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);
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 Type(int)")) { List list = new List (); 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；