Question

I am aware that you need to use a Dispatcher to update items in the UI thread from a worker thread. To confirm my understanding, when you get the Dispatcher associated with current object is it always the UI dispatcher if my class inherits from the UserControl class? In which cases is it not the UI dispatcher?

Anyway, in the following code, I am creating a query and starting it asynchronously and when it completes, it sets the itemsource on one of my UI elements. I also am adding items to an observable collection that a UI element uses as its itemsource. When this is ran, it works fine and isn’t fussing at me to use a dispatcher and update the UI that way. Why is that?

private void UserControl_Loaded(object sender, RoutedEventArgs e)
{
    QueryTask queryTask = new QueryTask(URL);
    queryTask.ExecuteCompleted += new EventHandler<QueryEventArgs>(queryTask_ExecuteCompleted);
    queryTask.Failed += new EventHandler<TaskFailedEventArgs>(queryTask_Failed);
    Query query = new Query();
    query.Where = "Field <> 'XXX'";
    query.OutFields.Add("*");
    queryTask.ExecuteAsync(query);
    BuildingsOrganizationList.ItemsSource = organizationList;
}


void queryTask_ExecuteCompleted(object sender, QueryEventArgs e)
{
    FeatureSet featureSet = e.FeatureSet;
    foreach (KeyValuePair<string, string> columns in featureSet.FieldAliases)
    {
        TypeGrid.Columns.Add(new DataGridTextColumn()
        {
            Header = columns.Key,
            Binding = new System.Windows.Data.Binding("Attributes[" + columns.Key + "]"),
            CanUserSort = true
        });
    }
    TypeGrid.ItemsSource = featureSet.Features;
    TypeBusyIndicator.IsBusy = false;

    testing();
}

private void testing()
{
    List<string> temp = new List<string>();
    temp.Add("Item 1");
    temp.Add("Item 2");
    temp.Add("Item 3");

    foreach (string org in temp)
    {
        organizationList.Add(org);
    }
}

Answer 1

Because even though the processing is done asynchronously, you retrieve the result in your UI thread (an event is NOT thread), and update it from there.

If, however, you put the code inside queryTask_ExecuteCompleted in a Task:

Task.Factory.StartNew(() => 
{
    //code of queryTask_ExecuteCompleted here
});

You will get your exception.