Ages ago, when Unity 3 was still in beta, I wrote about using UnityEditor.Macros to run JavaScript UnityScript snippets in the editor. Today you have a better option in the form of Immediate Window.

Immediate Window provides a more fully fledged REPL — you can inspect returned objects, import assemblies on the fly, and reference variables you previously defined. It’s a useful tool for quick debugging or to experiment with an API. Maybe some editor automation? Oh baby oh baby.

Under the hood it depends on Unity’s Code Analysis package, which includes Microsoft.CodeAnalysis DLLs. In particular, Microsoft.CodeAnalysis.CSharp.Scripting.CSharpScript.EvaluateAsync does the heavy lifting of evaluating arbitrary C# code.

Caveat: At present the Immediate Window package includes an unneeded using statement that causes a compilation error in Unity 2021.1. I created a fork of the repository that removes the offending line. To use this package instead, add this URL in Unity’s package manager: https://github.com/mminer/com.unity.immediate-window.git