单例模式在多线程环境下的lazy模式为什么要加两个if(instance==null)

using System;
using System.Threading;
 
namespace SingletonPattern
{
    class Program
    {
        static Thread[] threads;
 
        static void Main(string[] args)
        {
            int threadsCapacity;
 
            Console.WriteLine("请输入开启的线程数：");
            string s;
            do
            {
                s = Console.ReadLine();
                if (!int.TryParse(s, out threadsCapacity))
                {
                    Console.WriteLine("数字格式不正确，请输入开启的线程数：");
                    continue;
                }
                else if (1 > threadsCapacity)
                {
                    Console.WriteLine("数字应不小于 1，请输入开启的线程数：");
                    continue;
                }
                else
                {
                    break;
                }
            } while (true);
 
            threads = new Thread[threadsCapacity];
            Program pg = new Program();
            for (int i = 0; i < threads.Length; i++)
            {
                Thread thread = new Thread(new ThreadStart(Singleton.GetIns));
                thread.Name = "线程 " + i;
                threads[i] = thread;
            }
 
            for (int i = 0; i < threads.Length; i++)
            {
                threads[i].Start();
            }
 
            Console.ReadKey();
        }
    }
 
    class Singleton
    {
        private static Singleton instance;
        private static object _lock = new object();
        private static int instanceQuantity = 0;
 
        private Singleton()
        {
 
        }
 
        public static Singleton GetInstance()
        {
            if (instance == null)
            {
                // 挂起线程，确保所有的线程都执行到这并等待
                Thread.Sleep(1000);
 
                lock (_lock)
                {
                    //if (instance == null)
                    //{
                    //    instance = new Singleton();
                    //    ++instanceQuantity;
                    //    Console.WriteLine(Thread.CurrentThread.Name + " 生成第 " + instanceQuantity + " 实例！");
                    //}
 
                    instance = new Singleton();
                    ++instanceQuantity;
                    Console.WriteLine(Thread.CurrentThread.Name + " 生成第 " + instanceQuantity + " 实例！");
                }
            }
            return instance;
        }
 
        public static void GetIns()
        {
            GetInstance();
        }
    }
}

public static Singleton GetInstance()
        {
            if (instance == null)
            {
                // 挂起线程，确保所有的线程都执行到这并等待
                Console.WriteLine("我是" + Thread.CurrentThread.Name + " ，instance等于null，哦，对象还没创建呢，看来我有机会哦！");
                Thread.Sleep(1000);

                lock (_lock)
                {
                    Console.WriteLine("我是" + Thread.CurrentThread.Name + " ，instance等于null，哈哈，我锁，我是第一个到的！创建对象的任务就交给我了！");
                    if (instance == null)
                    {
                        instance = new Singleton();
                        ++instanceQuantity;
                        Console.WriteLine("我是" + Thread.CurrentThread.Name + " ，哼哼，对象是我创建的！");
                    }
                    else
                    {
                        Console.WriteLine("我是"+Thread.CurrentThread.Name + "，虽然我之前判断instance等于null，但现在判断的时候，却不等null了，唉，还是被别人快了一步！不过好在我判断了，要不就多创建了一个，失去了单例模式的意义！");
                    }
                    //Console.WriteLine(Thread.CurrentThread.Name + " 生成第 " + instanceQuantity + " 实例！");
                    //instance = new Singleton();
                    //++instanceQuantity;
                    //Console.WriteLine(Thread.CurrentThread.Name + " 生成第 " + instanceQuantity + " 实例！");
                }
            }
            return instance;
        }