再议C#中的装箱与拆箱的问题详解

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