Is it bad practice to put a custom Editor class in the same file as the Monobehaviour it affects?

Questions & Answers
1

So the documentation says to put your Editor scripts into an Editor folder to stop them compiling at runtime.

However, if I place the MonoBehaviour and Editor classes into a single file with the name of the MonoBehaviour for the file name, it seems to work anyway (and not when in an Editor folder). This reduces clutter for small scripts that you want to give custom inspectors, but is it bad practice?

using UnityEngine;
using UnityEditor;

public class Example : MonoBehaviour {
// Stuff here
}

[CustomEditor(typeof(Example))]
public class ExampleEditor : Editor {
// Stuff here
}
2

(A bit late, but was searching the same thing and would like to help someone here)

In my opinion, as long as it’s not too complex, it’s not a bad practice. But if your editor is a really big file, things can get messy.

JUST REMEMBER: If you put your Editor script inside the same file, remember to check if you’re in the editor first. Otherwise your game will not build, because de UnityEditor namespace is not added in builds.

Just do something like this:

// Import your regular stuff
using UnityEngine;
using UnityEngine.SceneManagement;
// using blábláblá

// Only import UnityEditor if we're in the editor.
#if UNITY_EDITOR
using UnityEditor;
#endif

public class MyClass : MonoBehaviour
{
    // Cool stuff here 
}

// Again, class will be declared only if we're in the editor.
#if UNITY_EDITOR
[CustomEditor(typeof(MyClass))]
public class MyClassEditor : Editor
{
   // Cool editor stuff here
}
#endif

Sad to answer a 4 y.o. question, but hope it helps someone!