Preserving a Variable's Values between Procedure Calls in VB.net

By: Steven Holzner Emailed: 1758 times Printed: 2449 times    

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You've written a function named Counter to keep track of the number of times the user clicks a particular button. Each time through a loop, you call the Counter function to increment the count, but when the program ends, it just displays 0 counts. Why? Let's look at the code:

Module Module1
    Sub Main()
        Dim intLoopIndex As Integer, intValue = 0
        For intLoopIndex = 0 To 4
            intValue = Counter()
        Next intLoopIndex
        System.Console.WriteLine(intValue)
    End Sub

    Function Counter() As Integer
        Dim intCountValue As Integer
        intCountValue += 1
        Return intCountValue
    End Function
End Module

The problem here is that the counter variable, intCountValue, in the Counter function is reinitialized each time the Counter function is called (because a new copy of all the variables local to procedures is allocated each time you call that procedure). The solution is to declare intCountValue as static. This means it will retain its value between calls to the Counter function. Here's the working code:

Module Module1
    Sub Main()
        Dim intLoopIndex As Integer, intValue = 0
        For intLoopIndex = 0 To 4
            intValue = Counter()
        Next intLoopIndex
        System.Console.WriteLine(intValue)
    End Sub

    Function Counter() As Integer
        Static intCountValue As Integer
        intCountValue += 1
        Return intCountValue
    End Function
End Module

Running this code displays a value of 5, as it should.

Tip 

You can also make intCountValue preserve its value between procedure calls by making it a module-level variable-just declare it outside any procedure. But note that you should restrict the scope of your variables as much as possible (to avoid inadvertent conflicts with variables of the same name), so making this variable a static variable in a procedure is probably a better choice.

Note 

You were able to declare a whole function static in VB6, which meant that all the variables in it would be static, but you can't do that in VB .NET.


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