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C#研究系列List<>与ArrayList的几个研究心得及问题（上）

代码放在https://gist.github.com/921076 上了，看不到的请留言。

第一个心得，是我看某本书提到，IList用起来要比ArrayList快。

这里面用到了我上一篇博客提到的高精度计时器（在这里能看到代码 http://sunxiunan.com/?p=1829 ）

在开始定义了两个类。

//---------------------

// public class List : IList, ICollection, IEnumerable, IList, ICollection, IEnumerable
class CFromList : List

{}

// public class ArrayList : IList, ICollection, IEnumerable, ICloneable
class CFromArrayList : ArrayList

{}

//---------------------

List和ArrayList的定义在注释中给出，可以看出来其实都差不多。ArrayList只是多了ICloneable，还少了几个泛型接口继承。

在后面代码中都用Add方法向list中添加int类型数据，然后通过foreach形式枚举数据，注意！枚举部分的代码是有问题的，我们在（下）中会提到。

这里还要推荐一个非常棒的工具ILSpy，是sharpdevelop开发的，强烈建议dotnet程序员都下载使用。

我把ILSpy disassemble出来的C#代码和IL代码分别列在后面。注意对于ArrayList的foreach语句，C#形式的代码与源代码有些差别（79 到96行），编译器加入一个IEnumerator enumerator2 = cFromArrayList.GetEnumerator();本地变量。

另外使用int num5 = (int)enumerator2.Current;这样访问iterator。而且还加入了IDisposable的finally部分。

再继续看IL代码部分，对于List形式，IL代码没有box装箱指令，而ArrayList在145行有个box指令，这是性能差别之一。
但是奇怪的是，在枚举部分，ILSpy生成的（以及ILDasm）IL代码，对于ArrayList和List而言，基本上差别不大，一样也有对 MoveNext和Current以及IDisposable接口的调用。

