Okay. I had no idea as I was just following a tutorial that was linked to some time ago. It's funny that of all 6 of the references that appear to be used in what I was following, only:
using Autodesk.AutoCAD.DatabaseServices;
appears to be used.
This is everything that was told to be used:
using System.Runtime.InteropServices;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using acadApp = Autodesk.AutoCAD.ApplicationServices.Application;
Reference