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));
            }
        }
    });
}

you sure that's working? your closing brackets seem to be out of sync ... — Noctis
using Task.Run to remove an item from a collection (assuming the collection is of reasonable size) is redundant. — Yuval Itzchakov
@YuvalItzchakov No, it's not. The OP is doing that to make sure the removal happens in the right synchronisation context. Calling Remove directly would do it in the background thread, and would cause any events raised by it to run in the background thread as well. Now, what the OP has in the question isn't entirely the right way to do it, hence the error message, but the way you're suggesting isn't either. — user743382
@hvd The OP is currently using a non-existing overload. If his intention was to invoke Remove on the UI thread, consider my comment void :) — Yuval Itzchakov

Yuval Itzchakov Yuval Itzchakov · Accepted Answer · 2015-06-21T08:01:25

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);

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

2 Answers