Visual Basic: Variable that references another variable changes when original vairable changes

Question

Example:

Dim a As Integer = 1
Dim b As Integer = a
Console.WriteLine(b)
a = 0
Console.WriteLine(b)

Output:

1
0

For some reason changing a also changes b even though I am not actually changing b. Why does this happen, and how do I get around it.

Answer 1

Integer is a Value type so when you assign 'a' to 'b' a COPY is made. Further changes to one or the other will only affect that particular copy in its own variable:

Module Module1

    Sub Main()
        Dim a As Integer = 1
        Dim b As Integer = a

        Console.WriteLine("Initial State:")
        Console.WriteLine("a = " & a)
        Console.WriteLine("b = " & b)

        a = 0
        Console.WriteLine("After changing 'a':")
        Console.WriteLine("a = " & a)
        Console.WriteLine("b = " & b)

        Console.Write("Press Enter to Quit...")
        Console.ReadLine()
    End Sub

End Module

If we are talking about Reference types, however, then it's a different story.

For example, that Integer might be encapsulated in a Class, and Classes are Reference types:

Module Module1

    Public Class Data
        Public I As Integer
    End Class

    Sub Main()
        Dim a As New Data
        a.I = 1
        Dim b As Data = a

        Console.WriteLine("Initial State:")
        Console.WriteLine("a.I = " & a.I)
        Console.WriteLine("b.I = " & b.I)

        a.I = 0
        Console.WriteLine("After changing 'a.I':")
        Console.WriteLine("a.I = " & a.I)
        Console.WriteLine("b.I = " & b.I)

        Console.Write("Press Enter to Quit...")
        Console.ReadLine()
    End Sub

End Module

In this second example assigning 'a' to 'b' makes 'b' a REFERENCE to the same instance of Data() that 'a' points to. Therefore changes to the 'I' variable from either 'a' or 'b' will be seen by both, since they both point to the same instance of Data().

See: "Value Types and Reference Types"