Question

Can anyone explain why, when I step through my unit tests with the debugger, that I get null references whenlooking at objects or properties. For example:

1      [TestMethod] 2            [Description('Test to confirm that upon initial class creation, the login view is loaded as the default content for the TaskRegion.')] 3            public void Shell_Initialisation_LoginViewIsLoadedByDefault() 4            { 5                Shell shell = new Shell(); 6     7                TestPanel.Children.Add(shell); 8     9                Shell_ViewModel viewModel = shell.DataContext as Shell_ViewModel; 10    11               Assert.IsTrue(viewModel.TaskRegionContent is ContentControl); 12    13               EnqueueTestComplete(); 14           } 

[Line 9] When I set my viewModel field to the DataContext of the shell view I get a ‘object not set to instance…’ exception. I know for sure that my datacontext is being set in my shell.xaml.cs; entire file:

1    using System.Windows; 2     3    namespace eg.WorkManager.UI.Shell 4    { 5        public partial class Shell 6        { 7     8            public Shell() 9            { 10               InitializeComponent(); 11               this.Loaded += new RoutedEventHandler(Shell_Loaded); 12           } 13    14           void Shell_Loaded(object sender, RoutedEventArgs e) 15           { 16               this.DataContext = new Shell_ViewModel(); 17           } 18       } 19   } 20    

I know I’m doing something wrong, but can anyone explain what?

Thanks, Mark

Answer 1

I am guessing that the problem is that you are instantiating the Shell object in isolation. Have you confirmed that Shell_Loaded (the Loaded event) is even being called?

Why are you not creating your view model as a static resource in your xaml? With MVVM, I usually create it as a static resource in the xaml and then bind it as the data context in the LayoutRoot… all in xaml.