The Main Window

The main window is the window passed to ApplicationLifetime.MainWindow in the OnFrameworkInitializationCompleted method of your App.axaml.cs file:

public override void OnFrameworkInitializationCompleted()
{
    if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
    {
        desktopLifetime.MainWindow = new MainWindow();
    }
}

It can be retrieved at any time by casting Application.Current.ApplicationLifetime to IClassicDesktopStyleApplicationLifetime.

Worth mentioning, developers should keep in mind, that using static globals and accessing MainWindow from any place of the app can be dangerous and sometimes cause bad UX. All top-level (window) related APIs should be used from the most specific top-level, usually, it's the latest active one. In this way, user dialogs won't be opened from the wrong window, for example.

warning

Mobile and browser platforms don't have a concept of Window in Avalonia. Instead, you need to set MainView control in Application.ApplicationLifetime when it implements ISingleViewApplicationLifetime interface.