State in VB.NET

   1:      ' State Design Pattern.

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

   3:      Class MainApp

   4:          ' Entry point into console application.

   5:          Public Shared Sub Main()

   6:              ' Setup context in a state

   7:              Dim c As New Context(New ConcreteStateA())

   8:              ' Issue requests, which toggles state

   9:              c.Request()

  10:              c.Request()

  11:              c.Request()

  12:              c.Request()

  13:              ' Wait for user

  14:              Console.ReadKey()

  15:          End Sub

  16:      End Class

  17:   

  18:      ' The 'State' abstract class

  19:      MustInherit Class State

  20:          Public MustOverride Sub Handle(context As Context)

  21:      End Class

  22:   

  23:      ' A 'ConcreteState' class

  24:      Class ConcreteStateA

  25:          Inherits State

  26:          Public Overrides Sub Handle(context As Context)

  27:              context.State = New ConcreteStateB()

  28:          End Sub

  29:      End Class

  30:   

  31:      ' A 'ConcreteState' class

  32:      Class ConcreteStateB

  33:          Inherits State

  34:          Public Overrides Sub Handle(context As Context)

  35:              context.State = New ConcreteStateA()

  36:          End Sub

  37:      End Class

  38:   

  39:      ' The 'Context' class

  40:      Class Context

  41:          Private _state As State

  42:          ' Constructor

  43:          Public Sub New(state As State)

  44:              Me.State = state

  45:          End Sub

  46:          ' Gets or sets the state

  47:          Public Property State() As State

  48:              Get

  49:                  Return _state

  50:              End Get

  51:              Set(value As State)

  52:                  _state = value

  53:                  Console.WriteLine("State: " + _state.[GetType]().Name)

  54:              End Set

  55:          End Property

  56:          Public Sub Request()

  57:              _state.Handle(Me)

  58:          End Sub

  59:      End Class