上一篇寫了一下裝箱拆箱的定義和IL分析，這一篇我們看下使用泛型和不使用泛型引發裝箱拆箱的情況
1. 使用非泛型集合時引發的裝箱和拆箱操作
看下面的一段代碼：
1 var array = new ArrayList();
2 array.Add(1);
3 array.Add(2);
4
5 foreach (int value in array)
6 {
7 Console.WriteLine(“value is {0}”,value);
8 }
代碼聲明了一個ArrayList對象，向ArrayList中添加兩個數字1，2；然後使用foreach將ArrayList中的元素打印到控制台。
在這個過程中會發生兩次裝箱操作和兩次拆箱操作，在向ArrayList中添加int類型元素時會發生裝箱，在使用foreach枚舉ArrayList中的int類型元素時會發生拆箱操作，將object類型轉換成int類型，在執行到Console.WriteLine時，還會執行兩次的裝箱操作；這一段代碼執行了6次的裝箱和拆箱操作；如果ArrayList的元素個數很多，執行裝箱拆箱的操作會更多。
你可以通過使用ILSpy之類的工具查看IL代碼的box，unbox指令查看裝箱和拆箱的過程
2. 使用泛型集合的情況
請看如下代碼：
var list = new List<int>();
list.Add(1);
list.Add(2);
foreach (int value in list)
{
Console.WriteLine("value is {0}", value);
}
代碼和1中的代碼的差別在於集合的類型使用了泛型的List，而非ArrayList；我們同樣可以通過查看IL代碼查看裝箱拆箱的情況，上述代碼只會在Console.WriteLine()方法時執行2次裝箱操作，不需要拆箱操作。
可以看出泛型可以避免裝箱拆箱帶來的不必要的性能消耗；當然泛型的好處不止於此，泛型還可以增加程序的可讀性，使程序更容易被復用等等。
本文使用的C#代碼如下：
01 using System;
02 using System.Collections;
03 using System.Collections.Generic;
04
05 namespace boxOrUnbox
06 {
07 class Program
08 {
09 static void Main(string[] args)
10 {
11 //do nothing
12 }
13
14 static void Box()
15 {
16 object objValue = 9;
17 }
18
19 static void Unbox()
20 {
21 object objValue = 4;
22 int value = (int)objValue;
23 }
24
25 static void LookatArrayList()
26 {
27 var array = new ArrayList();
28 array.Add(1);
29 array.Add(2);
30
31 foreach (int value in array)
32 {
33 Console.WriteLine("value is {0}", value);
34 }
35 }
36
37 static void LookatGenericList()
38 {
39 var list = new List<int>();
40 list.Add(1);
41 list.Add(2);
42
43 foreach (int value in list)
44 {
45 Console.WriteLine("value is {0}", value);
46 }
47 }
48 }
49 }
C#的IL代碼如下：
001 .class private auto ansi beforefieldinit boxOrUnbox.Program
002 extends [mscorlib]System.Object
003 {
004 // Methods
005 .method private hidebysig static
006 void Main (
007 string[] args
008 ) cil managed
009 {
010 // Method begins at RVA 0x2050
011 // Code size 2 (0x2)
012 .maxstack 8
013 .entrypoint
014
015 IL_0000: nop
016 IL_0001: ret
017 } // end of method Program::Main
018
019 .method private hidebysig static
020 void Box () cil managed
021 {
022 // Method begins at RVA 0x2054
023 // Code size 10 (0xa)
024 .maxstack 1
025 .locals init (
026 [0] object objValue
027 )
028
029 IL_0000: nop
030 IL_0001: ldc.i4.s 9
031 IL_0003: box [mscorlib]System.Int32
032 IL_0008: stloc.0
033 IL_0009: ret
034 } // end of method Program::Box
035
036 .method private hidebysig static
037 void Unbox () cil managed
038 {
039 // Method begins at RVA 0x206c
040 // Code size 16 (0x10)
041 .maxstack 1
042 .locals init (
043 [0] object objValue,
044 [1] int32 'value'
045 )
046
047 IL_0000: nop
048 IL_0001: ldc.i4.4
049 IL_0002: box [mscorlib]System.Int32
050 IL_0007: stloc.0
051 IL_0008: ldloc.0
052 IL_0009: unbox.any [mscorlib]System.Int32
053 IL_000e: stloc.1
054 IL_000f: ret
055 } // end of method Program::Unbox
056
057 .method private hidebysig static
058 void LookatArrayList () cil managed
059 {
060 // Method begins at RVA 0x2088
061 // Code size 114 (0x72)
062 .maxstack 2
063 .locals init (
064 [0] class [mscorlib]System.Collections.ArrayList 'array',
065 [1] int32 'value',
066 [2] class [mscorlib]System.Collections.IEnumerator CS$5$0000,
067 [3] bool CS$4$0001,
068 [4] class [mscorlib]System.IDisposable CS$0$0002
069 )
070
071 IL_0000: nop
072 IL_0001: newobj instance void [mscorlib]System.Collections.ArrayList::.ctor()
073 IL_0006: stloc.0
074 IL_0007: ldloc.0
075 IL_0008: ldc.i4.1
076 IL_0009: box [mscorlib]System.Int32
077 IL_000e: callvirt instance int32 [mscorlib]System.Collections.ArrayList::Add(object)
078 IL_0013: pop
079 IL_0014: ldloc.0
080 IL_0015: ldc.i4.2
081 IL_0016: box [mscorlib]System.Int32
082 IL_001b: callvirt instance int32 [mscorlib]System.Collections.ArrayList::Add(object)
083 IL_0020: pop
084 IL_0021: nop
085 IL_0022: ldloc.0
