再谈C#的装箱和拆箱

上一篇写了一下装箱拆箱的定义和IL分析，这一篇我们看下使用泛型和不使用泛型引发装箱拆箱的情况

1. 使用非泛型集合时引发的装箱和拆箱操作 

看下面的一段代码：

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var array = new ArrayList(); array.Add(1); array.Add(2);   foreach (int value in array) { Console.WriteLine(“value is {0}”,value); }

代码声明了一个ArrayList对象，向ArrayList中添加两个数字1，2；然后使用foreach将ArrayList中的元素打印到控制台。

在这个过程中会发生两次装箱操作和两次拆箱操作，在向ArrayList中添加int类型元素时会发生装箱，在使用foreach枚举ArrayList中的int类型元素时会发生拆箱操作，将object类型转换成int类型，在执行到Console.WriteLine时，还会执行两次的装箱操作；这一段代码执行了6次的装箱和拆箱操作；如果ArrayList的元素个数很多，执行装箱拆箱的操作会更多。

你可以通过使用ILSpy之类的工具查看IL代码的box，unbox指令查看装箱和拆箱的过程

2. 使用泛型集合的情况

请看如下代码：

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var list = new List<int>();

list.Add(1);

list.Add(2);

 

foreach (int value in list)

{

Console.WriteLine("value is {0}", value);

}

代码和1中的代码的差别在于集合的类型使用了泛型的List，而非ArrayList；我们同样可以通过查看IL代码查看装箱拆箱的情况，上述代码只会在Console.WriteLine()方法时执行2次装箱操作，不需要拆箱操作。

可以看出泛型可以避免装箱拆箱带来的不必要的性能消耗；当然泛型的好处不止于此，泛型还可以增加程序的可读性，使程序更容易被复用等等。

本文使用的C#代码如下：

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using System; using System.Collections; using System.Collections.Generic;   namespace boxOrUnbox {     class Program     {         static void Main(string[] args)         {             //do nothing         }           static void Box()         {             object objValue = 9;         }           static void Unbox()         {             object objValue = 4;             int value = (int)objValue;         }           static void LookatArrayList()         {             var array = new ArrayList();             array.Add(1);             array.Add(2);               foreach (int value in array)             {                 Console.WriteLine("value is {0}", value);             }         }           static void LookatGenericList()         {             var list = new List<int>();             list.Add(1);             list.Add(2);               foreach (int value in list)             {                 Console.WriteLine("value is {0}", value);             }         }     } }

