Null reference error in App.xaml MVVM light

Question

I'll make a WPF application whit one window and by changing the content of the Frame I'll navigate troth my application. For this I'm using MVVM light.

But on App.xaml I've got this error in the error list of Visual Studio.

Object reference not set to an instance of an object.

Here is the code where the error happens:

<Application 
    x:Class="Project.App" 
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:local="clr-namespace:Project" 
    StartupUri="MainWindow.xaml" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
    d1p1:Ignorable="d" 
    xmlns:vm="clr-namespace:Project.ViewModel"
    xmlns:services="clr-namespace:Project.Services"
    xmlns:d1p1="http://schemas.openxmlformats.org/markup-compatibility/2006">
    <Application.Resources>
        <ResourceDictionary>
            <services:IocContainer x:Key="ioc" />
            <vm:ApplicationViewModel x:Key="appvm" d:IsDataSource="True" /> <!-- error happens on this line -->
        </ResourceDictionary>
    </Application.Resources>
</Application>

This is my MainWindow:

<Window x:Class="Project.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Project"
        mc:Ignorable="d"
        DataContext="{StaticResource appvm}"
        Title="Project" Height="450" Width="800">
    <Frame>
        <Frame.Content>
            <Page Content="{Binding CurrentPage, Mode=TwoWay}" />
        </Frame.Content>
    </Frame>
</Window>

Here is my ApplicationViewModel that inherits from ViewModelBase:

public class ApplicationViewModel : ViewModelBase
{
    private Page _currentPage = IocContainer.Ioc.StartScreenPage;

    public Page CurrentPage
    {
        get
        {
            return _currentPage;
        }
        set
        {
            if (_currentPage != value)
            {
                _currentPage = value;
                OnPropertyChanged();
            }
        }
    }

    public StartScreenViewModel StartScreenViewModel
    {
        get
        {
            return (App.Current.Resources["ioc"] as IocContainer)?.StartScreenViewModel;
        }
    }

    public void Navigate(Type sourcePageType)
    {
    }
}

Here is the ViewModelBase that implements INotifyPropertyChanged.

public class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            Debug.WriteLine("PropertyChanged is niet null ☺");
        }
        else
        {
            Debug.WriteLine("PropertyChanged is null");
        }
    }
}

Here is my IoC container:

public class IocContainer
{
    static IocContainer()
    {
        SimpleIoc.Default.Register<ApplicationViewModel>(false);
        SimpleIoc.Default.Register<StartScreenViewModel>(false);
        SimpleIoc.Default.Register<StartScreenPage>(false);
    }

    public static IocContainer Ioc
    {
        get { return App.Current.Resources["ioc"] as IocContainer; }
    }

    public ApplicationViewModel ApplicationViewModel
    {
        get { return SimpleIoc.Default.GetInstance<ApplicationViewModel>(); }
    }

    public StartScreenPage StartScreenPage
    {
        get { return SimpleIoc.Default.GetInstance<StartScreenPage>(); }
    }

    public StartScreenViewModel StartScreenViewModel
    {
        get { return SimpleIoc.Default.GetInstance<StartScreenViewModel>(); }
    }
}

Here is my StartScreenPage:

<Page x:Class="Project.StartScreenPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:Project"
      mc:Ignorable="d" 
      DataContext="{Binding StartScreenViewModel, Source={StaticResource ioc}}"
      Title="StartScreen">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="auto" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="auto" />
        </Grid.ColumnDefinitions>

        <Label Content="Hello world" Grid.Row="0" Grid.Column="0" />
    </Grid>
</Page>

And here is StartScreenViewModel.

public class StartScreenViewModel : ViewModelBase
{ }

All the application, window and pages have a default constructor that calls InitializeComponent.

I can run my application but I see an empty window.

Did I forgot anything?

Edit: Continuing on my anwser on this question: Page can have only Frame as parent and not Window, I've changed my code of the MainWindow to this:

The code on the MainWindow must be this:

<!-- Opening Window tag with all attributes -->
<Frame Content="{Binding CurrentPage, Mode=TwoWay}" />
<!-- Closing Window tag -->

This will also show the StartScreenPage on the window.

However the null reference error is still being thrown.

Answer 1

There's very little null checking in your code, which is where this is happening.

The best way to find the issue is to go to the Visual Studio tool panel

Debug → Windows → Exception Settings

and fully check the row labelled 'Common Language Runtime Exceptions'. When you run the code again, you should get more information about where the null exception is happening.