.NET 循环完整性 101
对于这个问题我一直很困惑。考虑以下循环：
int [] list = new int [] { 1, 2, 3 };
for (int i=0; i < list.Length; i++) { }
foreach (int i in list) { }
while (list.GetEnumerator().MoveNext()) { } // Yes, yes you wouldn't call GetEnumerator with the while. Actually never tried that.
- 上面的 [list] 是硬编码的。如果在循环迭代时列表被外部更改，会发生什么？
- 如果 [list] 是只读属性，例如
int List{get{return(new int [] {1,2,3});}}会怎样？这会扰乱循环吗？如果不是，它会在每次迭代中创建一个新实例吗？
I've always been confused about this one. Consider the following loops:
int [] list = new int [] { 1, 2, 3 };
for (int i=0; i < list.Length; i++) { }
foreach (int i in list) { }
while (list.GetEnumerator().MoveNext()) { } // Yes, yes you wouldn't call GetEnumerator with the while. Actually never tried that.
- The [list] above is hard-coded. If the list was changed externally while the loop was going through iterations, what would happen?
- What if the [list] was a readonly property e.g.
int List{get{return(new int [] {1,2,3});}}? Would this upset the loop. If not, would it create a new instance in each iteration?
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好吧：
for循环在每次迭代时都会检查list.Length；您实际上并没有在循环内访问list，因此内容无关紧要foreach循环仅使用list来获取迭代器；将其更改为引用不同的列表不会有任何区别，但如果您在结构上修改了列表本身（例如，通过添加一个值，如果这实际上是一个List而不是int[])，这将使迭代器无效从根本上讲，您需要了解数组内容与更改变量引用的对象之间的区别 - 然后给我们一个非常具体的情况解释。不过，一般来说，
foreach循环在获取迭代器时仅直接接触源表达式一次 - 而for循环并没有什么魔力，也没有访问它的频率仅取决于代码 -for循环的条件和“step”部分都会在每次迭代中执行，因此，如果您引用这些部分中任一部分中的变量，您将看到任何改变...Well:
forloop checks againstlist.Lengthon each iteration; you don't actually accesslistwithin the loop, so the contents would be irrevelantforeachloop only useslistto get the iterator; changing it to refer to a different list would make no difference, but if you modified the list itself structurally (e.g. by adding a value, if this were really aList<int>instead of anint[]), that would invalidate the iteratorFundamentally you need to understand the difference between the contents of an array vs changing which object a variable refers to - and then give us a very concrete situation to explain. In general though, a
foreachloop only directly touches the source expression once, when it fetches the iterator - whereas aforloop has no magic in it, and how often it's accessed simply depends on the code - both the condition and the "step" part of theforloop are executed on each iteration, so if you refer to the variable in either of those parts, you'll see any changes...