c# - 10000000个整数相加时为什么在ArrayList和List中赋值时间相同？

Question

我有以下代码，其中我通过添加 10000000 个整数来比较 ArrayList 和 List 的性能。在 ArrayList 中，我有装箱和拆箱，所以在第一个循环中，添加整数的时间比在 List 中多得多，但是稍后分配整数到元素形式列表和 arrayList 的时间是相似的，即使在 arrayLIst 中我必须做拆箱。为什么？是因为 ArrayList 中整数的装箱和拆箱发生在第二个循环之前吗？

int numbers = 10000000;
ArrayList integerArrayList = new ArrayList();
Stopwatch timer = new Stopwatch();
timer.Start();

for (int i = 0; i < numbers; i++)
{
    integerArrayList.Add(i); //niejawna konwersja na object
}
timer.Stop();
var timeTestingIntegerArrayList = timer.ElapsedMilliseconds;
Console.WriteLine($"The time for adding {numbers} numbers to Arraylist is: {timeTestingIntegerArrayList} ms");
timer.Reset();

timer.Start();

for (int i = 0; i < numbers; i++)
{

    int number = (int)integerArrayList[i]; //unboxing
}
timer.Stop();
var arrayListWriteTime = timer.ElapsedMilliseconds;
Console.WriteLine($"The time for writting {numbers} numbers from Arraylist is: {arrayListWriteTime} ms");
timer.Reset();

Console.WriteLine($"The whole time for adding and writting {numbers} numbers from Arraylist is: {arrayListWriteTime + timeTestingIntegerArrayList} ms");` 

int numbers = 10000000;

List<int> integerList = new List<int>();
Stopwatch timer = new Stopwatch();

timer.Start();
for (int i = 0; i < numbers; i++)
{
    integerList.Add(i);
}
timer.Stop();   
var timeTestingIntegerList = timer.ElapsedMilliseconds;
Console.WriteLine($"The time for adding {numbers} numbers to list is: {timeTestingIntegerList} ms");
timer.Reset();

timer.Start();
for (var i = 0; i < numbers; i++)
{

    int number = integerList[i];

}
timer.Stop();
var listWriteTime = timer.ElapsedMilliseconds;
Console.WriteLine($"The time for assigning {numbers} numbers from list is: {listWriteTime} ms");
timer.Reset();


Console.WriteLine($"The whole time for adding and writting the integer numbers is: {timeTestingIntegerList + listWriteTime}");

Answer 1

你的程序产生相似的输出

The time for adding 10000000 numbers to Arraylist is: 1824 ms
The time for writting 10000000 numbers from Arraylist is: 100 ms
The whole time for adding and writting 10000000 numbers from Arraylist is: 1924 ms
The time for adding 10000000 numbers to list is: 111 ms
The time for assigning 10000000 numbers from list is: 77 ms
The whole time for adding and writting the integer numbers is: 188

but the time of assignig later the integer number to elements form list and arrayList is similar even though in arrayLIst I have to do unboxing

不，列表赋值比数组列表赋值相对快。如果比较 IL 生成的代码，ArrayList 和 List 赋值之间的区别几乎只是一条指令。

数组列表

L_0083: ldloc.s num8
    L_0085: callvirt instance object [mscorlib]System.Collections.ArrayList::get_Item(int32)
    L_008a: unbox.any int32
    L_008f: stloc.s num9
    L_0091: nop 
    L_0092: ldloc.s num8
    L_0094: stloc.s num7
    L_0096: ldloc.s num7
    L_0098: ldc.i4.1 
    L_0099: add 
    L_009a: stloc.s num8
    L_009c: ldloc.s num8
    L_009e: ldloc.0 

列表

L_017d: callvirt instance !0 [mscorlib]System.Collections.Generic.List`1<int32>::get_Item(int32)
L_0182: stloc.s num12
L_0184: nop 
L_0185: ldloc.s num11
L_0187: stloc.s num7
L_0189: ldloc.s num7
L_018b: ldc.i4.1 
L_018c: add 
L_018d: stloc.s num11
L_018f: ldloc.s num11
L_0191: ldloc.0 

L_008a:unbox.any int32 是数组列表中出现的唯一一条指令，因此与普通列表相比，数组列表中有一些额外的时钟滴答。 因此，正如预期的那样，由于拆箱，arraylist 比列表慢一点，但是与装箱相比，拆箱更快。

Is it because boxing and unboxing of integers in ArrayList occurs before second loop?

NO!!。两个循环彼此独立，没有共享变量或实例会导致您的假设。