BeginInvoke and array of classes

Question

I have a class setup as shown - I understand a longrunning task would be better, but this is the original state in a very trimmed down fasion. I have trimmed out anything that doesn't seem necessary to the problem.

TLDR:

1. Code runs in a loop and the listed conditions (1/2/3) can be executed independently
2. Logs show that condition3 only appears to execute when currentParentRun.ChildRuns.Count is > 0
3. The error is thrown and caught indicating (along with other logs) that in the BeginInvoke's called function, that currentParentRun.ChildRuns[currentParentRun.ChildRuns.Count - 1] with a >0 count is somehow out of range.

    // Class created and owned by UI
    public class DataProcessor
    {
        private BackgroundWorker worker = new BackgroundWorker();

        private ParentRun currentParentRun = null;

        public DataProcessor()
        {
            // register background worker events
            worker.WorkerSupportsCancellation = false;
            worker.WorkerReportsProgress = false;
            worker.DoWork += worker_DoWork;
            worker.RunWorkerCompleted += worker_RunWorkerCompleted;

            worker.RunWorkerAsync();
        }

        private void worker_DoWork(object sender, DoWorkEventArgs e)
        {
            while (process)
            {
                try
                {
                    // Create a new parent
                    if (condition1)
                    {
                        currentParentRun = new ParentRun();
                    }

                    // Create a new child (parent will always be not null here)
                    if (condition2)
                    {
                        ChildRun childRun = new ChildRun();

                        currentParentRun.ChildRuns.Add(childRun);
                    }
                    
                    // Call the UI and update
                    if (condition3)
                    {
                        ShowFinishedChildUI();
                    }
                    
                    System.Threading.Thread.Sleep(loopDelayProcessor);
                }
                catch (Exception ex)
                {
                }
            }
        }
        
        public void ShowFinishedChildUI()
        {
            System.Windows.Application.Current.Dispatcher.BeginInvoke((Action)(() => ShowFinishedChildUIDelegate()));
        }

        public void ShowFinishedChildUIDelegate()
        {
            // Parent is the UI being updated
            parent.ShowFinishedChild(currentParentRun.ParentRunID, 
                currentParentRun.ChildRuns[currentParentRun.ChildRuns.Count - 1].ChildRunID);

            resetParentDisplay = false;
        }
    }

Here, "parent" is the owning UI control and we use BeginInvoke to send call a UI function to update.

What's occuring every 20-30k new "parent" creations is occasionally, while in the middle of processing, there is an error in the ShowFinishedChildUIDelegate function of Index was out of range. Must be non-negative and less than the size of the collection. As far as I can verify, it is always at least non-zero, which leads me to think that somewhere between in the same line of code, the number of children changes between

currentParentRun.ChildRuns[...]

and

currentParentRun.ChildRuns.Count - 1

Is it possible that if condition2 in the main code block is fulfilled while the delegate is processing, that the same line of code can have different values, where the count of the children is greater than the number of classes in the list? That's my best guess, but I'm not sure how I can verify that.

Thanks.

More detailed error:

2021-03-19 12:00:12.8174|ERROR|GeneralLogger|UnhandledException StackTrace :    at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource)
   at System.Collections.Generic.List`1.get_Item(Int32 index)
   at QASystem.Classes.DataProcessor.ShowFinishedChildUIDelegate() in D:\..\QASystem\QASystem\Classes\DataProcessor.cs:line 756
   at QASystem.Classes.DataProcessor.<ShowFinishedChildUI>b__76_0() in D:\..\QASystem\QASystem\Classes\DataProcessor.cs:line 751
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
   at MS.Internal.CulturePreservingExecutionContext.CallbackWrapper(Object obj)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.ProcessQueue()
   at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Application.RunDispatcher(Object ignore)
   at System.Windows.Application.RunInternal(Window window)
   at System.Windows.Application.Run(Window window)
   at System.Windows.Application.Run()
   at QASystem.App.Main()

Answer 1

A likely guess is that the code is not thread safe. The example contains no synchronization at all. For example, the code could be run in the following order:

1. Thread A: var count = currentParentRun.ChildRuns.Count - 1;
2. Thread B: currentParentRun = new ParentRun();
3. Thread A: currentParentRun.ChildRuns[count];

As you can see this can result in incorrect, thread-unsafe behavior.

One way to solve this would be to insert locks to prevent modification while the UI is updated. Another way would be to create a copy of the data and give this copy to the UI, thus preventing any risk that the same data is accessed by multiple threads. I.e.

 var parentId = currentParentRun.ParentRunID; 
 var childId = currentParentRun.ChildRuns[currentParentRun.ChildRuns.Count - 1].ChildRunID;
 Dispatcher.BeginInvoke((Action)(() => ShowFinishedChildUIDelegate(parentId, childId)));