我的同事说,在以前的一次面试中,他得知VB.Net中的foreach比c#中的foreach更快。他被告知这是因为两者具有不同的CLR实现。
从C ++的角度来看,我很好奇这是为什么,我被告知需要先了解CLR。在谷歌上搜索foreach和CLR并不能帮助我理解。
是否有人能够对为什么VB.Net中的foreach比c#中的foreach更快给出一个好的解释?或者我的同事被误导了吗?
我的同事说,在以前的一次面试中,他得知VB.Net中的foreach比c#中的foreach更快。他被告知这是因为两者具有不同的CLR实现。
从C ++的角度来看,我很好奇这是为什么,我被告知需要先了解CLR。在谷歌上搜索foreach和CLR并不能帮助我理解。
是否有人能够对为什么VB.Net中的foreach比c#中的foreach更快给出一个好的解释?或者我的同事被误导了吗?
C#和VB.Net在IL级别上没有显着的区别。在这两个版本之间会有一些额外的Nop指令,但并没有真正改变正在发生的事情。
这是方法:(用C#编写)
public void TestForEach()
{
List<string> items = new List<string> { "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
以下是C#版本的IL代码:
.method public hidebysig instance void TestForEach() cil managed
{
.maxstack 2
.locals init (
[0] class [mscorlib]System.Collections.Generic.List`1<string> items,
[1] string item,
[2] class [mscorlib]System.Collections.Generic.List`1<string> <>g__initLocal3,
[3] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<string> CS$5$0000,
[4] bool CS$4$0001)
L_0000: nop
L_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1<string>::.ctor()
L_0006: stloc.2
L_0007: ldloc.2
L_0008: ldstr "one"
L_000d: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_0012: nop
L_0013: ldloc.2
L_0014: ldstr "two"
L_0019: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_001e: nop
L_001f: ldloc.2
L_0020: ldstr "three"
L_0025: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::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<!0> [mscorlib]System.Collections.Generic.List`1<string>::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<string>::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<string>::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$0000
L_0059: constrained [mscorlib]System.Collections.Generic.List`1/Enumerator<string>
L_005f: callvirt instance void [mscorlib]System.IDisposable::Dispose()
L_0064: nop
L_0065: endfinally
L_0066: nop
L_0067: ret
.try L_0035 to L_0057 finally handler L_0057 to L_0066
}
以下是VB.Net版本的中间语言(IL):
.method public instance void TestForEach() cil managed
{
.maxstack 2
.locals init (
[0] class [mscorlib]System.Collections.Generic.List`1<string> items,
[1] string item,
[2] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<string> VB$t_struct$L0,
[3] bool VB$CG$t_bool$S0)
L_0000: nop
L_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1<string>::.ctor()
L_0006: stloc.0
L_0007: ldloc.0
L_0008: ldstr "one"
L_000d: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_0012: nop
L_0013: ldloc.0
L_0014: ldstr "two"
L_0019: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_001e: nop
L_001f: ldloc.0
L_0020: ldstr "three"
L_0025: callvirt instance void [mscorlib]System.Collections.Generic.List`1<string>::Add(!0)
L_002a: nop
L_002b: nop
L_002c: ldloc.0
L_002d: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> [mscorlib]System.Collections.Generic.List`1<string>::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<string>::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<string>::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<string>
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
}
L_0048: ldloca.s CS$5$0001
L_004a: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator<string>::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<string>::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$L0
L_0045: call instance !0 [mscorlib]System.Collections.Generic.List`1/Enumerator<string>::get_Current()
L_004a: stloc.s item
L_004c: ldloc.s item
L_004e: call void [mscorlib]System.Console::WriteLine(string)
L_0053: nop
L_0054: nop
L_0055: ldloca.s VB$t_struct$L0
L_0057: call instance bool [mscorlib]System.Collections.Generic.List`1/Enumerator<string>::MoveNext()
L_005c: stloc.s VB$CG$t_bool$S0
L_005e: ldloc.s VB$CG$t_bool$S0
L_0060: brtrue.s L_0043
对于一个简单的循环遍历字符串数组,这是VB生成的IL代码:
L_0007: ldloc.0
L_0008: stloc.3
L_0009: ldc.i4.0
L_000a: stloc.2
L_000b: br.s L_0019
L_000d: ldloc.3
L_000e: ldloc.2
L_000f: ldelem.ref
L_0010: stloc.1
...
L_0015: ldloc.2
L_0016: ldc.i4.1
L_0017: add.ovf
L_0018: stloc.2
L_0019: ldloc.2
L_001a: ldloc.3
L_001b: ldlen
L_001c: conv.ovf.i4
L_001d: blt.s L_000d
以下是C#生成的IL代码:
L_0007: ldloc.0
L_0008: stloc.2
L_0009: ldc.i4.0
L_000a: stloc.3
L_000b: br.s L_0019
L_000d: ldloc.2
L_000e: ldloc.3
L_000f: ldelem.ref
L_0010: stloc.1
...
L_0015: ldloc.3
L_0016: ldc.i4.1
L_0017: add
L_0018: stloc.3
L_0019: ldloc.3
L_001a: ldloc.2
L_001b: ldlen
L_001c: conv.i4
L_001d: blt.s L_000d
add.ovf
和conv.ovf.i4
代替add
和conv.i4
。这意味着VB代码执行了两个额外的溢出检查,可能会稍微慢一些。VB.NET和C#都使用相同的CLR。我刚刚使用了以下代码进行了快速的基准测试:
C#版本:
static void Main(string[] args)
{
List<string> myList = new List<string>();
for(int i = 0; i < 500000; i++)
{
myList.Add(i.ToString());
}
DateTime st = DateTime.Now;
foreach(string s in myList)
{
Console.WriteLine(s);
}
DateTime et = DateTime.Now;
Console.WriteLine(et - st);
Console.ReadLine();
}
VB.NET版本:
Module Module1
Sub Main()
Dim myList As List(Of String) = New List(Of String)
For i = 1 To 500000
myList.Add(i)
Next
Dim st, et
st = DateTime.Now
For Each s As String In myList
Console.WriteLine(s)
Next
et = DateTime.Now
Console.WriteLine(et - st)
Console.ReadLine()
End Sub
End Module
在发布版本中(最重要的版本),执行500000次迭代,C# 代码稍微快一点,但只是一点点。
调试版本:
C# - 1分40秒457毫秒 VB.NET - 1分42秒022毫秒
发布版本:
C# - 0分56秒179毫秒 VB.NET - 0分56秒327毫秒
你应该进行一个实验。使用 (棒极了的) .NET Reflector,在每种语言中构建一个简单的测试案例,并查看生成的 MSIL 是否相同。