脚踏实地学C#3-装箱和拆箱

装箱:一种接受值类型的值,根据这个值在堆中创建一个完整的引用类型对象并返回对象的引用(堆地址)的隐式转换

int i_number = 2; //在栈中声明int类型i_Number变量并初始化
object o_number = null; //在栈中声明object类型的o_Number变量并初始化值为null,不在堆中分配内存
o_number = i_number;   //装箱过程 如下图所示①→②→③ 
Console.WriteLine(o_number);     
box1


装箱过程:1.在托管堆分配好内存,分配的内存量是值类型的字段需要的内存量(类型对象指针和同步索引块);

              2.值类型的字段复制到新分配的堆内存;

              3.将新分配的堆地址放入栈中的变量内;

拆箱:将装箱的对象转换为值类型的过程(显示转换)

object o_Number = 1; //装箱
int i_Number = (int)o_Number; //拆箱
Console.WriteLine(i_Number);

拆箱过程:1.获取已装箱的对象各个字段的地址;

              2.将字段包含的值复制到值类型的实例中;

装箱和拆箱测试

class Program
     {
        private static void Main(string[] args)
        {
            ValueTypeTestMethod();
	    Console.ReadKey();
        }
        private static void ValueTypeTestMethod()
        {
            const int count = 10000000;  //循环的次数
            using (new OperationTimer("List<int> Type(int)"))
            {
                List<int> list = new List<int>();
                for (int i = 0; i < count; i++)
                {
                    list.Add(i);     //泛型不装箱
                    int x = list[i]; //不拆箱
                }
                list = null;
            }
            using (new OperationTimer("ArrayList Type(int)"))
            {
                ArrayList arrayList = new ArrayList();
                for (int i = 0; i < count; i++)
                {
                    arrayList.Add(i);           //会进行装箱
                    int x = (int)arrayList[i];  //进行拆箱
                }
                arrayList = null;
            }
        }       
    }
    internal sealed class OperationTimer : IDisposable
    {
        private long m_StartTime;
        private string m_MSG;
        private int m_CollectionCount;
        public OperationTimer(String msg)
        {
            PrepareForOperation(); //强制垃圾回收
            m_MSG = msg;
            m_CollectionCount = GC.CollectionCount(0);
            m_StartTime = Stopwatch.GetTimestamp();
        }
        public void Dispose()
        {
            Console.WriteLine("{0,6:###.00} seconds (GC={1,3}) {2}", (Stopwatch.GetTimestamp() - m_StartTime) / (double)Stopwatch.Frequency, GC.CollectionCount(0) - m_CollectionCount, m_MSG);
        }
        private static void PrepareForOperation()
        {
            GC.Collect();
            GC.WaitForPendingFinalizers();
            GC.Collect();
        }
    }
    
集合List<int>ArrayList
时间(s)垃圾回收次数时间(s)垃圾回收次数
64位程序 .20 3 1.37 44
32位程序 .22 3 1.57 23

注意:从上面测试看出List<int>和ArrayList在所耗时间和垃圾回收区别都是在很大的,所以用泛型List<int>代替ArrayList是一个很好的习惯!同时也避免在循环中大量的装箱和拆箱操作;

posted @ 2013-09-12 16:19  秋壶冰月  阅读(783)  评论(0编辑  收藏