vb.net - Variable declared inside a for loop. How do I make this to a compile time error?

Question

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?

Answer 1

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.