在《.NET,你忘记了么？（八）—— 从dynamic到特性误用》一文中，飞林沙同学提到，使用泛型会略微降低程序性能，因此在程序中使用List
泛型容器与非泛型容器的性能比较
首先，我们来比较一种最“纯粹”的泛型容器，它的目的是避免程序的其他方面对性能的影响。因此，这里我们构造两个最简单的容器，就是简单的模仿ArrayList和List
public class MyArrayList { public MyArrayList(int length) { this.m_items = new object[length]; } public object[] m_items; public object this[int index] { get { return this.m_items[index]; } set { this.m_items[index] = value; } } public IEnumerable InnerContainer { get { return this.m_items; } } } public class MyList{ public MyList(int length) { this.m_items = new T[length]; } public T[] m_items; public T this[int index] { get { return this.m_items[index]; } set { this.m_items[index] = value; } } public IEnumerable InnerContainer { get { return this.m_items; } } }
MyArrayList为直接使用Object类型的容器，而MyList
于是我们就可以编写测试代码了。首先，我们初始化两种容器，并进行“预热”：
int length = 1000; object value = new object(); MyArrayList myArrayList = new MyArrayList(length); for (int i = 0; i < length; i++) { myArrayList[i] = myArrayList[i] ?? value; } MyList<object> myList = new MyList<object>(length); for (int i = 0; i < length; i++) { myList[i] = myList[i] ?? value; }
然后我们使用CodeTimer来进行统计：
CodeTimer.Initialize(); int iteration = 300 * 1000; CodeTimer.Time("MyArrayList下标访问", iteration, () => { for (int i = 0; i < length; i++) { var o = myArrayList[i]; } }); CodeTimer.Time("MyList下标访问", iteration, () => { for (int i = 0; i < length; i++) { var o = myList[i]; } });
Release Build并运行。猜猜看，结果是什么？
MyArrayList下标访问
Time Elapsed: 2,398ms
CPU Cycles: 5,042,561,997
Gen 0: 0
Gen 1: 0
Gen 2: 0
MyList下标访问
Time Elapsed: 2,285ms
CPU Cycles: 4,935,066,741
Gen 0: 0
Gen 1: 0
Gen 2: 0
以上是在老赵的机器上得到的结果，从结果上看，泛型的MyList性能甚至略比MyArrayList有所提高。当然测试的结果其实是互有胜负，但是事实上，MyList的获胜的次数甚至还略有领先。
那么我们再来看看“遍历”的性能如何：
CodeTimer.Time("MyArrayList遍历", iteration, () => { foreach (object o in myArrayList.InnerContainer) { var o1 = o; } }); CodeTimer.Time("MyList遍历", iteration, () => { foreach (object o in myList.InnerContainer) { var o1 = o; } });
运行的结果颇有意思：
MyArrayList遍历
Time Elapsed: 21,367ms
CPU Cycles: 46,023,627,496
Gen 0: 2
Gen 1: 1
Gen 2: 0
MyList遍历
Time Elapsed: 3,463ms
CPU Cycles: 7,448,928,223
Gen 0: 2
Gen 1: 0
Gen 2: 0
直接使用Object的MyArrayList性能居然差了这么多！个中原因老赵有所猜测，在得到明确答案之后会接着与大家分享。
ArrayList和List的性能
刚才比较了最“纯粹”的性能，那么我们再来比较ArrayList和List
int length = 1000; object value = new object(); ArrayList arrayList = new ArrayList(length); for (int i = 0; i < length; i++) { arrayList.Add(value); arrayList[i] = arrayList[i] ?? value; } List<object> list = new List<object>(length); for (int i = 0; i < length; i++) { list.Add(value); list[i] = list[i] ?? value; } CodeTimer.Initialize(); int iteration = 300 * 1000; CodeTimer.Time("ArrayList下标访问", iteration, () => { for (int i = 0; i < length; i++) { var o = arrayList[i]; } }); CodeTimer.Time("List下标访问", iteration, () => { for (int i = 0; i < length; i++) { var o = list[i]; } }); CodeTimer.Time("ArrayList遍历", iteration, () => { foreach (object o in arrayList) { var o1 = o; } }); CodeTimer.Time("List遍历", iteration, () => { foreach (object o in list) { var o1 = o; } });
结果如下：
ArrayList下标访问
Time Elapsed: 2,282ms
CPU Cycles: 4,838,476,797
Gen 0: 0
Gen 1: 0
Gen 2: 0
List下标访问
Time Elapsed: 2,302ms
CPU Cycles: 4,979,267,920
Gen 0: 0
Gen 1: 0
Gen 2: 0
ArrayList遍历
Time Elapsed: 5,187ms
CPU Cycles: 11,145,830,014
Gen 0: 4
Gen 1: 0
Gen 2: 0
List遍历
Time Elapsed: 2,989ms
CPU Cycles: 6,459,825,955
Gen 0: 0
Gen 1: 0
Gen 2: 0
现在您还觉得泛型会降低性能，List
总结
从结果上已经可以看出，泛型并不会影响性能，而List
放心使用List