Singleton in VB.NET

Ensure a class has only one instance and provide a global point of access to it.

Please download project with this source code from https://github.com/ViacheslavUKR/StandardDisignOopPattern

   1:  ' Singleton Design Pattern.

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

   3:  Class MainApp

   4:      ' Entry point into console application.

   5:      Public Shared Sub Main()

   6:          ' Constructor is protected -- cannot use new

   7:          Dim s1 As Singleton = Singleton.Instance()

   8:          Dim s2 As Singleton = Singleton.Instance()

   9:          ' Test for same instance

  10:          If s1 Is s2 Then

  11:              Console.WriteLine("Objects are the same instance")

  12:          End If

  13:          ' Wait for user

  14:          Console.ReadKey()

  15:      End Sub

  16:  End Class

  17:   

  18:  ' The 'Singleton' class

  19:  Class Singleton

  20:      Private Shared _instance As Singleton

  21:      ' Constructor is 'protected'

  22:      Protected Sub New()

  23:      End Sub

  24:   

  25:      Public Shared Function Instance() As Singleton

  26:          ' Uses lazy initialization.

  27:          ' Note: this is not thread safe.

  28:          If _instance Is Nothing Then

  29:              _instance = New Singleton()

  30:          End If

  31:          Return _instance

  32:      End Function

  33:  End Class

• Abstract Factory in VB.NET - Creates an instance of several families of classes.
• Builder in VB.NET - Separates object construction from its representation.
• Factory Method in VB.NET - Creates an instance of several derived classes.
• Prototype in VB.NET - A fully initialized instance to be copied or cloned.
• Singleton in VB.NET - A class of which only a single instance can exist.

• Adapter in VB.NET - Match interfaces of different classes
• Bridge in VB.NET - Separates an object’s interface from its implementation
• Composite in VB.NET - A tree structure of simple and composite objects
• Decorator in VB.NET - Add responsibilities to objects dynamically
• Facade in VB.NET - A single class that represents an entire subsystem
• Flyweight in VB.NET - A fine-grained instance used for efficient sharing
• Proxy in VB.NET - An object representing another object

• Chain of Resp. in VB.NET - A way of passing a request between a chain of objects
• Command in VB.NET - Encapsulate a command request as an object
• Interpreter in VB.NET - A way to include language elements in a program
• Iterator in VB.NET - Sequentially access the elements of a collection
• Mediator in VB.NET - Defines simplified communication between classes
• Memento in VB.NET - Capture and restore an object's internal state
• Observer in VB.NET - A way of notifying change to a number of classes
• State in VB.NET - Alter an object's behavior when its state changes
• Strategy in VB.NET - Encapsulates an algorithm inside a class
• Template Method in VB.NET - Defer the exact steps of an algorithm to a subclass
• Visitor in VB.NET - Defines a new operation to a class without change

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