Working with App.xaml

App.xaml is the declarative starting point of your application. Visual Studio will automatically create it for you when you start a new WPF application, including a Code-behind file called App.xaml.cs. They work much like for a Window, where the two files are partial classes, working together to allow you to work in both markup (XAML) and Code-behind.

App.xaml.cs extends the Application class, which is a central class in a WPF Windows application. .NET will go to this class for starting instructions and then start the desired Window or Page from there. This is also the place to subscribe to important application events, like application start, unhandled exceptions and so on. More about that later.

One of the most commonly used features of the App.xaml file is to define global resources that may be used and accessed from all over an application, for instance global styles. This will be discussed in detail later on.

App.xaml structure

When creating a new application, the automatically generated App.xaml will look something like this:

<Application x:Class="WpfTutorialSamples.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>

    </Application.Resources>
</Application>

The main thing to notice here is the StartupUri property. This is actually the part that instructs which Window or Page to start up when the application is launched. In this case, MainWindow.xaml will be started, but if you would like to use another window as the starting point, you can simply change this.

In some situations, you want more control over how and when the first window is displayed. In that case, you can remove the StartupUri property and value and then do it all from Code-Behind instead. This will be demonstrated below.

App.xaml.cs structure

The matching App.xaml.cs will usually look like this for a new project:

using System;
using System.Collections.Generic;
using System.Windows;

namespace WpfTutorialSamples
{
	public partial class App : Application
	{

	}
}

You will see how this class extends the Application class, allowing us to do stuff on the application level. For instance, you can subscribe to the Startup event, where you can manually create your starting window.

Here's an example:

<Application x:Class="WpfTutorialSamples.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
			 Startup="Application_Startup">
    <Application.Resources></Application.Resources>
</Application>

Notice how the StartupUri has been replaced with a subscription to the Startup event (subscribing to events through XAML is explained in another chapter). In Code-Behind, you can use the event like this:

using System;
using System.Collections.Generic;
using System.Windows;

namespace WpfTutorialSamples
{
	public partial class App : Application
	{

		private void Application_Startup(object sender, StartupEventArgs e)
		{
			// Create the startup window
			MainWindow wnd = new MainWindow();
			// Do stuff here, e.g. to the window
			wnd.Title = "Something else";
			// Show the window
			wnd.Show();
		}
	}
}

The cool thing in this example, compared to just using the StartupUri property, is that we get to manipulate the startup window before showing it. In this, we change the title of it, which is not terribly useful, but you could also subscribe to events or perhaps show a splash screen. When you have all the control, there are many possibilities. We will look deeper into several of them in the next articles of this tutorial.