Invert the current Unity project / assets selection

InvertSelection.cs

using System;
using UnityEngine;
using UnityEditor;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Object = UnityEngine.Object;
public class InvertSelection : ScriptableWizard
{
    [MenuItem("Assets/Selection/Invert", false, 50)]
    static void InvertAssetSelection()
    {
        EditorUtility.DisplayProgressBar("Inverting Selection", "Working...", 0);

        var oldSelection = Selection.GetFiltered(typeof(Object), SelectionMode.Assets).ToList();
        var newSelection = new List<Object>();
        var allAssets = AssetDatabase.FindAssets(string.Empty);

        for (var i = 0; i < allAssets.Length; i++)
        {
            var guid = allAssets[i];
            var path = AssetDatabase.GUIDToAssetPath(guid);
            var asset = AssetDatabase.LoadAssetAtPath<Object>(path);

            if (!oldSelection.Contains(asset))
            {
                newSelection.Add(asset);
            }

            var progress = (i / (float) allAssets.Length);
            EditorUtility.DisplayProgressBar("Inverting Selection", $"{progress*100f:N0}% Complete", progress);
        }

        Selection.objects = newSelection.ToArray();
        EditorUtility.ClearProgressBar();        
    }
}