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：
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：
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代码如下：
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 |
本文转载自 玉开的技术博客
详细的页面链接：http://www.cnblogs.com/yukaizhao/archive/2011/10/19/csharp_box_unbox_2.html