哪个代码片段会提供更好的性能？以下代码段是用 C# 编写的。

1.

for(int tempCount=0;tempCount<list.count;tempcount++)
{
    if(list[tempCount].value==value)
    {
        // Some code.
    }
}

foreach(object row in list)
{
    if(row.value==value)
    {
        //Some coding
    }
}

最佳答案

嗯，这部分取决于 list 的确切类型.它还将取决于您使用的确切 CLR。​​

它是否以任何方式重要将取决于您是否在循环中做任何实际工作。在几乎所有 情况下，性能差异并不显着，但可读性差异有利于 foreach循环。

我个人也会使用 LINQ 来避免“if”:

foreach (var item in list.Where(condition))
{
}

编辑:对于那些声称迭代 List<T> 的人与 foreach生成与 for 相同的代码循环，这里有证据表明它没有:

static void IterateOverList(List<object> list)
{
    foreach (object o in list)
    {
        Console.WriteLine(o);
    }
}

产生 IL:

.method private hidebysig static void  IterateOverList(class [mscorlib]System.Collections.Generic.List`1<object> list) cil managed
{
  // Code size       49 (0x31)
  .maxstack  1
  .locals init (object V_0,
           valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<object> V_1)
  IL_0000:  ldarg.0
  IL_0001:  callvirt   instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> class [mscorlib]System.Collections.Generic.List`1<object>::GetEnumerator()
  IL_0006:  stloc.1
  .try
  {
    IL_0007:  br.s       IL_0017
    IL_0009:  ldloca.s   V_1
    IL_000b:  call       instance !0 valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<object>::get_Current()
    IL_0010:  stloc.0
    IL_0011:  ldloc.0
    IL_0012:  call       void [mscorlib]System.Console::WriteLine(object)
    IL_0017:  ldloca.s   V_1
    IL_0019:  call       instance bool valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<object>::MoveNext()
    IL_001e:  brtrue.s   IL_0009
    IL_0020:  leave.s    IL_0030
  }  // end .try
  finally
  {
    IL_0022:  ldloca.s   V_1
    IL_0024:  constrained. valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<object>
    IL_002a:  callvirt   instance void [mscorlib]System.IDisposable::Dispose()
    IL_002f:  endfinally
  }  // end handler
  IL_0030:  ret
} // end of method Test::IterateOverList

编译器以不同的方式对待数组，将foreach基本上循环到 for循环，但不是 List<T> .这是数组的等效代码:

static void IterateOverArray(object[] array)
{
    foreach (object o in array)
    {
        Console.WriteLine(o);
    }
}

// Compiles into...

.method private hidebysig static void  IterateOverArray(object[] 'array') cil managed
{
  // Code size       27 (0x1b)
  .maxstack  2
  .locals init (object V_0,
           object[] V_1,
           int32 V_2)
  IL_0000:  ldarg.0
  IL_0001:  stloc.1
  IL_0002:  ldc.i4.0
  IL_0003:  stloc.2
  IL_0004:  br.s       IL_0014
  IL_0006:  ldloc.1
  IL_0007:  ldloc.2
  IL_0008:  ldelem.ref
  IL_0009:  stloc.0
  IL_000a:  ldloc.0
  IL_000b:  call       void [mscorlib]System.Console::WriteLine(object)
  IL_0010:  ldloc.2
  IL_0011:  ldc.i4.1
  IL_0012:  add
  IL_0013:  stloc.2
  IL_0014:  ldloc.2
  IL_0015:  ldloc.1
  IL_0016:  ldlen
  IL_0017:  conv.i4
  IL_0018:  blt.s      IL_0006
  IL_001a:  ret
} // end of method Test::IterateOverArray

有趣的是，我在 C# 3 规范的任何地方都找不到这个记录...

关于c# - C# 中控制结构 'for' 和 'foreach' 的性能差异，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/1124753/