Question

Why I can’t resolve ContentControl ContentTemplateSelector on designtime?
(On run time it works)
Designer (VS2010) shows an exception:

System.Reflection.TargetInvocationException
Exception has been thrown by the
target of an invocation.

and

System.NullReferenceException Object
reference not set to an instance of an
object.

XAML:

<Window.Resources>
    <DataTemplate x:Key="Temp1">
       <TextBox TextWrapping="Wrap" Text="1" Height="20" Width="Auto"/>
    </DataTemplate>

    <local:TemplateSelector x:Key="mySelector"/>
    <Grid>
       <ContentControl ContentTemplateSelector="{StaticResource mySelector}">
          <ContentControl.Content>
               1
          </ContentControl.Content>
       </ContentControl>      
    </Grid>
 </Window.Resources>

C#:

    public class TemplateSelector : DataTemplateSelector
    {
        public override DataTemplate SelectTemplate(object item, DependencyObject container)
        {
             //int num = int.Parse(item.ToString());
             Window win = Application.Current.MainWindow;
             return win.FindResource("Temp1") as DataTemplate;//load template based on num...
         }

    }

Answer 1

H.B. is right about Application.Current.MainWindow being null at design time. Here’s a better way to retrieve the resource by name:

public override DataTemplate SelectTemplate( object item, DependencyObject container ) {
    var element = container as FrameworkElement;
    if ( element != null ) {
        var template = element.TryFindResource( "Temp1" ) as DataTemplate;
        if ( template != null ) {
            return template;
        }
    }
    return base.SelectTemplate( item, container );
}

Other parts of your code are still incomplete, though, so I hope you’re just in the middle of it. Your DataTemplate should have bindings, for example, rather than hard-coded values.