在for循环中声明的变量。我如何使这个编译时错误？ [英] Variable declared inside a for loop. How do I make this to a compile time error?

问题描述

今天我调查了我们软件中的一个逻辑错误，并发现这与VB.NET线程变量在循环中的方式有​​关。

下面的代码：

 昏暗的数字作为新列表（整数）从{1，2，3，4，5} 
 For每个数字As Integer数字
 
 Dim isEven作为布尔
 
如果数字Mod 2 = 0那么
 isEven = True 
 End If 
 
如果isEven Then 
 Console.WriteLine（number.ToString（）&is Even）
 else 
 Console.WriteLine（number.ToString（）& amp; ;is Odd）
 End If 
 
 Next 
  

产生以下结果：

$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $'是Even
5是Even

问题在于 isEven 是已声明但未分配的。
在这个特定情况下，将 dim isEven作为布尔值= false 是正确的，但是我还没有做到这一点。

在VB.NET中，在for循环中声明的变量为下一次操作保留了它的值。这是由设计： http://social.msdn.microsoft但是这对于程序员来说也是一个危险的陷阱。

不过，直到现在，我还没有意识到这个问题/行为。到现在。
我们的大部分代码都是C＃，不允许使用未初始化的变量，所以没有问题。但是我们有一些在VB.NET中编写的遗留代码，我们必须支持。

我不要以为我们的开发团队中的任何人都曾经有过这样的目的。如果我明确地想要在一个for循环中迭代一个变量，我声明它在范围之外。

所以最好的办法是产生一个警告，甚至是一个警告在这个特定情况下的错误。
但即使使用Option Explicit / Option Strict，也不会产生警告/错误。

有没有办法让这个编译时错误或者是一个如何用FxCop来检查这个问题？

解决方案

我不认为我们的开发团队曾经有过这样的目的。如果我明确地想要在for循环中迭代共享一个变量，我将它声明在范围之外。

我想整个点在循环中声明一个变量是明确地限制它的范围到那个块。为了使这个编译时间错误将从语言中删除块级作用域。虽然方法级别范围是合理的，但是块级范围的重要性无疑也是可以的。我不认为你可以很容易地从语言中删除，而不用引入一些新的句法来使用它。在这一点上，你正在进入重新设计VB.NET的领域 - 我不确定有一个简单的方法来做到这一点。

Today I investigated a logical bug in our software and figured out that this is related to the way VB.NET thread variables inside a loop.

Let's say I have the following code:

    Dim numbers As New List(Of Integer) From {1, 2, 3, 4, 5}
    For Each number As Integer In numbers

        Dim isEven As Boolean

        If number Mod 2 = 0 Then
            isEven = True
        End If

        If isEven Then
            Console.WriteLine(number.ToString() & " is Even")
        Else
            Console.WriteLine(number.ToString() & " is Odd")
        End If

    Next

produces the following output

1 is Odd
2 is Even
3 is Even
4 is Even
5 is Even

The problem is that isEven is declared but not assigned. In this specific case, it would be correct to write dim isEven as Boolean = false but I haven't done this.

In VB.NET, a variable that is declared inside a for loop keeps its value for the next itaration. This is by design: http://social.msdn.microsoft.com/Forums/en/vblanguage/thread/c9cb4c22-d40b-49ff-b535-19d47e4db38d but this is also dangerous pitfall for programmers.

However, until now, I haven't been aware of this problem/behaviour. Until now. Most of our code base is C# anyway, which doesn't allow the use of an uninitialized variable, so there is no problem.

But we have some legacy code that is written in VB.NET that we have to support.

I don't think that anyone of our dev team has ever used this with purpose. If I explicitly want to share a variable over iterations inside a for loop, I declare it outside the scope.

So the best thing would be to generate a warning or even an error in this specific case. But even with Option Explicit / Option Strict this does not generate a warning / an error.

Is there a way make this a compile time error or maybe a way to check this with FxCop?

解决方案

I don't think that anyone of our dev team has ever used this with purpose. If I explicitly want to share a variable over iterations inside a for loop, I declare it outside the scope.

I suppose the whole point of declaring a variable inside the loop is to explicitly restrict its scope to that block, though. To make this a compile time error would remove block-level scope from the language. While there are certainly cases where method level scope is sensible there can no doubt also be a case made for the importance of block level scope. I don't think you can easily excise this from the language without introducing some new syntactic method for employing it. At this point you are entering the realm of redesigning VB.NET - I'm not sure that there is an easy way to do this.

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