think moqcked DataService not excetiong at all) Test:

[Fact]
public async Task TestListCountAfterDelay()
{
    // Arrange
    var expectedCount = 3;
    var mockSystemClock = new Mock<ISystemClock>();
    var mockDataRepository = new Mock<IDataRepository>();

    var dataService = new DataService(mockSystemClock.Object, mockDataRepository.Object);
    dataService.StartEntityTask();


    // Wait for 6 seconds
    await Task.Delay(TimeSpan.FromSeconds(6));

    // Act
    dataService.StopEntityTask();
    // Assert
    Assert.Equal(expectedCount, dataService.Allentity.Count);
}
    

Service:

    public class DataService : IDataService
    {
        private readonly ISystemClock _systemClock;
        private readonly IDataRepository _dataRepository;
        private CancellationTokenSource cancellationTokenentity;
        private IList<DataEntity> _allEntity;
        public IList<DataEntity> Allentity => _allEntity;
        private readonly string[] entityArray = { "Entity1", "Entity2", "Entity3", "Entity4" };
        private int currentobject = 0;

        
        private DateTimeOffset _insertentity;
        private const int object_INTERVAL = 2000;
 

        public DataService(ISystemClock systemClock, IDataRepository dataInsertRepository)
        {
            _dataRepository = dataInsertRepository;
            _systemClock = systemClock;
            cancellationTokenentity = new CancellationTokenSource();
            _allEntity = new List<DataEntity>();

        }


        private void Processentity(CancellationToken cancellationToken)
        {

                while (!cancellationToken.IsCancellationRequested)
                {
                    if (_insertentity.Add(TimeSpan.FromMilliseconds(object_INTERVAL)) <= _systemClock.UtcNow)
                    {
                        var object = new DataEntity { Name = entityArray[currentobject], DateTime = dateTime_ };                        
                        _dataRepository.WriteobjectData(object);
                        if (!_allEntity.Contains(object))
                        {
                            _allEntity.Add(object);
                        }
                        currentobject = (currentobject >= (entityArray.Length - 1)) ? 0 : ++currentobject;
                        _insertentity = _systemClock.UtcNow;
                    }
                }
            }
        }

    public void StartEntityTask()
    {
        cancellationTokenentity = new CancellationTokenSource();
        Task.Run(() => Processentity(cancellationTokenentity.Token));
    }

    public void StopEntityTask()
    {
        cancellationTokenentity.Cancel();
    }
}

Could you help me, change this test correctly. Thanks.