编写高质量代码改善C#程序的157个建议：第17个建议之多数情况下使用foreach进行循环遍历

今天是我看《编写高质量代码：改善C#程序的157个建议》第二遍的时候了，看完这本书的确是受益匪浅，学到了很多东西，也明白了很多道理。

里面的代码我每个都调试了一遍，有时候是有些出入的，可能是作者写的书比较早，使用的开发环境比较旧，也许是我的学习还不到家，今天在看建议17的时候，发现了一些小问题，不是很大，是小问题，记录下来，当别人看到的时候可以起到修正的作用。

可能很多人和我一样，没有太在乎for和foreach的区别，也没有深究其底层发生的一些东西，看了文章才发现里面的东西还真是不少。

好了，我们从头聊一下，我对foreach的认识。

Gof的23种设计模式，我相信大家都看过，我现在也正在写有关《设计模式》的一些文章。在这些设计模式种，有一个设计模式叫“迭代器”，这种设计模式就是针对集合对象的迭代而产生的。具体的模式我就不介绍了，FCL框架中也有针对的此模式的具体实现，具体的接口分别是IEnumerable和IEnumerator接口。迭代器模式屏蔽了各种集合的内部实现，为客户对集合的迭代生成了一个统一接口。foreach的使用语法就是针对FCL框架中实现的迭代器的统一操作实现，简化了语法，方便了客户的实现。

通过该文章和对IL代码的分析，foreach除了可以提供简化语法外，还有另外两个有事：

1、自动将代码置入try-finally块。

2、若类型实现了IDisposable接口，他会在循环结束后自动调用Dispose方法。

这是书里的原话，但是这两大优点是有出入的，并不是所有的集合类型都会将代码自动放入try-finally块中。

我们先来了解一下集合概况吧，集合类型主要分为两种，一种是线性集合，另一种是非线性集合，如图：



1、我们先针对线性集合测试，看看结果怎么样：

1）、线性集合里面的直接存取类型的代表：数组

int[] arra = new int[] { 1, 2, 3, 4, 5, 6, 7 };

foreach (int item in arra)
{
Console.WriteLine(item);
}

代码运行结果是：



我们再来看看他们所生成的IL代码：

.method private hidebysig static void  Main(string[] args) cil managed
{
.entrypoint
// 代码大小       56 (0x38)
.maxstack  3
.locals init ([0] int32[] arra,
[1] int32[] V_1,
[2] int32 V_2,
[3] int32 item)
IL_0000:  nop
IL_0001:  ldc.i4.7
IL_0002:  newarr     [mscorlib]System.Int32
IL_0007:  dup
IL_0008:  ldtoken    field valuetype '<PrivateImplementationDetails>'/'__StaticArrayInitTypeSize=28' '<PrivateImplementationDetails>'::'447E020739FB3351B9350DB35F297A3AD27669A4'
IL_000d:  call       void [mscorlib]System.Runtime.CompilerServices.RuntimeHelpers::InitializeArray(class [mscorlib]System.Array,
valuetype [mscorlib]System.RuntimeFieldHandle)
IL_0012:  stloc.0
IL_0013:  nop
IL_0014:  ldloc.0
IL_0015:  stloc.1
IL_0016:  ldc.i4.0
IL_0017:  stloc.2
IL_0018:  br.s       IL_002b
IL_001a:  ldloc.1
IL_001b:  ldloc.2
IL_001c:  ldelem.i4
IL_001d:  stloc.3
IL_001e:  nop
IL_001f:  ldloc.3
IL_0020:  call       void [mscorlib]System.Console::WriteLine(int32)
IL_0025:  nop
IL_0026:  nop
IL_0027:  ldloc.2
IL_0028:  ldc.i4.1
IL_0029:  add
IL_002a:  stloc.2
IL_002b:  ldloc.2
IL_002c:  ldloc.1
IL_002d:  ldlen
IL_002e:  conv.i4
IL_002f:  blt.s      IL_001a
IL_0031:  call       int32 [mscorlib]System.Console::Read()
IL_0036:  pop
IL_0037:  ret
} // end of method Program::Main

我们针对数组执行foreach迭代，但是在所生成的IL代码中并没有看到try-finally中，最起码连try和finally这个两个字样都没看到，书中说foreach遍历集合会自动生成try-finally块，这是不准确的，最起码数组没有生成try-finally代码块。