C#的IL代码如下：

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.class private auto ansi beforefieldinit boxOrUnbox.Program     extends [mscorlib]System.Object {     // Methods     .method private hidebysig static         void Main (             string[] args         ) cil managed     {         // Method begins at RVA 0x2050         // Code size 2 (0x2)         .maxstack 8         .entrypoint           IL_0000: nop         IL_0001: ret     } // end of method Program::Main       .method private hidebysig static         void Box () cil managed     {         // Method begins at RVA 0x2054         // Code size 10 (0xa)         .maxstack 1         .locals init (             [0] object objValue         )           IL_0000: nop         IL_0001: ldc.i4.s 9         IL_0003: box [mscorlib]System.Int32         IL_0008: stloc.0         IL_0009: ret     } // end of method Program::Box       .method private hidebysig static         void Unbox () cil managed     {         // Method begins at RVA 0x206c         // Code size 16 (0x10)         .maxstack 1         .locals init (             [0] object objValue,             [1] int32 'value'         )           IL_0000: nop         IL_0001: ldc.i4.4         IL_0002: box [mscorlib]System.Int32         IL_0007: stloc.0         IL_0008: ldloc.0         IL_0009: unbox.any [mscorlib]System.Int32         IL_000e: stloc.1         IL_000f: ret     } // end of method Program::Unbox       .method private hidebysig static         void LookatArrayList () cil managed     {         // Method begins at RVA 0x2088         // Code size 114 (0x72)         .maxstack 2         .locals init (             [0] class [mscorlib]System.Collections.ArrayList 'array',             [1] int32 'value',             [2] class [mscorlib]System.Collections.IEnumerator CS$5$0000,             [3] bool CS$4$0001,             [4] class [mscorlib]System.IDisposable CS$0$0002         )           IL_0000: nop         IL_0001: newobj instance void [mscorlib]System.Collections.ArrayList::.ctor()         IL_0006: stloc.0         IL_0007: ldloc.0         IL_0008: ldc.i4.1         IL_0009: box [mscorlib]System.Int32         IL_000e: callvirt instance int32 [mscorlib]System.Collections.ArrayList::Add(object)         IL_0013: pop         IL_0014: ldloc.0         IL_0015: ldc.i4.2         IL_0016: box [mscorlib]System.Int32         IL_001b: callvirt instance int32 [mscorlib]System.Collections.ArrayList::Add(object)         IL_0020: pop         IL_0021: nop         IL_0022: ldloc.0         IL_0023: callvirt instance class [mscorlib]System.Collections.IEnumerator [mscorlib]System.Collections.ArrayList::GetEnumerator()         IL_0028: stloc.2         .try         {             IL_0029: br.s IL_004a             // loop start (head: IL_004a)                 IL_002b: ldloc.2                 IL_002c: callvirt instance object [mscorlib]System.Collections.IEnumerator::get_Current()                 IL_0031: unbox.any [mscorlib]System.Int32                 IL_0036: stloc.1                 IL_0037: nop                 IL_0038: ldstr "value is {0}"                 IL_003d: ldloc.1                 IL_003e: box [mscorlib]System.Int32                 IL_0043: call void [mscorlib]System.Console::WriteLine(string, object)                 IL_0048: nop                 IL_0049: nop                   IL_004a: ldloc.2                 IL_004b: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()                 IL_0050: stloc.3                 IL_0051: ldloc.3                 IL_0052: brtrue.s IL_002b             // end loop               IL_0054: leave.s IL_0070         } // end .try         finally         {             IL_0056: ldloc.2             IL_0057: isinst [mscorlib]System.IDisposable             IL_005c: stloc.s CS$0$0002             IL_005e: ldloc.s CS$0$0002             IL_0060: ldnull             IL_0061: ceq             IL_0063: stloc.3             IL_0064: ldloc.3             IL_0065: brtrue.s IL_006f               IL_0067: ldloc.s CS$0$0002             IL_0069: callvirt instance void [mscorlib]System.IDisposable::Dispose()             IL_006e: nop               IL_006f: endfinally         } // end handler           IL_0070: nop         IL_0071: ret     } // end of method Program::LookatArrayList       .method private hidebysig static         void LookatGenericList () cil managed     {         // Method begins at RVA 0x2118         // Code size 90 (0x5a)         .maxstack 2         .locals init (             [0] class [mscorlib]System.Collections.Generic.List`1<int32> list,             [1] int32 'value',             [2] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32> CS$5$0000,             [3] bool CS$4$0001         )           IL_0000: nop         IL_0001: newobj instance void class [mscorlib]System.Collections.Generic.List`1<int32>::.ctor()         IL_0006: stloc.0         IL_0007: ldloc.0         IL_0008: ldc.i4.1         IL_0009: callvirt instance void class [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0)         IL_000e: nop         IL_000f: ldloc.0         IL_0010: ldc.i4.2         IL_0011: callvirt instance void class [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0)         IL_0016: nop         IL_0017: nop         IL_0018: ldloc.0         IL_0019: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> class [mscorlib]System.Collections.Generic.List`1<int32>::GetEnumerator()         IL_001e: stloc.2         .try         {             IL_001f: br.s IL_003c             // loop start (head: IL_003c)                 IL_0021: ldloca.s CS$5$0000                 IL_0023: call instance !0 valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::get_Current()                 IL_0028: stloc.1                 IL_0029: nop                 IL_002a: ldstr "value is {0}"                 IL_002f: ldloc.1                 IL_0030: box [mscorlib]System.Int32                 IL_0035: call void [mscorlib]System.Console::WriteLine(string, object)                 IL_003a: nop                 IL_003b: nop                   IL_003c: ldloca.s CS$5$0000                 IL_003e: call instance bool valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::MoveNext()                 IL_0043: stloc.3                 IL_0044: ldloc.3                 IL_0045: brtrue.s IL_0021             // end loop               IL_0047: leave.s IL_0058         } // end .try         finally         {             IL_0049: ldloca.s CS$5$0000             IL_004b: constrained. valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>             IL_0051: callvirt instance void [mscorlib]System.IDisposable::Dispose()             IL_0056: nop             IL_0057: endfinally         } // end handler           IL_0058: nop         IL_0059: ret     } // end of method Program::LookatGenericList       .method public hidebysig specialname rtspecialname         instance void .ctor () cil managed     {         // Method begins at RVA 0x2190         // Code size 7 (0x7)         .maxstack 8           IL_0000: ldarg.0         IL_0001: call instance void [mscorlib]System.Object::.ctor()         IL_0006: ret     } // end of method Program::.ctor   } // end of class boxOrUnbox.Program