The Main Window

The main window is the window passed to ApplicationLifetime.MainWindow in the OnFrameworkInitializationCompleted method of your 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.ApplicationLifetime IClassicDesktopStyleApplicationLifetime.

danger

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.