ByVal, ByRef

In VB.NET a parameter passed ByVal—by value—can be changed in the new method. Its value will not be changed elsewhere. ByRef means the variable location itself is copied.

The ByVal and ByRef keywords change how parameters are received. They can be used to specify exactly how a function uses its arguments.

This program introduces 2 subs other than Main. It shows the Example1 method, which receives an integer parameter ByVal, and the Example2 method, which receives an integer ByRef.

Module Module1
    Sub Main()
        Dim value As Integer = 1

        ' Part 1: the integer value does not change here when passed ByVal.
        Example1(value)
        Console.WriteLine(value)

        ' Part 2: the integer value does change when passed ByRef.
        Example2(value)
        Console.WriteLine(value)
    End Sub

    Sub Example1(ByVal test As Integer)
        test = 10
    End Sub

    Sub Example2(ByRef test As Integer)
        test = 10
    End Sub
End Module
1
10

Objects

The program here used Integers, which are a value type. With object references, you are dealing with a value that indicates a memory location.

Notes, continued

If you access or mutate fields or methods on a copied reference, the changes will be reflected everywhere in the program.

ByVal is often useful for references and also values. ByRef is typically more useful for values because you more often need to change the original values.