Question

Today I discovered that something I had assumed about VB.NET for many years was not true (worrying!). I assumed that a variable declared within a loop had a lifetime of the iteration it was declared in, but in fact it seems it has a lifetime of the whole procedure.

For example:

        For i As Integer = 0 To 1
            Dim var1 As Boolean
            Console.WriteLine(var1.ToString())
            var1 = True
            Console.WriteLine(var1.ToString())
        Next
        Console.ReadKey()

I had assumed an output of False, True, False, True but instead it is actually False, True, True, True.

In C# the equivalent code would not compile as you would get a compile time error of Error “Use of unassigned local variable ‘var1′”.

I realise there are many ways to fix this and that best practice would be to declare the variable outside of the loop and reset it at the beginning of every loop through.

I find this behaviour so counter-intuitive to me that I would like at least a compile time warning in VB.NET when/if I do this. (I could also then set this on any projects I already have and get warning that would allow me to check that my assumptions aren’t causing errors).

Does anyone know how/if I can get this to generate a compile time warning in VB.NET? Am I the only one that finds this counter-intuitive?

Answer 1

We’ll have to work on fixing your intuition because getting an error out of the compiler is not an option. It is partially implemented, you can get this warning:

error BC42104: Variable ‘mumble’ is used before it has been assigned a value. A null reference exception could result at runtime.

And elevate it from a warning to an error with Project + Properties, Compile tab. However, as the warning message indicates, this is only supported for reference type references, it won’t budge for a variable of a value type.

Okay, intuition. If the runtime would implement your desired behavior then it would have to allocate a new variable for each iteration of the loop. Which implies that the number of local variables is bounded only by the number of iterations. This is very wasteful and a very easy trigger for StackOverflowException. The JIT compiler doesn’t do this, it re-uses the variable. This happens in C# as well, minus the option of letting you not initialize the value explicitly of course.

Fwiw: I very much agree with you that this is unhelpful behavior. You’ll probably find receptive ears at connect.microsoft.com, post your feature request there and the VB.NET team will see it. There has been strong backing from customers as well as within MSFT to make VB.NET and C# feature comparable. If you post a link to your feedback report then I’ll be happy to vote it up.