UnitySelectionOutlinePreference.cs

// SelectionOutlinePreference
using System;
using System.Reflection;
using UnityEngine;
using UnityEditor;

// Put in a folder called 'Editor' anywhere inside project Assets folder.
// Combines ideas based on forum posts:
//     https://discussions.unity.com/t/dsiable-selection-outlines-on-object/869598/2
//     https://discussions.unity.com/t/editor-how-to-add-checkmarks-to-menuitems/114525/6

[InitializeOnLoad]
public static class SelectionOutlinePreference
{
    private const string MENUITEM_NAME =
        "Edit/Selection/Show Selection Outline";

    private static bool enabled = true;

    static SelectionOutlinePreference()
    {
        SelectionOutlinePreference.enabled =
            EditorPrefs.GetBool(MENUITEM_NAME, true);
        EditorApplication.delayCall += () =>
            {
                PerformAction(SelectionOutlinePreference.enabled);
            };
    }

    [MenuItem(MENUITEM_NAME)]
    private static void ToggleAction()
    {
        PerformAction(!enabled);
    }

    public static void PerformAction(bool enable)
    {
        Menu.SetChecked(MENUITEM_NAME, enable);
        EditorPrefs.SetBool(MENUITEM_NAME, enable);
        SelectionOutlinePreference.enabled = enable;
        ToggleSelectionGizmo(enable);
    }

    private static void ToggleSelectionGizmo(bool enable)
    {
        Type AnnotationUtility =
            Type.GetType("UnityEditor.AnnotationUtility, UnityEditor");
        var ShowOutlineOption = AnnotationUtility.GetProperty(
            "showSelectionOutline",
            BindingFlags.NonPublic |
            BindingFlags.Public |
            BindingFlags.Static);
        ShowOutlineOption.SetValue(null, enable);
    }
}