Programming Tutorials

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

By: Steven Holzner  

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.

Archived Comments