In most cases, I write as a library. It makes life sooooooo much easier and can be tested at different levels with unit tests. I will always remember a relativ

In most cases, I write as a library. It makes life sooooooo much easier and can be tested at different levels with unit tests. I will always remember a relatively large GUI application... one big EXE project. All content was hard coded inside Windows/Forms, Button Click methods, etc. I started to break the GUI app down into smaller components. OK.. It has (A), (B), (C), (D), etc. Started to build isolated libraries. I could then test it without having to run the GUI app and get to the respected Window, etc. It made life sooooooooo much easier. Now, it was a bunch of libraries which the GUI apps includes and wraps with presentation. The great thing is that we need to upgrade our tools, opening the door to using a more modern GUI -- as everything is in their own library.. this becomes so much easier to do. This was a C# application using WinForms using .NET framework (v4 something). We upgraded to .NET Core. Once the libraries had upgraded, we decided to use WPF. It was a relatively pleasant experience. Now imagine trying to upgrade .NET version + WinForms to WPF as it currently stood -- all scattered inside the GUI app. My default is to create as library first.