ComboBox drop down menu doesn't disapper immediately after selection changed

Hi, in Visual C# 2010 Express, WPF project, I have a ComboBox item that is populated with initial data (no binding involved).

To change the selected combobox item, I first pull down the combobox drop down menu and then choose a different item.

On combo box items selection changed event  DoSomeWork is called. Until this task is done, the combobox drop down menu stays 'open' and this is annoying.

I tried Refreshing the ComboBox and even the main Grid (that hosts the ComboBox) immediately on Selection Changed event but of no help.

The Refresh works on other controls like Label

private delegate void MyDelegate();
private static void Refresh(DependencyObject obj)
{
    obj.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Render, (MyDelegate)delegate { });
}

private void cbList_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    Refresh(cbList);
    Refresh(gridMain);

    DoSomeWork(); // takes several seconds
}

private void DoSomeWork()
{
    lblStatus.Content = "Running call1 ....";
    Refresh(lblStatus);

    call1();    // takes several seconds

    lblStatus.Content = "Finished call1";
    Refresh(lblStatus);
}

Thanks,

-srinivas yelamanchili

sri