ARTICLE

Singleton Pattern

Posted by rajesh mg in Articles | Design & Architecture on February 07, 2007

Tags: .NET Framework, ASP.NET, Singelton Pattern, SingleTon, VB.NET

The singleton design pattern is designed to restrict instantiation of a class to one (or a few) objects. This article talks about building .NET applications using singleton pattern.

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Singleton pattern:

In software engineering, the singleton design pattern is designed to restrict instantiation of a class to one (or a few) objects. This is useful when exactly one object is needed to coordinate actions across the system. Sometimes it is generalized to systems that operate more efficiently when only one or a few objects exist.

The singleton pattern is implemented by creating a class with a method that creates a new instance of the object if one does not exist. If an instance already exists, it simply returns a reference to that object. To make sure that the object cannot be instantiated any other way, the constructor is made private.

Example implementation

I have created a class called SingleTon. the constructor of this class is made as private to make sure the object cannot be instantiated any other way. In this constructor I created three objects of Job class (which is listed later in this article) add added to a job queue.

The function GetObject returns an instance of SingleTon class. If it exists it returns the existing object reference.

The function GetJOB returns a job object based on the index it receives.

The function Release decrements the count of reference.

Public Class SingleTon

#Region "declaration"

Private Shared instance As SingleTon

Private Shared m_nNofReference As Integer

Private m_ArrJob As ArrayList

#End Region

''' <summary>

'''

''' </summary>

Private Sub New()

' TODO: Add constructor logic here

m_nNofReference = 0

m_ArrJob = New ArrayList()

'''

'''Add three job types in the queue for

'''the time being

'''

m_ArrJob.Add(New Job(1,"INSERT INTO TABLE"))

m_ArrJob.Add(New Job(2,"DELETE FROM TABLE"))

m_ArrJob.Add(New Job(2,"UPDATE TABLE"))

End Sub

''' <summary>

''' Return the object if it is existing

''' OR create new ONE and return

''' </summary>

Public Shared Function GetObject() As SingleTon

If instance Is Nothing Then

instance = New SingleTon()

End If

m_nNofReference += 1

Return instance

End Function

''' <summary>

'''

''' </summary>

Public Shared Sub Release()

m_nNofReference -= 1

End Sub

''' <summary>

'''

''' </summary>

''' <param name="nJob"></param>

''' <returns></returns>

Public Function GetJOB(ByVal nJob As Integer) As Object

If nJob < m_ArrJob.Count Then

Return m_ArrJob(nJob)

Else

Return Nothing

End If

End Function

End Class

The following is the implementation of class Job.

Public Class Job

#Region "declaration"

Private m_nJobID As Integer

Private m_strJobName As String

#End Region

''' <summary>

'''

''' </summary>

''' <param name="nJID"></param>

''' <param name="strJName"></param>

Public Sub New(ByVal nJID As Integer, ByVal strJName As String)

' TODO: Add constructor logic here

m_nJobID = nJID

m_strJobName = strJName

End Sub

''' <summary>

'''

''' </summary>

Public ReadOnly Property JOBID() As Integer

Get

Return m_nJobID

End Get

End Property

''' <summary>

'''

''' </summary>

Public ReadOnly Property JOBNAME() As String

Get

Return m_strJobName

End Get

End Property

End Class

The following statement can be used to check functionality of the above code

Private objSingleTon As SingleTon = SingleTon.GetObjesct

Private objJob As Job = CType(objSingleTon.GetJOB(0), Job)
Dim objSingleTon2 As SingleTon = SingleTon.GetObject()

objJob = CType(objSingleTon.GetJOB(1), Job)

The first line retrieves an object of SingleTon class. In the second line we are requesting for the SingleTon object once again. But at this time it retrieves the existing SingleTon object reference only.

NOTE: THIS ARTICLE IS CONVERTED FROM C# TO VB.NET USING A CONVERSION TOOL. ORIGINAL ARTICLE CAN BE FOUND ON C# CORNER (http://www.c-sharpcorner.com/).