Question

As usual, I’m trying to use a new technology and having problems right off the bat.

I have a Silverlight Business Application + MvvmLight.

In my viewmodel I try to get the logged in users’ roles:

    public HomeViewModel()
    {
        if (IsInDesignMode)
        {
            // Code runs in Blend --> create design time data.
        }
        else
        {
            // Code runs "for real"                
            DetermineStartableProcesses();
        }
    }

    private void DetermineStartableProcesses()
    {
        _startableProcesses = new ObservableCollection<WorkflowProcess>(
            WebContext.Current.User.Roles.SelectMany(r =>
                WorkflowProcess.GetStartableByRole(r))
                .Distinct());
    }

At runtime, I get this exception:

System.Windows.Markup.XamlParseException occurred
  Message=The invocation of the constructor on type 'CompanyHR.ViewModel.ViewModelLocator' that matches the specified binding constraints threw an exception. [Line: 18 Position: 57]
  LineNumber=18
  LinePosition=57
  StackTrace:
       at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
       at CompanyHR.App.InitializeComponent()
       at CompanyHR.App..ctor()
  InnerException: System.ArgumentNullException
       Message=Value cannot be null.
Parameter name: source
       StackTrace:
            at System.Linq.Enumerable.SelectMany[TSource,TResult](IEnumerable`1 source, Func`2 selector)
            at CompanyHR.ViewModel.HomeViewModel.DetermineStartableProcesses()
            at CompanyHR.ViewModel.HomeViewModel..ctor()
            at CompanyHR.ViewModel.ViewModelLocator..ctor()
       InnerException: 

Looks like the ViewModelLocator is instantiating the ViewModels at app startup before the webcontext get’s created, which means it’s a bad idea for me to do a lot of work in my viewmodel constructors.

So, where at in the viewmodel should I be retrieving data that will get databound?

Answer 1

Instantiate your WebContext in the App constructor. then add it to your resources in the App_startup before you call

public App()
        {
            Startup += Application_Startup;
            Exit += Application_Exit;
            UnhandledException += Application_UnhandledException;

            if (IsInDesignModeStatic)
            {
                Services.ServiceLoader.LoadDesignTimeServices();
                DispatcherHelper.Initialize();
            }
            else
            {
                try
                {

                    ServiceLoader.LoadRunTimeServices();
                    DispatcherHelper.Initialize();

                    WebContext webContext = new WebContext();
                    ApplicationLifetimeObjects.Add(WebContext.Current);

                    FormsAuthentication fa = new FormsAuthentication();
                    fa.DomainContext = new Web.Services.AuthenticationDomainContext();
                    WebContext.Current.Authentication = fa;
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.ToString());
                }
            }

            InitializeComponent();
        }



private void Application_Startup(object sender, StartupEventArgs e)
        {
            this.Resources.Add("WebContext", WebContext.Current);

            RootVisual = new MainPage();

        }

I find its easier to do this part in the code behind because of my custom AuthenticationDomainContext and Membershipprovider…But Dereks works well too, i just was using the code behind while i was getting everything working.