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