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：
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：
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：
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：
.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
