WPF: What is App.xaml's Purpose?

I've done .Net development for awhile but I'm new to the WPF technology. What is the supposed purpose of App.xaml? Also, what type of xaml code do you usually put in it? It seems like for simple applications it could be ignored and left untouched. Is this true?

App.xaml is the declarative portion of your code (usually generated by Visual Studio) extending System.Windows.Application. For example, Expression Blend can use App.xaml to share a Resource Dictionary or a design-time data set with your entire application. And, because we are using Microsoft products, whatever Expression Blend can do auto-magically, we can do by hand in Visual Studio.

Now the tangent: To me, to ask about the purpose of App.xaml is to ask about the purpose for System.Windows.Application. Feel free to accuse me of changing the original question (let the digital brutality ensue).

You can’t just open a System.Windows.Controls.Window in any Assembly you like… Chris Sells is likely telling me this in his book. I began to understand the purpose of System.Windows.Application while using MEF and MVVM Light to display WPF windows in DLLs (not EXEs). I got errors like this:

The type 'System.Windows.Markup.IComponentConnector' is defined in an assembly that is not referenced.

or

The type 'System.Windows.Markup.IQueryAmbient' is defined in an assembly that is not referenced.

The above error is simply saying that I’m trying to open a WPF Window inside of a DLL and not an EXE. Then, there’s this error:

The component 'Songhay.Wpf.WordWalkingStick.Views.ClientView' does not have a resource identified by the URI '/Songhay.Wpf.WordWalkingStick;component/views/clientview.xaml'.

This boils down to the absence of a facility that associates WPF Window XAML with the WPF “code” (an instance). This facility is associated with WPF EXEs and not WPF DLLs. Visual Studio auto-generates a WPF EXE class called App.g.cs (in your \obj\Debug folder) with this call in it: System.Windows.Application.LoadComponent(this, resourceLocater) where resourceLocater is a badly named variable containing a System.Uri pointing to the XAML like ClientView.xaml mentioned above.

I’m sure Chris Sells has a whole chapter written on how WPF depends on System.Windows.Application for its very life. It is my loss (quite literally of time) for not having read about it.

I have shown myself a little something with this unit test:

[STAThread]
[TestMethod]
public void ShouldOpenWindow()
{
    Application app = new Application();
    app.Run(new Window());
}

Failing to wrap a new Window in the System.Windows.Application.Run() method will throw an error from the land of COM talking about, “Why did you pull the rug from underneath me?”

For simple applications, it is true, it can be ignored. The major purpose for App.xaml is for holding resources (style, pens, brushes, etc.) that would would like to be available through out all of the windows in your application.

It is true. App.Xaml is some sort of central starting point. You CAN use it, or you CAN start your first window (it is defined in the app.xaml) manually. There are some lifetime events there centralls (like application start).

Storing resources that are used across the whole application.

Application is the root of the logical tree.