State Design Pattern in Visual Basic .NET

Definition

The State Design Pattern allows an object to alter its behavior when its internal state changes. The object will appear to change
its class.

State Design Pattern is one of the Behavioral Design Patterns. Some objects may have several states in its life cycle, and also it has different behavior depending on the state where the object lives. In such cases you can use State Design Patter

First of all we will see the UML diagram for State Design Pattern

Participants

Context

• Provide interface to client
• Keep track of the current state which is an instance of the ConcreteSate

State
This is the interface which defines all the states of the Context class
Concrete State
You can have sub class for each state. Each state has different behaviors

Example

We will see an example to understand State Design Pattern and implement it in VB.NET

A file can have three different states Open,Close,Delete. If file is opened you can not delete it but you can close it. If file is closed you can open it or you can delete it. If file is deleted you can not open it or you can not close it

To implement above example we can use the following UML diagram

Here File is the Context .

Now we will see how we can code this State Design Pattern in VB.NET

File (context) Class

Public Class File

    Private m_state As State
    Public Property State() As State
        Get
            Return m_state
        End Get
        Set(ByVal value As State)
            m_state = value
        End Set
    End Property

    Public Sub New()
        m_state = New CloseState(Me)
    End Sub

    Public Sub Open()
        m_state.Open()

    End Sub


    Public Sub Delete()
        m_state.Delete()

    End Sub

    Public Sub Close()
        m_state.Colse()

    End Sub

End Class

State Interface

Public Interface State
    Sub Open()
    Sub Delete()
    Sub Colse()
End Interface

DeleteState Concrete Class

Public Class DeleteState
    Implements State

    Dim f As File
    Public Sub New(ByVal obj As File)
        f = obj
    End Sub

    Public Sub Colse() Implements State.Colse
        Console.WriteLine("No file exists to close")
    End Sub

    Public Sub Delete() Implements State.Delete
        If (f.State.ToString() = "Delete State") Then
            Console.WriteLine("File already deleted")
        Else
            f.State = New DeleteState(f)
            Console.WriteLine("File deleted")
        End If
    End Sub

    Public Sub Open() Implements State.Open
        Console.WriteLine("No file exists to open")
    End Sub

    Public Overrides Function ToString() As String
        Return "Delete State"
    End Function
End Class

OpenState Concrete Class

Public Class OpenState
    Implements State

    Dim f As File
    Public Sub New(ByVal obj As File)
        f = obj
    End Sub


    Public Sub Colse() Implements State.Colse
        f.State = New CloseState(f)
        Console.WriteLine("File is on Closed State now")
    End Sub

    Public Sub Delete() Implements State.Delete
        Console.WriteLine("You can not delete file because file is on " + f.State.ToString())
    End Sub

    Public Sub Open() Implements State.Open

    End Sub

    Public Overrides Function ToString() As String
        Return "Open State"
    End Function
End Class

CloseState Concrete Class

Public Class CloseState
    Implements State

    Dim f As File
    Public Sub New(ByVal obj As File)
        f = obj
    End Sub


    Public Sub Colse() Implements State.Colse
        Console.WriteLine("File is already closed")
    End Sub

    Public Sub Delete() Implements State.Delete
        Console.WriteLine("File Deleted")
        f.State = New DeleteState(f)
    End Sub

    Public Sub Open() Implements State.Open
        f.State = New OpenState(f)
    End Sub

    Public Overrides Function ToString() As String
        Return "Close State"
    End Function
End Class

Now you can test the above code with following code

Module Module1

    Sub Main()
        Dim f As New File()
        f.Open()
        f.Delete()
        f.Close()
        f.Delete()
        f.Delete()
        Console.ReadLine()
    End Sub

End Module

Finally, you will see the following output

You can not delete file because file is on Open State
File is on Closed State now
File Deleted
File already deleted