Throw exception when access image from App.xaml.cs - wpf

Question

First, try this.
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Height="80" Width="100" Loaded="Window_Loaded">
<Image x:Name="image"/>
</Window>
and
public partial class MainWindow : Window
{
public static BitmapImage okImage = new BitmapImage(new Uri("pack://Application:,,,/ok.png"));
public MainWindow()
{
InitializeComponent();
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
image.Source = okImage;
}
}
REMEMBER put any image named ok.png to the project directory. This runs well and shows an image in the window.
Now try to move public static BitmapImage okImage = new BitmapImage(new Uri("pack://Application:,,,/ok.png")); to App.xaml.cs.
Run again, we get TypeInitializationException. Why?
I know I can define okImage in MainWindow.xaml.cs, but if I insist it must be defined and initilized in App.xaml.cs, how should I do?

Answer 1

Usually TypeInitializationException iteslf is not very helpful - always check it's InnerException property.
In your case, it says "Invalid URI: Invalid port specified.", which is much more helpful.
It tells that the problem isn't image itself, it's Uri constructor which throws exception.
But why it succeeds in MainWindow.xaml.cs and fails in App.xaml.cs?
It's all about UriParser schemes. UriParser defines way of parsing some parts of Uri and is involved in Uri object initialization. When your MainWindow object is accessed for the first time, it's static constructor is called and your image is created. At this moment, your application has already registered a UriParser for pack:// scheme, and Uri parser succeeds. But when you try to do the same thing in Application.xaml.cs, custom UriParser for pack:// scheme is not yet registered, and default one fails.
Solution:
Do not instantiate your images in Application static constructor. You may still keep fields or properties and make them static if you want, but move all your image initialization code to OnStartup():
public partial class App : Application
{
public static BitmapImage okImage;
protected override void OnStartup(StartupEventArgs e)
{
okImage = new BitmapImage(new Uri("pack://application:,,,/ok.png"));
base.OnStartup(e);
}
}