Cannot implicitly convert type 'bool' to 'System.Threading.Tasks.Task'

Question

This method is called from wpf main thread. Throwing - Cannot implicitly convert type 'bool' to 'System.Threading.Tasks.Task' error. What am I doing wrong? DocCollection is of type ObservableCollection.

Task TaskProcesQueue(SynchronizationContext _SyncContext)
{
    return Task.Run(() =>
    {
        if (DocCollection != null)
        {
            foreach (var item in DocCollection.ToList())
            {
                ProcessCurrentDocument(item);
                var t = Task.Run(() => DocCollection.Remove(item), _SyncContext));
            }
        }
    });
}

Answer 1

Task.Run has no overload accepting a SynchronizationContext . Hence, when overload resolution kicks in, it fails to find the appropriate overload. If you remove the synchronization context, it compiles:

var t = Task.Run(() => DocCollection.Remove(item));

Note i'd advise you not to use a dedicated threadpool thread to remove an item from a collection. That seems redundant. Instead, let the thread-pool thread already dedicated to removing items to it's job:

while (DocCollection.Count > 0)
{
    ProcessCurrentDocument(item);
    DocCollection.Remove(item);
}

Edit:

If you want to post on the SynchronizationContext :

_SyncContext.Post(_ => { DocCollection.Remove(item) }, null);

Answer 2

Now that your code is in sync, your problem is that DocCollection.Remove(item) is actually returning true/false, being a boolean method that indicates whether or not the remove succeeded or not.

If you'll remove that whole line, all will work nicely, and items will be processed (but not removed).

I think what you want is to actually use the Result of the running the task (note that it'll block though).

Have a look at the official page on MSDN for more.