我的同事说,在之前的一次采访中,他了解到VB中的foreach比c#的foreach更快.他被告知这是因为两者都有不同的CLR实现.
从C++的角度来看,我很好奇为什么会这样,而且我被告知我需要先阅读CLR.谷歌搜索foreach和CLR并不能帮助我理解.
有没有人能够很好地解释为什么foreach在VB.Net中比在c#中更快？还是我的同事误导了？
C#和VB.Net之间的IL级别没有显着差异.在这两个版本之间有一些额外的Nop指令,但实际上并没有改变发生的事情.
这是方法:(在C#中)
public void TestForEach()
{
List items = new List { "one", "two", "three" };
foreach (string item in items)
{
Debug.WriteLine(item);
}
}
在VB.Net中:
Public Sub TestForEach
Dim items As List(Of String) = New List(Of String)()
items.Add("one")
items.Add("two")
items.Add("three")
For Each item As string In items
Debug.WriteLine(item)
Next
End Sub
这是C#版本的IL:
.method public hidebysig instance void TestForEach() cil managed
{
.maxstack 2
.locals init (
[0] class [mscorlib]System.Collections.Generic.List`1 items,
[1] string item,
[2] class [mscorlib]System.Collections.Generic.List`1 <>g__initLocal3,
[3] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator CS$5$0000,
[4] bool CS$4$0001)
L_0000: nop
L_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1::.ctor()
L_0006: stloc.2
L_0007: ldloc.2
L_0008: ldstr "one"
L_000d: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)
L_0012: nop
L_0013: ldloc.2
L_0014: ldstr "two"
L_0019: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)
L_001e: nop
L_001f: ldloc.2
L_0020: ldstr "three"
L_0025: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)
L_002a: nop
L_002b: ldloc.2
L_002c: stloc.0
L_002d: nop
L_002e: ldloc.0
L_002f: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator [mscorlib]System.Collections.Generic.List`1::GetEnumerator()
L_0034: stloc.3
L_0035: br.s L_0048
L_0037: ldloca.s CS$5$0000
L_0039: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator::get_Current()
L_003e: stloc.1
L_003f: nop
L_0040: ldloc.1
L_0041: call void [System]System.Diagnostics.Debug::WriteLine(string)
L_0046: nop
L_0047: nop
L_0048: ldloca.s CS$5$0000
L_004a: call instance bool [mscorlib]System.Collections.Generic.List`1/Enumerator::MoveNext()
L_004f: stloc.s CS$4$0001
L_0051: ldloc.s CS$4$0001
L_0053: brtrue.s L_0037
L_0055: leave.s L_0066
L_0057: ldloca.s CS$5$0000
L_0059: constrained [mscorlib]System.Collections.Generic.List`1/Enumerator
L_005f: callvirt instance void [mscorlib]System.IDisposable::Dispose()
L_0064: nop
L_0065: endfinally
L_0066: nop
L_0067: ret
.try L_0035 to L_0057 finally handler L_0057 to L_0066
}
这是VB.Net版本的IL:
.method public instance void TestForEach() cil managed
{
.maxstack 2
.locals init (
[0] class [mscorlib]System.Collections.Generic.List`1 items,
[1] string item,
[2] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator VB$t_struct$L0,
[3] bool VB$CG$t_bool$S0)
L_0000: nop
L_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1::.ctor()
L_0006: stloc.0
L_0007: ldloc.0
L_0008: ldstr "one"
L_000d: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)
L_0012: nop
L_0013: ldloc.0
L_0014: ldstr "two"
L_0019: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)
L_001e: nop
L_001f: ldloc.0
L_0020: ldstr "three"
L_0025: callvirt instance void [mscorlib]System.Collections.Generic.List`1::Add(!0)
L_002a: nop
L_002b: nop
L_002c: ldloc.0
L_002d: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator [mscorlib]System.Collections.Generic.List`1::GetEnumerator()
L_0032: stloc.2
L_0033: br.s L_0045
L_0035: ldloca.s VB$t_struct$L0
L_0037: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator::get_Current()
L_003c: stloc.1
L_003d: ldloc.1
L_003e: call void [System]System.Diagnostics.Debug::WriteLine(string)
L_0043: nop
L_0044: nop
L_0045: ldloca.s VB$t_struct$L0
L_0047: call instance bool [mscorlib]System.Collections.Generic.List`1/Enumerator::MoveNext()
L_004c: stloc.3
L_004d: ldloc.3
L_004e: brtrue.s L_0035
L_0050: nop
L_0051: leave.s L_0062
L_0053: ldloca.s VB$t_struct$L0
L_0055: constrained [mscorlib]System.Collections.Generic.List`1/Enumerator
L_005b: callvirt instance void [mscorlib]System.IDisposable::Dispose()
L_0060: nop
L_0061: endfinally
L_0062: nop
L_0063: ret
.try L_002c to L_0053 finally handler L_0053 to L_0062
}
我对这种说法有点怀疑.foreach构造对两种语言的工作方式相同,因为它从托管对象获取IEnumerator并在其上调用MoveNext().无论原始代码是用VB.NET还是用c#编写都没关系,它们都编译成同样的东西.
在我的测试时间中,VB.NET和c#中的相同foreach循环在很长的迭代中间隔不超过1%.
C#:
L_0048: ldloca.s CS$5$0001 L_004a: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator::get_Current() L_004f: stloc.3 L_0050: nop L_0051: ldloc.3 L_0052: call void [mscorlib]System.Console::WriteLine(string) L_0057: nop L_0058: nop L_0059: ldloca.s CS$5$0001 L_005b: call instance bool [mscorlib]System.Collections.Generic.List`1/Enumerator ::MoveNext() L_0060: stloc.s CS$4$0000 L_0062: ldloc.s CS$4$0000 L_0064: brtrue.s L_0048
VB.NET:
L_0043: ldloca.s VB$t_struct$L0 L_0045: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator::get_Current() L_004a: stloc.s item L_004c: ldloc.s item L_004e: call void [mscorlib]System.Console::WriteLine(string) L_0053: nop L_0054: nop L_0055: ldloca.s VB$t_struct$L0 L_0057: call instance bool [mscorlib]System.Collections.Generic.List`1/Enumerator ::MoveNext() L_005c: stloc.s VB$CG$t_bool$S0 L_005e: ldloc.s VB$CG$t_bool$S0 L_0060: brtrue.s L_0043
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