Preserving a Variable's Values between Procedure Calls in VB.net
By: Steven Holzner Printer Friendly Format
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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