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Tip of the Day
Language: VB4,VB5,VB6
Expertise: Intermediate
Apr 20, 1999

ByVal with object references

When you write a procedure and you want to be sure that the procedure does not affect the value of any variable passed to it as an argument in the procedure call, you just have to declare that parameter using the ByVal keyword. However, if you declare an object parameter using the ByVal keyword, the procedure is still capable to modify the object's properties. For example:


Sub ClearInvoice (ByVal inv As CInvoice)
  ' the next line of code will change a property of the INV
  ' object, even if it is passed by value
  inv.Total = 0
End Sub
If you used ByVal in your program hoping that it would protect the passed object from being modified by the called procedure you're going to experience several logical errors.

This is not VB a bug, rather it is a consequence of object variables being just pointers to data: even if the procedure cannot change the value of the pointer passed by value, it can still access (and modify) all the properties of the object pointed to by that variable. However, this doesn't mean that ByVal has no effect when used with an object parameter. For instance, the following procedures seem equivalent, but aren't:


Sub DestroyInvoice (inv As CInvoice)
  Set inv = Nothing    ' this works
End Sub

Sub DestroyInvoice2 (ByVal inv As CInvoice)
  Set inv = Nothing       ' this doesn't work
  Set inv = New Invoice   ' neither does this one
End Sub
Francesco Balena
 
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