Inject ViewModel with data throws Exception

Question

Hello,

this I got: A first chance exception of type 'System.NullReferenceException' occurred in PresentationFramework.dll

When I use a parameter for the constructor of my LessonPlannerViewModel class.

I use a datatemplateselector class to switch between weekly/daily view.

public class ApplicationNavigationTemplateSelector : DataTemplateSelector
{
    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        if (item is LessonPlannerViewModel)
        {
            var vm = item as LessonPlannerViewModel;
            Window window = Application.Current.MainWindow;                

            if (vm.IsDailyView)
                return window.FindResource("dailyViewTemplate") as DataTemplate;
            else
                return window.FindResource("weeklyViewTemplate") as DataTemplate;
        }
        return base.SelectTemplate(item, container);
    } 
}

public LessonPlannerViewModel(DateTime asOfDate)
    {
        _asOfDate = asOfDate;

        if(_isDailyView) 
            LoadDailyData();
        if(_isWeeklyView)
            LoadWeeklyData();

...

Is that not allowed? Without the parameter I get no exception...

What do I wrong?

EDIT: NOw I changed the parameter to an integer and got better message ;P

XamlParseException=> 'No matching constructor found on type 'TBM.ViewModel.LessonPlannerViewModel'. You can use the Arguments or FactoryMethod directives to construct this type.' Line number '13' and line position '10'.

ok this is now understandable, that

<UserControl.Resources>
    <ViewModel:LessonPlannerViewModel x:Key="LessonPlannerViewModelID" />
</UserControl.Resources>

has no parameter.

So what to do now?

Answer 1

You could use a mediator pattern to send the data to the viewmodel after its initialization. (Using a viewmodel constructor without any parameters)

I use the MVVM Light ToolKit of Laurent Bugnion. A really nice, light framework for mvvm. This includes a mediator called Messenger

Otherwise you will find many sources for the mediator pattern: Google Search mvvm mediator