Question

In my Caliburn Bootstrapper, I am trying to validate that a user has permissions to run my application. If they do not, I need to present them with a message and exit. The following code throws a NullReferenceException on the MessageBox.Show() call. Even if I move the ComposeMef() call before the ValidateUserHasPermissionsToRun() in the Configure() method, it still errors.

public class MyBootstrapper : Bootstrapper<DropWindowViewModel>
{

   // irrelevant methods omitted for brevity

   protected override void Configure()
   {
      this.InitializeSecurity();
      this.ValidateUserHasPermissionToRun();
      this.ComposeMef();
   }

   private void ComposeMef()
   {
      AggregateCatalog catalog = new AggregateCatalog(AssemblySource.Instance.Select(x => new AssemblyCatalog(x)).OfType<ComposablePartCatalog>());

      this.container = new CompositionContainer(catalog);

      CompositionBatch batch = new CompositionBatch();

      batch.AddExportedValue<IWindowManager>(new WindowManager());
      batch.AddExportedValue<IEventAggregator>(new EventAggregator());
      batch.AddExportedValue(this.container);
      batch.AddExportedValue(catalog);

      this.container.Compose(batch);
   }      

   private void ValidateUserHasPermissionToRun()
   {
      User user = SecurityContext.Current.SecurityUser;

      if (!user.HasPrivilege(Constants.PrivilegeLoadData))
      {
         // throws an exception
         MessageBox.Show("You do not have access to VIPER. Please contact the help desk if you need help.");
         this.TerminateApplication();
      }
   }
}

What is the proper way to handle this? For something this simple, I just want to show a message box. I don’t really want to get into composing a whole new ViewModel/View. If I have to use a different ViewModel, how would I switch which ViewModel Caliburn uses? Or should I set a property on the DropWindowViewModel to trigger a different interface?

Answer 1

I’d suggest this is something that should be done within your root viewmodel. The bootstrapper should be just that. It loads the resources needed for your app & configures them. After that it’s done. Checking whether the user can run the app is part of your application logic & should therefore go in the root viewmodel.