2）、然后我们再测试一下string类型，看看他的运行结果怎么样！

代码如下：

string dd = "我是中国人";
foreach (var item in dd)
{
Console.WriteLine(item);
}

执行效果如图：



让我们再来看看它所生成的IL代码吧，不要惊讶：

.method private hidebysig static void  Main(string[] args) cil managed
{
.entrypoint
// 代码大小       51 (0x33)
.maxstack  2
.locals init ([0] string dd,
[1] string V_1,
[2] int32 V_2,
[3] char item)
IL_0000:  nop
IL_0001:  ldstr      bytearray (11 62 2F 66 2D 4E FD 56 BA 4E )                   // .b/f-N.V.N
IL_0006:  stloc.0
IL_0007:  nop
IL_0008:  ldloc.0
IL_0009:  stloc.1
IL_000a:  ldc.i4.0
IL_000b:  stloc.2
IL_000c:  br.s       IL_0023
IL_000e:  ldloc.1
IL_000f:  ldloc.2
IL_0010:  callvirt   instance char [mscorlib]System.String::get_Chars(int32)
IL_0015:  stloc.3
IL_0016:  nop
IL_0017:  ldloc.3
IL_0018:  call       void [mscorlib]System.Console::WriteLine(char)
IL_001d:  nop
IL_001e:  nop
IL_001f:  ldloc.2
IL_0020:  ldc.i4.1
IL_0021:  add
IL_0022:  stloc.2
IL_0023:  ldloc.2
IL_0024:  ldloc.1
IL_0025:  callvirt   instance int32 [mscorlib]System.String::get_Length()
IL_002a:  blt.s      IL_000e
IL_002c:  call       int32 [mscorlib]System.Console::Read()
IL_0031:  pop
IL_0032:  ret
} // end of method Program::Main

这里面也没有生成try-finally代码块，说明并不是所有的集合都会自动生成try-finally代码块的。

3）、我们继续测试一下Dictionary<TKey,TValue>，List<T>，Queue<T>，Stack<T>，ArrayList类型：

（1）、Dictionary<TKey,TValue>测试代码：

Dictionary<string, string> dictionary = new Dictionary<string, string>();
dictionary.Add("1", "11");
dictionary.Add("2", "22");
dictionary.Add("3", "33");
dictionary.Add("4", "44");
dictionary.Add("5", "55");
dictionary.Add("6", "66");
dictionary.Add("7", "77");

foreach (var item in dictionary)
{
Console.WriteLine(item.Key + "----" + item.Value);
}

