Question

I have a wpf xbap application that downloads some data in background. There is a need to stop downloading and flush cache on application exit. Now it is implemented as follows :
App.xaml :

<Application Exit="ApplicationExit">

App.xaml.cs :

private void ApplicationExit(object sender, ExitEventArgs e)
{
    BackgroundImageLoader.Instance.Stop(); // target
    ViewModelLocator.Cleanup();
    FileCacheHelpers.FlushTemporaryFolder();
}

BackgroundImageLoader.cs :

// Thread-safe singleton
public sealed class BackgroundImageLoader
{
        private static volatile BackgroundImageLoader _instance;
        private static readonly object SyncRoot = new object();
        private BackgroundImageLoader() { }
        public static BackgroundImageLoader Instance
        {
            get
            {
                if (_instance == null)
                {
                    lock (SyncRoot)
                    {
                        if (_instance == null)
                            _instance = new BackgroundImageLoader();
                    }
                }
                return _instance;
            }
        }
        // other properties, omitted for conciseness
        private Task loaderTask;
        private CancellationTokenSource ts;
        public void Start()
        {
            ts = new CancellationTokenSource();
            var ct = ts.Token;
            loaderTask = Task.Factory.StartNew(() =>
            {
                 // TaskList is an array of tasks that cannot be canceled 
                 // if they were executed, so i want to actually cancel the 
                 // whole pool after last non-canceled operation was ended
                 foreach (var task in TasksList)
                 {
                      if (ct.IsCancellationRequested)
                      {
                           Debug.WriteLine("[BackgroundImageLoader] Cancel requested");
                           // wait for last operation to finish
                           Thread.Sleep(3000);
                           FileCacheHelpers.FlushTemporaryFolder();
                           break;
                      }
                      task.DoSomeHeavyWorkHere();                             
                 }
            }, ct);
        }

        public void Stop()
        {
            if (loaderTask != null)
            {
                if (loaderTask.Status == TaskStatus.Running)
                {
                    ts.Cancel();        
                }
            }
        }
    }

Stop is called, loaderTask is not null and Status is Running, but after ts.Cancel() IsCancellationRequested property is not changed (even after wrapping the whole task inside while(true){..} as described here). Loading stops, but i guess it happens only thanks to automatic GC. My flush method is not executed. What am i missing?
p.s. Also i need to refactor the functionality to run tasks in separate threads simultaniously, but afraid of side effects. Any help appreciated. Thanks in advance.

Answer 1

Added few manual flags which I set and check during processing and call to Stop() at the beginning of Start() so behaviour is acceptable.