Why is the SelectTemplate Method run 2 times in debug mode?

Question

debuging this class the SelectTemplate Method is run 2 times, but why?

The first time the item is always null.

public class PersonDataTemplateSelector : DataTemplateSelector
{
    public override DataTemplate SelectTemplate(object item,DependencyObject container)
    {
        if (item is Person)
        {
            Person person = item as Person;

            Window window = Application.Current.MainWindow;

            if (System.ComponentModel.DesignerProperties.GetIsInDesignMode( window))
                return null;

            if (person.Gender == "male")               
                return window.FindResource("boysViewTemplate") as DataTemplate;
            else                
                return window.FindResource("girlsViewTemplate") as DataTemplate;

        }
        return null;
    }
}

Mousse

Answer 1

You can set a break point and check the stack trace to verify but I believe it's called once with a null input when the visual tree is set up and the second time is when the bindings are actually populated.

Phanerozoic

Answer 2

If your selector were to provide a look for "Empty" or "Loading", the first call is what gives your selector the opportunity to provide that template while the elements are loading.

Cuzco

Hello MIke, do you have any info/links about your source about the empty/loading etc? I havent found anything. – Mousse