App.xaml is the declarative starting point of a WPF (Windows Presentation Foundation) application
App.xaml is the declarative starting point of a wpf (Windows Presentation Foundation) application. visual-studio automatically creates it when a new WPF project is created, including a code-behind file called App.xaml.cs. They work together to allow the developer 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 refer 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.
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.
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