C＃中“ for”和“ foreach”控制结构的性能差异

Z时代
2024-01-10
分类：问答

哪个代码段可以提供更好的性能？以下代码段是用C＃编写的。

1。

for(int counter=0; counter<list.Count; counter++)
{
    list[counter].DoSomething();
}

2。

foreach(MyType current in list)
{
    current.DoSomething();
}

回答：

好吧，部分取决于的确切类型list。它还取决于您使用的确切CLR。

它是否 有意义 ，将取决于您是否在循环中进行任何实际工作。在几乎所有

情况下，对性能的影响都不是很大，但对可读性的影响则有利于foreach循环。

我个人也会使用LINQ来避免出现“ if”：

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

编辑：对于那些声称对List<T>with进行迭代foreach产生与for循环相同代码的人来说，以下证据表明它不会：

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

编译器对 数组的 处理方式不同，foreach基本上将for循环转换为循环，但不转换为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