字典类型所生成IL代码如下：

.method private hidebysig static void  Main(string[] args) cil managed
{
.entrypoint
// 代码大小       209 (0xd1)
.maxstack  3
.locals init ([0] class [mscorlib]System.Collections.Generic.Dictionary`2<string,string> dictionary,
[1] valuetype [mscorlib]System.Collections.Generic.Dictionary`2/Enumerator<string,string> V_1,
[2] valuetype [mscorlib]System.Collections.Generic.KeyValuePair`2<string,string> item)
IL_0000:  nop
IL_0001:  newobj     instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::.ctor()
IL_0006:  stloc.0
IL_0007:  ldloc.0
IL_0008:  ldstr      "1"
IL_000d:  ldstr      "11"
IL_0012:  callvirt   instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::Add(!0,
!1)
IL_0017:  nop
IL_0018:  ldloc.0
IL_0019:  ldstr      "2"
IL_001e:  ldstr      "22"
IL_0023:  callvirt   instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::Add(!0,
!1)
IL_0028:  nop
IL_0029:  ldloc.0
IL_002a:  ldstr      "3"
IL_002f:  ldstr      "33"
IL_0034:  callvirt   instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::Add(!0,
!1)
IL_0039:  nop
IL_003a:  ldloc.0
IL_003b:  ldstr      "4"
IL_0040:  ldstr      "44"
IL_0045:  callvirt   instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::Add(!0,
!1)
IL_004a:  nop
IL_004b:  ldloc.0
IL_004c:  ldstr      "5"
IL_0051:  ldstr      "55"
IL_0056:  callvirt   instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::Add(!0,
!1)
IL_005b:  nop
IL_005c:  ldloc.0
IL_005d:  ldstr      "6"
IL_0062:  ldstr      "66"
IL_0067:  callvirt   instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::Add(!0,
!1)
IL_006c:  nop
IL_006d:  ldloc.0
IL_006e:  ldstr      "7"
IL_0073:  ldstr      "77"
IL_0078:  callvirt   instance void class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::Add(!0,
!1)
IL_007d:  nop
IL_007e:  nop
IL_007f:  ldloc.0
IL_0080:  callvirt   instance valuetype [mscorlib]System.Collections.Generic.Dictionary`2/Enumerator<!0,!1> class [mscorlib]System.Collections.Generic.Dictionary`2<string,string>::GetEnumerator()
IL_0085:  stloc.1
.try
{
IL_0086:  br.s       IL_00b0
IL_0088:  ldloca.s   V_1
IL_008a:  call       instance valuetype [mscorlib]System.Collections.Generic.KeyValuePair`2<!0,!1> valuetype [mscorlib]System.Collections.Generic.Dictionary`2/Enumerator<string,string>::get_Current()
IL_008f:  stloc.2
IL_0090:  nop
IL_0091:  ldloca.s   item
IL_0093:  call       instance !0 valuetype [mscorlib]System.Collections.Generic.KeyValuePair`2<string,string>::get_Key()
IL_0098:  ldstr      "----"
IL_009d:  ldloca.s   item
IL_009f:  call       instance !1 valuetype [mscorlib]System.Collections.Generic.KeyValuePair`2<string,string>::get_Value()
IL_00a4:  call       string [mscorlib]System.String::Concat(string,
string,
string)
IL_00a9:  call       void [mscorlib]System.Console::WriteLine(string)
IL_00ae:  nop
IL_00af:  nop
IL_00b0:  ldloca.s   V_1
IL_00b2:  call       instance bool valuetype [mscorlib]System.Collections.Generic.Dictionary`2/Enumerator<string,string>::MoveNext()
IL_00b7:  brtrue.s   IL_0088
IL_00b9:  leave.s    IL_00ca
}  // end .try
finally
{
IL_00bb:  ldloca.s   V_1
IL_00bd:  constrained. valuetype [mscorlib]System.Collections.Generic.Dictionary`2/Enumerator<string,string>
IL_00c3:  callvirt   instance void [mscorlib]System.IDisposable::Dispose()
IL_00c8:  nop
IL_00c9:  endfinally
}  // end handler
IL_00ca:  call       int32 [mscorlib]System.Console::Read()
IL_00cf:  pop
IL_00d0:  ret
} // end of method Program::Main

哈哈哈，终于出现了，自动生成try-finally代码块，并且调用了Dispose方法。

（2）List<T>，Queue<T>，Stack<T>，ArrayList测试代码如下：

List<int> list = new List<int>() { 1, 2 };

foreach (var item in list)
{
Console.WriteLine(item);
}

.method private hidebysig static void  Main(string[] args) cil managed
{
.entrypoint
// 代码大小       83 (0x53)
.maxstack  3
.locals init ([0] class [mscorlib]System.Collections.Generic.List`1<int32> list,
[1] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32> V_1,
[2] int32 item)
IL_0000:  nop
IL_0001:  newobj     instance void class [mscorlib]System.Collections.Generic.List`1<int32>::.ctor()
IL_0006:  dup
IL_0007:  ldc.i4.1
IL_0008:  callvirt   instance void class [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0)
IL_000d:  nop
IL_000e:  dup
IL_000f:  ldc.i4.2
IL_0010:  callvirt   instance void class [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0)
IL_0015:  nop
IL_0016:  stloc.0
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.1
.try
{
IL_001f:  br.s       IL_0032
IL_0021:  ldloca.s   V_1
IL_0023:  call       instance !0 valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::get_Current()
IL_0028:  stloc.2
IL_0029:  nop
IL_002a:  ldloc.2
IL_002b:  call       void [mscorlib]System.Console::WriteLine(int32)
IL_0030:  nop
IL_0031:  nop
IL_0032:  ldloca.s   V_1
IL_0034:  call       instance bool valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::MoveNext()
IL_0039:  brtrue.s   IL_0021
IL_003b:  leave.s    IL_004c
}  // end .try
finally
{
IL_003d:  ldloca.s   V_1
IL_003f:  constrained. valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>
IL_0045:  callvirt   instance void [mscorlib]System.IDisposable::Dispose()
IL_004a:  nop
IL_004b:  endfinally
}  // end handler
IL_004c:  call       int32 [mscorlib]System.Console::Read()
IL_0051:  pop
IL_0052:  ret
} // end of method Program::Main

