Patterns

Patterns. The Singleton pattern. Features of implementation in C#

The Singleton pattern. Features of implementation in C#

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1. The structure of the Singleton pattern. Figure

The structure of the Singleton pattern is shown in Figure 1.

 

The structure of Singleton pattern

Figure 1. The structure of Singleton pattern

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2. The structure of the Singleton pattern with reference to C# code. Figure with explanation

 

The structure of the Singleton pattern. Example in C#

Figure 2. The structure of the Singleton pattern. Example in C#

An instance (object) of the Singleton class is obtained by calling the Instance() method. In the Instance() method, an instance is created by calling the class constructor using the new operator. Since the constructor of the Singleton class is implemented with the protected access modifier, it will not be possible to create an instance of this class from the methods of other classes. However, you can expand the capabilities of this class.

If the task allows the possibility of extending a single class by inherited subclasses, then the constructor is declared as protected. If you want to prohibit the extension of a single class, then the constructor must be declared as private.

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3. C# code that implements the Singleton pattern

 

using System;
using static System.Console;

namespace ConsoleApp9
{
  // Implementation of the Singleton pattern
  class Singleton
  {
    // A static method that returns an instance of the Singleton class.
    // This method can be replaced with the corresponding property.
    public static Singleton Instance()
    {
      if (_instance == null)
      {
        _instance = new Singleton();
        return _instance;
      }
      else
      {
        return null;
      }
    }

    // The class constructor declared as protected, in order to:
    // - prohibit the creation of an instance of the class using the new operator;
    // - it was possible to inherit this class.
    protected Singleton()
    {
    }

    // A static internal variable that holds an instance of the class.
    // This variable can be accessed from the methods of this class.
    // There is no access to the variable from the methods of other classes.
    private static Singleton _instance = null;

    // -------------------------------------------------
    // Other internal fields of the class
    private int d;

    // Property for accessing the d field
    public int D
    {
      get { return d; }
      set { d = value; }
    }

    // Method that outputs the value of the d field
    public void Print(string text)
    {
      WriteLine("------------------");
      WriteLine("{0}. d = {1}", text, d);
    }
  }

  // Client class
  class Program
  {
    static void Main(string[] args)
    {
      // This is the client code.
      // Create a single instance of the Singleton class
      Singleton obj1 = Singleton.Instance();

      // Check obj1 for null
      if (obj1 != null)
      {
        obj1.D = 25;
        obj1.Print("obj1");
      }
      else
        WriteLine("obj1 == null");

      // Trying to create another instance of a class
      Singleton obj2 = Singleton.Instance();

      if (obj2 != null)
      {
        obj2.D = 77;
        obj2.Print("obj2");
      }
      else
        WriteLine("obj2 == null");
    }
  }
}

The result of the program

------------------
obj1. d = 25
obj2 == null

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4. Using a static property instead of a method

C# allows the use of properties instead of methods. Therefore, if desired, in the above code the Instance() method can be replaced with a property. Below is an example of declaring such a property

 

// A static property that returns an instance of the Singleton class
public static Singleton Instance
{
  get
  {
    if (_instance == null)
    {
      _instance = new Singleton();
      return _instance;
    }
    else
      return null;
  }
}

Using a property in a client method could be like the following

...

// This is the client code.
// Use the Instance property
Singleton obj1 = Singleton.Instance;

// Check obj1 for null
if (obj1 != null)
{
  obj1.D = 25;
  obj1.Print("obj1");
}
else
  WriteLine("obj1 == null");

// Trying to create another instance of a class
Singleton obj2 = Singleton.Instance;

if (obj2 != null)
{
  obj2.D = 77;
  obj2.Print("obj2");
}
else
  WriteLine("obj2 == null");

...

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Related topics

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