Composite in VB.NET

   1:      ' Composite Design Pattern.

   2:      ' See description in //www.vb-net.com/ProgramTheory/Composite.htm

   3:      Class MainApp

   4:          ' Entry point into console application.

   5:          Public Shared Sub Main()

   6:              ' Create a tree structure

   7:              Dim root As New Composite("root")

   8:              root.Add(New Leaf("Leaf A"))

   9:              root.Add(New Leaf("Leaf B"))

  10:              Dim comp As New Composite("Composite X")

  11:              comp.Add(New Leaf("Leaf XA"))

  12:              comp.Add(New Leaf("Leaf XB"))

  13:              root.Add(comp)

  14:              root.Add(New Leaf("Leaf C"))

  15:              ' Add and remove a leaf

  16:              Dim leaf As New Leaf("Leaf D")

  17:              root.Add(leaf)

  18:              root.Remove(leaf)

  19:              ' Recursively display tree

  20:              root.Display(1)

  21:              ' Wait for user

  22:              Console.ReadKey()

  23:          End Sub

  24:      End Class

  25:   

  26:      ' The 'Component' abstract class

  27:      MustInherit Class Component

  28:          Protected name As String

  29:          ' Constructor

  30:          Public Sub New(name As String)

  31:              Me.name = name

  32:          End Sub

  33:          Public MustOverride Sub Add(c As Component)

  34:          Public MustOverride Sub Remove(c As Component)

  35:          Public MustOverride Sub Display(depth As Integer)

  36:      End Class

  37:   

  38:      ' The 'Composite' class

  39:      Class Composite

  40:          Inherits Component

  41:          Private _children As New List(Of Component)()

  42:          ' Constructor

  43:          Public Sub New(name As String)

  44:              MyBase.New(name)

  45:          End Sub

  46:          Public Overrides Sub Add(component As Component)

  47:              _children.Add(component)

  48:          End Sub

  49:          Public Overrides Sub Remove(component As Component)

  50:              _children.Remove(component)

  51:          End Sub

  52:          Public Overrides Sub Display(depth As Integer)

  53:              Console.WriteLine(New [String]("-"c, depth) & name)

  54:              ' Recursively display child nodes

  55:              For Each component As Component In _children

  56:                  component.Display(depth + 2)

  57:              Next

  58:          End Sub

  59:      End Class

  60:   

  61:      ' The 'Leaf' class

  62:      Class Leaf

  63:          Inherits Component

  64:          ' Constructor

  65:          Public Sub New(name As String)

  66:              MyBase.New(name)

  67:          End Sub

  68:          Public Overrides Sub Add(c As Component)

  69:              Console.WriteLine("Cannot add to a leaf")

  70:          End Sub

  71:          Public Overrides Sub Remove(c As Component)

  72:              Console.WriteLine("Cannot remove from a leaf")

  73:          End Sub

  74:          Public Overrides Sub Display(depth As Integer)

  75:              Console.WriteLine(New [String]("-"c, depth) & name)

  76:          End Sub

  77:      End Class