其他的代码我就不贴了，都生成了try-finally代码块，如果类型实现了IDisposable接口，也会调用Dispose方法。

2、我们再测试一下非线性集合是否会自动产生相关代码

我们测试一下HashSet<T>代码，看看效果怎么样。

HashSet<string> hash = new HashSet<string>();
hash.Add("1");
hash.Add("2");
hash.Add("2");
hash.Add("3");
hash.Add("4");
hash.Add("5");

foreach (var item in hash)
{
Console.WriteLine(item);
}

运行效果图：



最后我们看看IL代码：

.method private hidebysig static void  Main(string[] args) cil managed
{
.entrypoint
// 代码大小       139 (0x8b)
.maxstack  2
.locals init ([0] class [System.Core]System.Collections.Generic.HashSet`1<string> hash,
[1] valuetype [System.Core]System.Collections.Generic.HashSet`1/Enumerator<string> V_1,
[2] string item)
IL_0000:  nop
IL_0001:  newobj     instance void class [System.Core]System.Collections.Generic.HashSet`1<string>::.ctor()
IL_0006:  stloc.0
IL_0007:  ldloc.0
IL_0008:  ldstr      "1"
IL_000d:  callvirt   instance bool class [System.Core]System.Collections.Generic.HashSet`1<string>::Add(!0)
IL_0012:  pop
IL_0013:  ldloc.0
IL_0014:  ldstr      "2"
IL_0019:  callvirt   instance bool class [System.Core]System.Collections.Generic.HashSet`1<string>::Add(!0)
IL_001e:  pop
IL_001f:  ldloc.0
IL_0020:  ldstr      "2"
IL_0025:  callvirt   instance bool class [System.Core]System.Collections.Generic.HashSet`1<string>::Add(!0)
IL_002a:  pop
IL_002b:  ldloc.0
IL_002c:  ldstr      "3"
IL_0031:  callvirt   instance bool class [System.Core]System.Collections.Generic.HashSet`1<string>::Add(!0)
IL_0036:  pop
IL_0037:  ldloc.0
IL_0038:  ldstr      "4"
IL_003d:  callvirt   instance bool class [System.Core]System.Collections.Generic.HashSet`1<string>::Add(!0)
IL_0042:  pop
IL_0043:  ldloc.0
IL_0044:  ldstr      "5"
IL_0049:  callvirt   instance bool class [System.Core]System.Collections.Generic.HashSet`1<string>::Add(!0)
IL_004e:  pop
IL_004f:  nop
IL_0050:  ldloc.0
IL_0051:  callvirt   instance valuetype [System.Core]System.Collections.Generic.HashSet`1/Enumerator<!0> class [System.Core]System.Collections.Generic.HashSet`1<string>::GetEnumerator()
IL_0056:  stloc.1
.try
{
IL_0057:  br.s       IL_006a
IL_0059:  ldloca.s   V_1
IL_005b:  call       instance !0 valuetype [System.Core]System.Collections.Generic.HashSet`1/Enumerator<string>::get_Current()
IL_0060:  stloc.2
IL_0061:  nop
IL_0062:  ldloc.2
IL_0063:  call       void [mscorlib]System.Console::WriteLine(string)
IL_0068:  nop
IL_0069:  nop
IL_006a:  ldloca.s   V_1
IL_006c:  call       instance bool valuetype [System.Core]System.Collections.Generic.HashSet`1/Enumerator<string>::MoveNext()
IL_0071:  brtrue.s   IL_0059
IL_0073:  leave.s    IL_0084
}  // end .try
finally
{
IL_0075:  ldloca.s   V_1
IL_0077:  constrained. valuetype [System.Core]System.Collections.Generic.HashSet`1/Enumerator<string>
IL_007d:  callvirt   instance void [mscorlib]System.IDisposable::Dispose()
IL_0082:  nop
IL_0083:  endfinally
}  // end handler
IL_0084:  call       int32 [mscorlib]System.Console::Read()
IL_0089:  pop
IL_008a:  ret
} // end of method Program::Main

好了，总结一下吧，如果不涉及到修改集合元素，一般情况下使用Foreach比较好，在我测试的代码中，数组和String数据类型不会生成try-finally代码块，里面有一些出入而已，但是这并不影响Foreach的使用。所以我们在具体的编码过程中，尽量多的使用Foreach吧，没关系。