在循环内还是在循环外声明变量更好? [英] Is it better to declare a variable inside or outside a loop?
问题描述
最好这样做:
variable1Type foo;
variable2Type baa;
foreach(var val in list)
{
foo = new Foo( ... );
foo.x = FormatValue(val);
baa = new Baa();
baa.main = foo;
baa.Do();
}
或者:
foreach(var val in list)
{
variable1Type foo = new Foo( ... );
foo.x = FormatValue(val);
variable2Type baa = new Baa();
baa.main = foo;
baa.Do();
}
问题是:1 case 和 2 case 哪个更快?差别是微不足道的吗?在实际应用中是否相同?这可能是一个微优化，但我真的很想知道哪个更好.
The question is: What is faster 1 case or 2 case? Is the difference is insignificant? Is it the same in real applications? This may be a optimization-micro, but I really want know which is better.
推荐答案
性能方面，让我们尝试具体的例子:
Performance-wise, let's try concrete examples:
public void Method1()
{
foreach(int i in Enumerable.Range(0, 10))
{
int x = i * i;
StringBuilder sb = new StringBuilder();
sb.Append(x);
Console.WriteLine(sb);
}
}
public void Method2()
{
int x;
StringBuilder sb;
foreach(int i in Enumerable.Range(0, 10))
{
x = i * i;
sb = new StringBuilder();
sb.Append(x);
Console.WriteLine(sb);
}
}
我特意选择了值类型和引用类型，以防影响事情.现在，他们的 IL:
I deliberately picked both a value-type and a reference-type in case that affects things. Now, the IL for them:
.method public hidebysig instance void Method1() cil managed
{
.maxstack 2
.locals init (
[0] int32 i,
[1] int32 x,
[2] class [mscorlib]System.Text.StringBuilder sb,
[3] class [mscorlib]System.Collections.Generic.IEnumerator`1<int32> enumerator)
L_0000: ldc.i4.0
L_0001: ldc.i4.s 10
L_0003: call class [mscorlib]System.Collections.Generic.IEnumerable`1<int32> [System.Core]System.Linq.Enumerable::Range(int32, int32)
L_0008: callvirt instance class [mscorlib]System.Collections.Generic.IEnumerator`1<!0> [mscorlib]System.Collections.Generic.IEnumerable`1<int32>::GetEnumerator()
L_000d: stloc.3
L_000e: br.s L_002f
L_0010: ldloc.3
L_0011: callvirt instance !0 [mscorlib]System.Collections.Generic.IEnumerator`1<int32>::get_Current()
L_0016: stloc.0
L_0017: ldloc.0
L_0018: ldloc.0
L_0019: mul
L_001a: stloc.1
L_001b: newobj instance void [mscorlib]System.Text.StringBuilder::.ctor()
L_0020: stloc.2
L_0021: ldloc.2
L_0022: ldloc.1
L_0023: callvirt instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::Append(int32)
L_0028: pop
L_0029: ldloc.2
L_002a: call void [mscorlib]System.Console::WriteLine(object)
L_002f: ldloc.3
L_0030: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
L_0035: brtrue.s L_0010
L_0037: leave.s L_0043
L_0039: ldloc.3
L_003a: brfalse.s L_0042
L_003c: ldloc.3
L_003d: callvirt instance void [mscorlib]System.IDisposable::Dispose()
L_0042: endfinally
L_0043: ret
.try L_000e to L_0039 finally handler L_0039 to L_0043
}
.method public hidebysig instance void Method2() cil managed
{
.maxstack 2
.locals init (
[0] int32 x,
[1] class [mscorlib]System.Text.StringBuilder sb,
[2] int32 i,
[3] class [mscorlib]System.Collections.Generic.IEnumerator`1<int32> enumerator)
L_0000: ldc.i4.0
L_0001: ldc.i4.s 10
L_0003: call class [mscorlib]System.Collections.Generic.IEnumerable`1<int32> [System.Core]System.Linq.Enumerable::Range(int32, int32)
L_0008: callvirt instance class [mscorlib]System.Collections.Generic.IEnumerator`1<!0> [mscorlib]System.Collections.Generic.IEnumerable`1<int32>::GetEnumerator()
L_000d: stloc.3
L_000e: br.s L_002f
L_0010: ldloc.3
L_0011: callvirt instance !0 [mscorlib]System.Collections.Generic.IEnumerator`1<int32>::get_Current()
L_0016: stloc.2
L_0017: ldloc.2
L_0018: ldloc.2
L_0019: mul
L_001a: stloc.0
L_001b: newobj instance void [mscorlib]System.Text.StringBuilder::.ctor()
L_0020: stloc.1
L_0021: ldloc.1
L_0022: ldloc.0
L_0023: callvirt instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::Append(int32)
L_0028: pop
L_0029: ldloc.1
L_002a: call void [mscorlib]System.Console::WriteLine(object)
L_002f: ldloc.3
L_0030: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
L_0035: brtrue.s L_0010
L_0037: leave.s L_0043
L_0039: ldloc.3
L_003a: brfalse.s L_0042
L_003c: ldloc.3
L_003d: callvirt instance void [mscorlib]System.IDisposable::Dispose()
L_0042: endfinally
L_0043: ret
.try L_000e to L_0039 finally handler L_0039 to L_0043
}
如您所见，除了编译器碰巧选择的堆栈顺序之外——这也可能是不同的顺序——它绝对没有任何影响.反过来，确实没有任何东西可以让一个人充分利用另一个人没有给予的抖动.
As you can see, apart from the order on the stack the compiler happened to choose - which could just as well have been a different order - it had absolutely no effect. In turn, there really isn't anything that one is giving the jitter to make much use of that the other isn't giving it.
除此之外，还有一种区别.
Other than that, there is one sort-of difference.
在我的 Method1() 中，x 和 sb 的作用域是 foreach，并且无法访问有意或无意地在它之外.
In my Method1(), x and sb are scoped to the foreach, and cannot be accessed either deliberately or accidentally outside of it.
在我的 Method2() 中，x 和 sb 在编译时并不知道在 foreach(编译器不知道foreach 将执行至少一个循环)，因此禁止使用它.
In my Method2(), x and sb are not known at compile-time to be reliably assigned a value within the foreach (the compiler doesn't know the foreach will perform at least one loop), so use of it is forbidden.
到目前为止，没有真正的区别.
So far, no real difference.
我可以但是在foreach之外分配和使用x和/或sb.通常我会说这在大多数情况下可能是糟糕的范围，所以我更喜欢 Method1，但我可能有一些合理的理由想要引用它们(如果它们不是更现实的话)'不可能未分配)，在这种情况下，我会选择 Method2.
I can however assign and use x and/or sb outside of the foreach. As a rule I would say that this is probably poor scoping most of the time, so I'd favour Method1, but I might have some sensible reason to want to refer to them (more realistically if they weren't possibly unassigned), in which case I'd go for Method2.
不过，这是每个代码如何扩展或不扩展的问题，而不是编写的代码的差异.真的，没什么区别.
Still, that's a matter of how the each code can be extended or not, not a difference of the code as written. Really, there's no difference.
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