I have 2 main threads to my program:

1. Thread checkForNewItemsThread Adds new objects to a volatile List<Object>
2. Thread workOnListThread Works on the List<Object> and creates another new Thread doWorkThread for each item in list that needs work.

I am running into an ArgumentNull Exception during my workOnListThread because (what I think is happening) either:

1. My checkForNewItemsThread is adding to the list while workOnListThread is iterating through the list.
2. My doWorkThreadis removing the item from the list once it is finished working on that object.

I believe the right hing to do here would be to put a lock on the List<Object> while workOnListThread is working on the list, however, I'm having a hard time determining the right place to place the lock on the List<Object>, I think this should be correct, however could use some guidance.

volatile List<Object> List1 = new List<Object>();
private static readonly object _lock = new object();

Thread checkForNewItemsThread;
Thread workOnListThread;
Thread doWorkThread;

public void OnTimerCheckNewItems(object sender, ElapsedEventArgs args)
    {
        List<Object> List1Temp = new List<Object>();
        List1Temp = getList();
        addNewItems(List1Temp);
    }
private void addNewItems(List<Object> list)
    {
        foreach (Object obj in list)
        {
            if (!List1.Any(o => o.ID == obj.ID)) //check if object already exists
            {
                List1.Add(obj);
            }
        }
    }
private void workOnList()
    {
        while (true) //loop forever
        {
            while (List1.Count != 0) //while items exist
            {
                lock (_lock) //lock while iterating through list
                {
                    try
                    {
                        for (int i = 0; i < List1.Count; i++)
                        {
                            if (List1[i].Status == Status.ready && List1[i] != null)
                            {
                                doWorkThread = new Thread(() => doWork(List1[i]));
                                doWorkThread.Start();
                                Thread.Sleep(3000); // wait for process to start
                            }
                        }
                    }
                    catch (Exception e)
                    {
                        Console.WriteLine(DateTime.Now.ToString(), " : ", e);
                        Log(e.ToString());
                    }
                }
            }
        }
    }
private void doWork(Object obj)
    {
        lock (_lock) //lock during update to status
        {
            int i = List.IndexOf(obj);
            List1[i].Status = Status.pending;
        }

        //work done here

        List1.Remove(alert);
    }