Question

I have a question about threading.

I have a WPF application, that should show the status of some queries. For this I have created a class, that runs multiple queries in multiple thread (backgroundworker). I’ve also created an event, so I can react while the queries are running.

    private void cmdStart(object sender, RoutedEventArgs e)
    {
        QuerySetup qs = new QuerySetup();
        //Filling qs with some data
        //...
        SqlProxy sql = new SqlProxy();
        sql.QueryCompleted += new SqlProxy.QueryCompletedEventHandler(sql_QueryCompleted);

        //Starts the backgroundworker
        sql.RunSQL(qs);
    }

    private void sql_QueryCompleted(QueryResult qr)
    {
        lstStatus.Items.Clear();
        lstStatus.Items.Add(qr.RuntimeTotal);
        //... and some more...
    }

Now I get an error message that tells me, that I can’t access lstStatus because it’s owned by a different thread.

But why? Shouldn’t the event be in the same thread as the GUI? How can I solve my problem?

Answer 1

No.
Most non-UI components will raise callbacks in non-UI threads (typcially ThreadPool threads)

You can call Dispactcher.BeginInvoke to run code on the UI thread:

Dispactcher.BeginInvoke(new Action(delegate {
    ...
}));