Weird case-insensitive behaviour in WPF - can you please explain - wpf

Question

Can someone please explain this to me:
I have removed the startupUri in my application and moved it to the app class.
<Application x:Class="My.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
><!-- StartupUri="/Forms/MainWindow.xaml"-->
<Application.Resources>
</Application.Resources>
</Application>
I accidently typed "mainWindow" with a lower case in "var mainWindow" but I reference it with an upper case in "MainWindow.Show();".
public partial class App : Application
{
public App()
{
var mainWindow = new MainWindow();
MainWindow.Show();
}
}
Please note that MainWindow doesn't refer to the class here, but in fact to the lowercase mainWindow object.
Why is this then working fine? I thought it should throw an compiler error?
How can mainWindow and MainWindow refer to the same object?

Answer 1

The application has a main window of type MainWindow which initially is "MainWindow" generated in your project. If you use a instance, or this property the result should be the same.
Here you have a picture with the application content and the MainWindow property:

Answer 2

The MainWindow object you're accessing with MainWindow.Show() is actually a property of the Application class. It's just a coincidence that your main window is also called 'MainWindow'.
According to MSDN, the Application.MainWindow property "is automatically set with a reference to the first Window object to be instantiated in the AppDomain."
Your code var mainWindow = new MainWindow(); is effectively doing that: instantiating the first instance of a Window object in your AppDomain. So the subsequent attempt to access Application.MainWindow will return your newly created Window object.
The behaviour is correct, but it is a little confusing.
MSDN: https://msdn.microsoft.com/en-us/library/system.windows.application.mainwindow(v=vs.110).aspx