c# - VB.NET 中的 foreach 是否比 c# 中的更快？

Question

我的同事说，在之前的采访中，他了解到 VB.Net 中的 foreach 比 c# 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，我被告知我需要先阅读 CLR。谷歌搜索 foreach 和 CLR 无法帮助我理解。

有没有人能很好地解释为什么 foreach 在 VB.Net 中比在 C# 中更快？还是我的同事误导了我？

Answer 1

C# 和 VB.Net 在 IL 级别上没有显着差异。在这两个版本之间各处添加了一些额外的 Nop 指令，但实际上并没有改变正在发生的事情。

这是方法:(在 C# 中)

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

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<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
}

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<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
}