FirstRun example

FirstRun.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

[InitializeOnLoad]
[Location("our_cache.txt", LocationAttribute.Location.LibraryFolder)]
public class FirstRun : ScriptObjectSingleton<FirstRun>
{
    [SerializeField] private string lastTimeWeRan;
    public string LastTimeWeRan
    {
        get { return lastTimeWeRan; }
        set
        {
            lastTimeWeRan = value;
            Save(true);
        }
    }

    static FirstRun()
    {
        EditorApplication.delayCall += EntryPoint;
    }

    static void EntryPoint()
    {
        var isFirstRun = instance == null;
        var theRealInstance = Instance;
        if (isFirstRun)
        {
            Debug.LogFormat("Running for the first time! The last time we ran was '{0}'", theRealInstance.LastTimeWeRan);
            theRealInstance.LastTimeWeRan = DateTimeOffset.UtcNow.ToString("yyyyMMddHHmmss");
        }
        else
            Debug.LogFormat("I saw what you did there Unity! The last time we ran was '{0}'", theRealInstance.LastTimeWeRan);
    }
}