086 IL_0023: callvirt instance class [mscorlib]System.Collections.IEnumerator [mscorlib]System.Collections.ArrayList::GetEnumerator()
087 IL_0028: stloc.2
088 .try
089 {
090 IL_0029: br.s IL_004a
091 // loop start (head: IL_004a)
092 IL_002b: ldloc.2
093 IL_002c: callvirt instance object [mscorlib]System.Collections.IEnumerator::get_Current()
094 IL_0031: unbox.any [mscorlib]System.Int32
095 IL_0036: stloc.1
096 IL_0037: nop
097 IL_0038: ldstr "value is {0}"
098 IL_003d: ldloc.1
099 IL_003e: box [mscorlib]System.Int32
100 IL_0043: call void [mscorlib]System.Console::WriteLine(string, object)
101 IL_0048: nop
102 IL_0049: nop
103
104 IL_004a: ldloc.2
105 IL_004b: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
106 IL_0050: stloc.3
107 IL_0051: ldloc.3
108 IL_0052: brtrue.s IL_002b
109 // end loop
110
111 IL_0054: leave.s IL_0070
112 } // end .try
113 finally
114 {
115 IL_0056: ldloc.2
116 IL_0057: isinst [mscorlib]System.IDisposable
117 IL_005c: stloc.s CS$0$0002
118 IL_005e: ldloc.s CS$0$0002
119 IL_0060: ldnull
120 IL_0061: ceq
121 IL_0063: stloc.3
122 IL_0064: ldloc.3
123 IL_0065: brtrue.s IL_006f
124
125 IL_0067: ldloc.s CS$0$0002
126 IL_0069: callvirt instance void [mscorlib]System.IDisposable::Dispose()
127 IL_006e: nop
128
129 IL_006f: endfinally
130 } // end handler
131
132 IL_0070: nop
133 IL_0071: ret
134 } // end of method Program::LookatArrayList
135
136 .method private hidebysig static
137 void LookatGenericList () cil managed
138 {
139 // Method begins at RVA 0x2118
140 // Code size 90 (0x5a)
141 .maxstack 2
142 .locals init (
143 [0] class [mscorlib]System.Collections.Generic.List`1<int32> list,
144 [1] int32 'value',
145 [2] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32> CS$5$0000,
146 [3] bool CS$4$0001
147 )
148
149 IL_0000: nop
150 IL_0001: newobj instance void class [mscorlib]System.Collections.Generic.List`1<int32>::.ctor()
151 IL_0006: stloc.0
152 IL_0007: ldloc.0
153 IL_0008: ldc.i4.1
154 IL_0009: callvirt instance void class [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0)
155 IL_000e: nop
156 IL_000f: ldloc.0
157 IL_0010: ldc.i4.2
158 IL_0011: callvirt instance void class [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0)
159 IL_0016: nop
160 IL_0017: nop
161 IL_0018: ldloc.0
162 IL_0019: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> class [mscorlib]System.Collections.Generic.List`1<int32>::GetEnumerator()
163 IL_001e: stloc.2
164 .try
165 {
166 IL_001f: br.s IL_003c
167 // loop start (head: IL_003c)
168 IL_0021: ldloca.s CS$5$0000
169 IL_0023: call instance !0 valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::get_Current()
170 IL_0028: stloc.1
171 IL_0029: nop
172 IL_002a: ldstr "value is {0}"
173 IL_002f: ldloc.1
174 IL_0030: box [mscorlib]System.Int32
175 IL_0035: call void [mscorlib]System.Console::WriteLine(string, object)
176 IL_003a: nop
177 IL_003b: nop
178
179 IL_003c: ldloca.s CS$5$0000
180 IL_003e: call instance bool valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::MoveNext()
181 IL_0043: stloc.3
182 IL_0044: ldloc.3
183 IL_0045: brtrue.s IL_0021
184 // end loop
185
186 IL_0047: leave.s IL_0058
187 } // end .try
188 finally
189 {
190 IL_0049: ldloca.s CS$5$0000
191 IL_004b: constrained. valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>
192 IL_0051: callvirt instance void [mscorlib]System.IDisposable::Dispose()
193 IL_0056: nop
194 IL_0057: endfinally
195 } // end handler
196
197 IL_0058: nop
198 IL_0059: ret
199 } // end of method Program::LookatGenericList
200
201 .method public hidebysig specialname rtspecialname
202 instance void .ctor () cil managed
203 {
204 // Method begins at RVA 0x2190
205 // Code size 7 (0x7)
206 .maxstack 8
207
208 IL_0000: ldarg.0
209 IL_0001: call instance void [mscorlib]System.Object::.ctor()
210 IL_0006: ret
211 } // end of method Program::.ctor
212
213 } // end of class boxOrUnbox.Program
摘自：玉開的技術博客
