Structures and Enums

Enums

An enumeration gives names to a fixed set of related values:

Public Enum OrderStatus
    Pending
    Paid
    Shipped
    Cancelled
End Enum

Module Program
    Function Describe(status As OrderStatus) As String
        Select Case status
            Case OrderStatus.Pending : Return "waiting for payment"
            Case OrderStatus.Paid : Return "ready to ship"
            Case OrderStatus.Shipped : Return "on its way"
            Case Else : Return "no longer active"
        End Select
    End Function

    Sub Main()
        Dim s = OrderStatus.Paid
        Console.WriteLine(s)
        Console.WriteLine(CInt(s))
        Console.WriteLine(Describe(s))
        Console.WriteLine([Enum].Parse(Of OrderStatus)("Shipped") = OrderStatus.Shipped)
        For Each value In [Enum].GetValues(Of OrderStatus)()
            Console.Write(value.ToString() & " ")
        Next
        Console.WriteLine()
    End Sub
End Module

1
1
ready to ship
True
Pending Paid Shipped Cancelled

• Enum values are integers underneath, starting at 0.
• The colon : puts two statements on one line — handy for short Case branches.
• [Enum] is in brackets because Enum is also a VB keyword.

Flags

Mark an enum <Flags> and give its values powers of two to combine them:

<Flags>
Public Enum Permissions
    None = 0
    Read = 1
    Write = 2
    Delete = 4
End Enum

Module Program
    Sub Main()
        Dim p = Permissions.Read Or Permissions.Write
        Console.WriteLine(p)
        Console.WriteLine(p.HasFlag(Permissions.Write))
        Console.WriteLine(p.HasFlag(Permissions.Delete))
    End Sub
End Module

3
True
False

<Flags> is an attribute — extra information attached to code, in angle brackets.

Structures

A Structure is like a class, but it's a value type: assigning it copies the whole value.

Public Structure Point
    Public X As Integer
    Public Y As Integer

    Public Sub New(x As Integer, y As Integer)
        Me.X = x
        Me.Y = y
    End Sub

    Public Overrides Function ToString() As String
        Return $"({X}, {Y})"
    End Function
End Structure

Module Program
    Sub Main()
        Dim a As New Point(1, 2)
        Dim b = a          ' a copy
        b.X = 99
        Console.WriteLine($"{a} {b}")
        Console.WriteLine(a.Equals(New Point(1, 2)))
    End Sub
End Module

(1, 2) (99, 2)
True

Changing b didn't change a — compare that with the class example in Classes and Objects. Structures also compare by value with Equals.

Structure or class?

Use a Structure for small, simple values that behave like numbers — points, colors, money amounts — ideally immutable. Use a Class for everything else: objects with identity, behavior, or a lot of data. .NET's own Integer, Date, and Decimal are structures.