@craftworkgames put it well together, but just for the sake of completeness I also want to mention the awesome imgui library (C++).
There is also a .Net wrapper called ImGui.Net.
From the readme file:
This is a .NET wrapper for the immediate mode GUI library, Dear ImGui (https://github.com/ocornut/imgui).
ImGui.NET lets you build graphical interfaces using a simple
immediate-mode style. ImGui.NET is a .NET Standard library, and can be
used on all major .NET runtimes and operating systems.
There is a sample program which shows you how to use it together with MonoGame.
The advantage here is that you can integrate it into a DesktopGL project which makes your editor available on multiple platforms without extra work.
I also managed to make it work with MonoGame.Forms so you would have additional designing options in a WindowsForms environment, where it's often painful to design things (like a property grid).
Another advantage is that you can still use WindowsForms controls / user controls or file dialogs which makes working with directories and files pretty easy.
But in the end it really depends on your ambition and technology knowledge. I would say:
- Are you skilled with WPF? Then try to create your editor with the MonoGame.WpfInterop.
- You just want a little tool window for manipulating level files? Maybe then MonoGame.Forms is simple enough.
- You want a full blown multi platform editor? Maybe then you should try the ImGui.Net library with the MonoGame.Framework.
- Are you experienced with WindowsForms but not with WPF and you want fast results without missing designing capabilities? Well, I can show you how to integrate ImGui.Net into a MonoGame.Forms project.
Hope this helps with your decision.