xamarin-macios/dotnet/Microsoft.MacCatalyst.Templates/maccatalyst/AppDelegate.cs

using Foundation;
using UIKit;

namespace MacCatalystApp1 {
	[Register ("AppDelegate")]
	public class AppDelegate : UIApplicationDelegate {
		public override UIWindow Window {
			get;
			set;
		}

		public override bool FinishedLaunching (UIApplication application, NSDictionary launchOptions)
		{
			// create a new window instance based on the screen size
			Window = new UIWindow (UIScreen.MainScreen.Bounds);

			// create a UIViewController with a single UILabel
			var vc = new UIViewController ();
			vc.View.AddSubview (new UILabel (Window.Frame) {
				BackgroundColor = UIColor.White,
				TextAlignment = UITextAlignment.Center,
				Text = "Hello, Catalyst!"
			});
			Window.RootViewController = vc;

			// make the window visible
			Window.MakeKeyAndVisible ();

			return true;
		}
	}
}