只不过ArrayList多出unbox和box的指令。

运行结果如我们所料，List要比ArrayList快不少。

但是我们在枚举部分的代码是有问题的，我明天在（下）中会介绍。

    // public class List<T> : IList<T>, ICollection<T>, IEnumerable<T>, IList, ICollection, IEnumerable
    class CFromList : List<int>{}
    // public class ArrayList : IList, ICollection, IEnumerable, ICloneable
    class CFromArrayList : ArrayList{}
    public partial class Form1 : Form
    {
        private void button1_Click(object sender, EventArgs e)
        {
            CFromList list1 = new CFromList();
            CFromArrayList list2 = new CFromArrayList();
            ////////////////////////////
            HighResolutionTimer timera = new HighResolutionTimer();
            for (int i = 0; i < 100000; i++)
            {
                list1.Add(i - 99999);
            }
            Int64 reta = timera.Stop();
            ////////////////////////////
            HighResolutionTimer timerb = new HighResolutionTimer();
            for (int i = 0; i < 100000; i++)
            {
                list2.Add(i - 99999);
            }
            Int64 retb = timerb.Stop();
            ////////////////////////////
            int index = 0;
            HighResolutionTimer timer1 = new HighResolutionTimer();
            foreach (int elem1 in list1)
            {
                list1[index++] = elem1 + 99;                
            }
            Int64 ret1 = timer1.Stop();
            ////////////////////////////
            index = 0;
            HighResolutionTimer timer2 = new HighResolutionTimer();
            foreach (int elem2 in list2)
            {
                list2[index++] = elem2 + 99;
            }
            Int64 ret2 = timer2.Stop();
            ////////////////////////////
      }
   }
///////////////////////////////////////////////////////
// result from ILSpy
	CFromList cFromList = new CFromList();
	CFromArrayList cFromArrayList = new CFromArrayList();
	HighResolutionTimer highResolutionTimer = new HighResolutionTimer();
	for (int i = 0; i < 100000; i++)
	{
		cFromList.Add(i - 99999);
	}
	long num = highResolutionTimer.Stop();
	HighResolutionTimer highResolutionTimer2 = new HighResolutionTimer();
	for (int i = 0; i < 100000; i++)
	{
		cFromArrayList.Add(i - 99999);
	}
	long num2 = highResolutionTimer2.Stop();
	int num3 = 0;
	HighResolutionTimer highResolutionTimer3 = new HighResolutionTimer();
	foreach (int current in cFromList)
	{
		cFromList[num3++] = current + 99;
	}
	long num4 = highResolutionTimer3.Stop();
	num3 = 0;
	HighResolutionTimer highResolutionTimer4 = new HighResolutionTimer();
	IEnumerator enumerator2 = cFromArrayList.GetEnumerator();
	try
	{
		while (enumerator2.MoveNext())
		{
			int num5 = (int)enumerator2.Current;
			cFromArrayList[num3++] = num5 + 99;
		}
	}
	finally
	{
		IDisposable disposable = enumerator2 as IDisposable;
		if (disposable != null)
		{
			disposable.Dispose();
		}
	}
	long num6 = highResolutionTimer4.Stop();
//////////////////////////////////////////////////
// result from ILSpy IL format
	IL_0000: nop
	IL_0001: newobj instance void WindowsFormsApplication1.CFromList::.ctor()
	IL_0006: stloc.0
	IL_0007: newobj instance void WindowsFormsApplication1.CFromArrayList::.ctor()
	IL_000c: stloc.1
	IL_000d: newobj instance void WindowsFormsApplication1.HighResolutionTimer::.ctor()
	IL_0012: stloc.2
	IL_0013: ldc.i4.0
	IL_0014: stloc.3
	IL_0015: br.s IL_002b
	// loop start (head: IL_002b)
		IL_0017: nop
		IL_0018: ldloc.0
		IL_0019: ldloc.3
		IL_001a: ldc.i4 99999
		IL_001f: sub
		IL_0020: callvirt instance void [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0)
		IL_0025: nop
		IL_0026: nop
		IL_0027: ldloc.3
		IL_0028: ldc.i4.1
		IL_0029: add
		IL_002a: stloc.3
		IL_002b: ldloc.3
		IL_002c: ldc.i4 100000
		IL_0031: clt
		IL_0033: stloc.s 17
		IL_0035: ldloc.s 17
		IL_0037: brtrue.s IL_0017
	// end loop
	IL_0039: ldloc.2
	IL_003a: callvirt instance int64 WindowsFormsApplication1.HighResolutionTimer::Stop()
	IL_003f: stloc.s 4
	IL_0041: newobj instance void WindowsFormsApplication1.HighResolutionTimer::.ctor()
