This lesson is part of an ongoing tutorial. The first part is here: Create your own Subs in VB .NET
The word ByVal is short for "By Value". What it means is that you are passing a copy of a variable to your Subroutine. You can make changes to the copy and the original will not be altered. If you have 2012/13 of VB NET Express then ByVal is hidden. It's hidden because ByVal is the default when you're passing variables over to a function or Sub.
ByRef is the alternative. This is short for By Reference. This means that you are not handing over a copy of the original variable but pointing to the original variable. Let's see a coding example.
Add a new button the form you created in the previous section. Double click the button and add the following code:
Dim Number1 As Integer
Number1 = 10
You'll get a wiggly line under IncrementVariable(Number1). To get rid of it, add the following Subroutine to your code (again, 2012 users don't need to add the ByVal):
Private Sub IncrementVariable(ByVal Number1 As Integer)
Number1 = Number1 + 1
When you're done, run the programme and click your new button. What answer was displayed in the message box?
It should have been 10. But hold on. Didn't we increment the variable Number1 with this line?
Number1 = Number1 + 1
So Number1 started out having a value of 10. After our Sub got called, we added 1 to Number1. So we should have 11 in the message box, right?
The reason Number1 didn't get incremented was because we specified ByVal in the Sub:
ByVal Number1 As Integer
This means that only a copy of the original variable got passed over. When we incremented the variable, only the copy got 1 added to it. The original stayed the same - 10.
Change the parameter to the this (2012/13 users should add ByRef, as well):
ByRef Number1 As Integer
Run your programme again. Click the button and see what happens.
This time, you should see 11 displayed in the message box. The variable has now been incremented!
It was incremented because we used ByRef. We're referencing the original variable. So when we add 1 to it, the original will change.
The default is ByVal - which means a copy of the original variable. If you need to refer to the original variable, use ByRef.
In the next part, we'll take a look at Functions in VB .NET