c#多线程的Interkock.Exchange方法执行问题

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;

namespace 多线程LOCK.InterLock
{//打印机：有三台计算机同事发出打印指令，使用Exchange实现锁
class Program
{

    class Printerwithinterlocktest
    {


        public static int computernum = 3;//计算机数量
        public static void openthread()
        {
            Thread thread;
            Random random = new Random();
            thread = new Thread(printerproc);
            thread.Name = "mythread";
            thread.Start();

       }
        private static void printerproc()

        {
            userprint();
            Thread.Sleep(1000);
        }

        public static int usingprint = 0;//初始化正在打印状态
        private static bool userprint()
        {
            if ( Interlocked.Exchange(ref usingprint, 1) ==0)
            {
                int c = 1;
                Console.WriteLine(Thread.CurrentThread.Name + "acquired the lock");
                Thread.Sleep(5000);
                Console.WriteLine(Thread.CurrentThread.Name + "existed the lock");
                Interlocked.Exchange(ref usingprint, 0);

                return true;
            }
            else
            {
                int c = 2;
                Console.WriteLine(Thread.CurrentThread.Name + "is denited the lock");
                return false;
            }
        }
    }
  static void Main(string[] args)
    {
        Printerwithinterlocktest.openthread();
        Console.ReadKey();
    }
}

}
这段代码在执行到if ( Interlocked.Exchange(ref usingprint, 1) ==0)时，按照调试结果看是继续执行而不是跳入到Else,想问问到底是先执行Exchange方法还是先比较呢，为什么

运行结果