c#赋值的执行顺序


主要代码

有下面一段代码,

/*@Magma 2014/05/22 
 *This code is testing the performance of boxing
 */
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;


namespace Service
{
    public class Boxing
    {
        /// <summary>
        /// Test the performance of boxing 
        /// </summary>
        /// <param name="cycleTimes">the times of cycles</param>
        /// <returns></returns>
        public void PerformanceTest(int cycleTimes) {
            //Normal operation
            DateTime begin = DateTime.Now;
            for (int i = 0; i < cycleTimes; i++)
            {
                int ita = 63;
                //int itb = ita;
            }
            Console.WriteLine("the time of Normal:" + (DateTime.Now - begin).Seconds + "s," + (DateTime.Now - begin).Milliseconds + "ms"); 


            //boxing
            begin = DateTime.Now;            
            for (int i = 0; i < cycleTimes; i++)
            {
                object ob = 1;//inboxing
                //int it = (int)ob;//unboxing
            }
            Console.WriteLine("the time of Boxing:" + (DateTime.Now - begin).Seconds+"s,"+(DateTime.Now - begin).Milliseconds+"ms"); 
        }
    }
}


循环4000次时

用如下代码调用:

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

namespace KindsIntegration
{
    class Program
    {
        static void Main(string[] args)
        {
            Boxing box = new Boxing();
            box.PerformanceTest(4000);
            Console.ReadLine();
        }
    }
}

输出(反复运行多次):


这个结果很难理解,装箱操作的时间竟然比普通赋值时间还要短。

在循环次数小于400000次时一直这样,为了消除执行先后的影响,我改变了Performance函数中装箱与普通操作的先后顺序,结果一样(装箱时间<普通赋值)


循环1000000

输出(反复运行多次):


这个结果比较正常,装箱的时间超过正常赋值


原因:

论坛上有人帮忙回答,结果真是奇怪。原因是下面这一句:

DateTime begin = DateTime.Now;

赋值前,要先计算出等号右边的值,再赋值给左边。在得到 datetime.now后,DateTime begin这是一个新对象,需要创建,时间就是消耗在这上面,改成如下的代码,则会清楚地展示原因:

/*@Magma 2014/05/22 
 *This code is testing the performance of boxing
 */
using System;

namespace Service
{
    public class Boxing
    {
        /// <summary>
        /// Test the performance of boxing 
        /// </summary>
        /// <param name="cycleTimes">the times of cycles</param>
        /// <returns></returns>
        public void PerformanceTest(int cycleTimes) {
            //Normal operation
            DateTime begin = DateTime.Now;
            Console.WriteLine("begin:"+begin.Millisecond);
            begin = DateTime.Now;
            Console.WriteLine("begin:" + begin.Millisecond);
            begin = DateTime.Now;
            Console.WriteLine("begin:" + begin.Millisecond);
            begin = DateTime.Now;
            Console.WriteLine("begin:" + begin.Millisecond);
            begin = DateTime.Now;
            Console.WriteLine("begin:" + begin.Millisecond);
            for (int i = 0; i < cycleTimes; i++)
            {
                int x = 63;
                int y = x;
            }
            Console.WriteLine("the time of Normal:" + (DateTime.Now - begin).Seconds + "s," + (DateTime.Now - begin).Milliseconds + "ms"); 

            //boxing
            begin = DateTime.Now;            
            for (int i = 0; i < cycleTimes; i++)
            {
                int x = 1;
                object ob = x;//inboxing
            }
            Console.WriteLine("the time of Boxing:" + (DateTime.Now - begin).Seconds+"s,"+(DateTime.Now - begin).Milliseconds+"ms"); 
        }
    }
}





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