	IL_0046: stloc.s 5
	IL_0048: ldc.i4.0
	IL_0049: stloc.3
	IL_004a: br.s IL_0065
	// loop start (head: IL_0065)
		IL_004c: nop
		IL_004d: ldloc.1
		IL_004e: ldloc.3
		IL_004f: ldc.i4 99999
		IL_0054: sub
		IL_0055: box int32
		IL_005a: callvirt instance int32 [mscorlib]System.Collections.ArrayList::Add(object)
		IL_005f: pop
		IL_0060: nop
		IL_0061: ldloc.3
		IL_0062: ldc.i4.1
		IL_0063: add
		IL_0064: stloc.3
		IL_0065: ldloc.3
		IL_0066: ldc.i4 100000
		IL_006b: clt
		IL_006d: stloc.s 17
		IL_006f: ldloc.s 17
		IL_0071: brtrue.s IL_004c
	// end loop
	IL_0073: ldloc.s 5
	IL_0075: callvirt instance int64 WindowsFormsApplication1.HighResolutionTimer::Stop()
	IL_007a: stloc.s 6
	IL_007c: ldc.i4.0
	IL_007d: stloc.s 7
	IL_007f: newobj instance void WindowsFormsApplication1.HighResolutionTimer::.ctor()
	IL_0084: stloc.s 8
	IL_0086: nop
	IL_0087: ldloc.0
	IL_0088: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> [mscorlib]System.Collections.Generic.List`1<int32>::GetEnumerator()
	IL_008d: stloc.s 18
	.try {
		IL_008f: br.s IL_00af
		// loop start (head: IL_00af)
			IL_0091: ldloca.s 18
			IL_0093: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::get_Current()
			IL_0098: stloc.s 9
			IL_009a: nop
			IL_009b: ldloc.0
			IL_009c: ldloc.s 7
			IL_009e: dup
			IL_009f: ldc.i4.1
			IL_00a0: add
			IL_00a1: stloc.s 7
			IL_00a3: ldloc.s 9
			IL_00a5: ldc.i4.s 99
			IL_00a7: add
			IL_00a8: callvirt instance void [mscorlib]System.Collections.Generic.List`1<int32>::set_Item(int32, !0)
			IL_00ad: nop
			IL_00ae: nop
			IL_00af: ldloca.s 18
			IL_00b1: call instance bool [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::MoveNext()
			IL_00b6: stloc.s 17
			IL_00b8: ldloc.s 17
			IL_00ba: brtrue.s IL_0091
		// end loop
		IL_00bc: leave.s IL_00cd
	} // end .try
	finally {
		IL_00be: ldloca.s 18
		IL_00c0: constrained. valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>
		IL_00c6: callvirt instance void [mscorlib]System.IDisposable::Dispose()
		IL_00cb: nop
		IL_00cc: endfinally
	} // end handler
	IL_00cd: nop
	IL_00ce: ldloc.s 8
	IL_00d0: callvirt instance int64 WindowsFormsApplication1.HighResolutionTimer::Stop()
	IL_00d5: stloc.s 10
	IL_00d7: ldc.i4.0
	IL_00d8: stloc.s 7
	IL_00da: newobj instance void WindowsFormsApplication1.HighResolutionTimer::.ctor()
	IL_00df: stloc.s 11
	IL_00e1: nop
	IL_00e2: ldloc.1
	IL_00e3: callvirt instance class [mscorlib]System.Collections.IEnumerator [mscorlib]System.Collections.ArrayList::GetEnumerator()
	IL_00e8: stloc.s 19
	.try {
		IL_00ea: br.s IL_0114
		// loop start (head: IL_0114)
			IL_00ec: ldloc.s 19
			IL_00ee: callvirt instance object [mscorlib]System.Collections.IEnumerator::get_Current()
			IL_00f3: unbox.any int32
			IL_00f8: stloc.s 12
			IL_00fa: nop
			IL_00fb: ldloc.1
			IL_00fc: ldloc.s 7
			IL_00fe: dup
			IL_00ff: ldc.i4.1
			IL_0100: add
			IL_0101: stloc.s 7
			IL_0103: ldloc.s 12
			IL_0105: ldc.i4.s 99
			IL_0107: add
			IL_0108: box int32
			IL_010d: callvirt instance void [mscorlib]System.Collections.ArrayList::set_Item(int32, object)
			IL_0112: nop
			IL_0113: nop
			IL_0114: ldloc.s 19
			IL_0116: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
			IL_011b: stloc.s 17
			IL_011d: ldloc.s 17
			IL_011f: brtrue.s IL_00ec
		// end loop
		IL_0121: leave.s IL_0140
	} // end .try
	finally {
		IL_0123: ldloc.s 19
		IL_0125: isinst class [mscorlib]System.IDisposable
		IL_012a: stloc.s 20
		IL_012c: ldloc.s 20
		IL_012e: ldnull
		IL_012f: ceq
		IL_0131: stloc.s 17
		IL_0133: ldloc.s 17
		IL_0135: brtrue.s IL_013f
		IL_0137: ldloc.s 20
		IL_0139: callvirt instance void [mscorlib]System.IDisposable::Dispose()
		IL_013e: nop
		IL_013f: endfinally
	} // end handler
	IL_0140: nop
	IL_0141: ldloc.s 11
	IL_0143: callvirt instance int64 WindowsFormsApplication1.HighResolutionTimer::Stop()
	IL_0148: stloc.s 13

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原文地址：https://www.cnblogs.com/lua5/p/2017644.html

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