Interface Builder: Behind the Scenes

For the sake of demystifying what is happening behind the scenes, double click on the MainWindow.designer.xib.cs file you will see that behind the scenes MonoDevelop has generated one property accessor for each outlet:

This file is automatically updated every time you modify your XIB file using Interface Builder. Do not make any changes to this file as any changes that you make will be lost the next time you update your XIB file. This is a MonoDevelop generated file, you can completely ignore it while developing.

The properties are stored in a partial class so the compiler will combine your AppDelegate in Main.cs with these autogenerated properties.