VB.NET For 循环函数作用域与块作用域 [英] VB.NET For loop function scope vs block scope

问题描述

鉴于下面的代码示例，变量 currOn 似乎是在循环之外引发的，并且只实例化了一次.例如，假设 itemList 中有三个项目，并且在第二次迭代 SomeFunctionThatDoesSomeStuff 中返回 true.currOn 的值将是 true.在第三次迭代中，我会认为给定的 VB.NET 是一种块作用域语言，currOn 将被重新实例化并默认为 false；但是，我看到它仍然是 true，因此无论 sOn 的值如何，都不会在进一步的迭代中更新.这似乎是 javascript 的函数范围，其中 currOn 的声明将被拉到循环之外.有人知道这里发生了什么吗?

Given the code sample below, it seems that the variable currOn is raised up outside of the loop and only instantiated once. For example, say there are three items in itemList and on the second iteration SomeFunctionThatDoesSomeStuff returns true. The value of currOn will then be true. On the third iteration, I would have thought that given VB.NET is a block scope language that currOn would be re-instantiated and default to false; however, I see that it remains true and therefore no matter the value of sOn, does not get updated in further iterations. It seems like javascript's function scope where the declaration of currOn would be pulled up outside the loop. Anyone know what's going on here?

        For Each item As MyItem In itemList
            Dim currOn As Boolean
            Dim sOn As Boolean = SomeFunctionThatDoesStuff(item)
            currOn = currOn OrElse sOn

            Debug.Print(String.Format("the value of currOn is: {0}", currOn))
        Next

再举个例子，明确设置 currOn = false 每次迭代似乎都像我预期的那样工作.

As a further example, explicitly setting currOn = false every iteration seems to work as I would have expected the above to work.

            For Each item As MyItem In itemList

                Dim currOn As Boolean = False
                Dim sOn As Boolean = SomeFunctionThatDoesStuff()
                currOn = currOn OrElse sOn

                Debug.Print(String.Format("the value of currOn is: {0}", currOn))
            Next

推荐答案

在 For 循环中声明变量时，您就是在块范围内声明它.在块内声明的对象只能在该块内访问，但在整个过程中都有生命周期.

When declaring a variable within a For loop you are declaring it within a block scope. The objects that have been declared within a block will only be accessible within that block but will have a lifetime across the entire procedure.

来自 MSDN:

即使变量的作用域限于一个块，它的生命周期仍然是整个过程的生命周期.如果在该过程中多次输入块，则每个块变量都会保留其先前的值.在这种情况下，为避免出现意外结果，明智的做法是在块的开头初始化块变量.

Even if the scope of a variable is limited to a block, its lifetime is still that of the entire procedure. If you enter the block more than once during the procedure, each block variable retains its previous value. To avoid unexpected results in such a case, it is wise to initialize block variables at the beginning of the block.

MSDN 链接:https://msdn.microsoft.com/en-us/library/1t0wsc67.aspx

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