Singleton Design Pattern in C#: A Beginner’s Guide with Examples

Last updated on December 30th, 2023 at 10:06 pm

In this article, We will explore the Singleton design pattern in C#, its use cases, and various ways to implement singleton class.

What is the Singleton Design Pattern in C#?

A Singleton pattern is a Creational design pattern that ensures a class has only one instance of itself throughout the lifetime of an application, and it provides a global access point to that instance.

Singleton class is often used for logging, shared objects, caching, thread pool, and database connections.

Here are the key features of a Singleton Pattern in C#:

• Private and parameterless constructor: The class has a special kind of private constructor that doesn’t take any parameters.
• Sealed class: The class is sealed, meaning other classes cannot inherit it. This helps in maintaining the single instance rule.
• Static variable for a single instance: A static variable inside the class holds the reference to the single instance of the class. It makes sure there’s only one copy of the class.
• Public and static method to access the instance: To interact with the singleton class, there’s a public and static method that allows you to easily get the reference to the created instance.

Example: Non-thread-safe Singleton Class

In the following code, the getInstance() property creates a Singleton object when called for the first time and subsequently returns the same object. However, it’s important to note that this example does not implement thread safety. If multiple threads run simultaneously, two Singleton objects can be created.

// This singleton is not thread-safe.
public sealed class Singleton
{
  private static Singleton obj = null;

  // private constructor.
  private Singleton()
  {
  }

  // public static property for creating a single instance.
  public static Singleton getInstance
  {
    get
    {
      if (obj==null)
      {
        obj = new Singleton();
      }
      return obj;
    }
  }
}

Advantages of Singleton Design Pattern

Here are the advantages of using the Singleton Design Pattern in C#:

• Single Instance: The primary advantage is that a singleton ensures only one instance of the class and prevents unnecessary multiple instantiations.
• Global Access Point: Singleton provides a global access point to the single instance, making it easily accessible from any part of the program.
• Lazy Loading: Singleton allows for lazy loading, meaning the instance is created only when it is first requested. It can improve performance by deferring instantiation until necessary.
• Thread Safety: Properly implemented singleton class in C# can ensure thread safety and prevent multiple threads from creating separate instances concurrently.
• Reduced Memory Footprint: Having a single instance reduces the overall memory footprint of the program, especially in scenarios where the class is resource-intensive.
• Improved Control: Singleton provides a centralized point for managing and controlling the instance, allowing for better coordination in scenarios like resource sharing.
• Simplified Global State Management: When a single instance manages a global state, a singleton pattern simplifies handling this shared state across the application.
• Single Point of Control: In situations where having a single point of control is critical, such as managing configuration settings, a singleton pattern is beneficial.

Disadvantages of Singleton Design Pattern

Here are the disadvantages of using the Singleton Design Pattern in C#:

• Tight Coupling: Dependency on a singleton creates tight coupling, making it challenging to replace the singleton with another implementation or perform unit testing.
• Difficult to Subclass: Singleton classes are often sealed, which makes it difficult for the subclasses to extend their functionality through inheritance.
• Concurrency Concerns: Implementing a singleton in a multi-threaded environment requires careful consideration to avoid concurrency issues, and incorrect implementation can lead to race conditions.
• Singleton Pattern Violation: Developers may inadvertently violate the singleton pattern by creating additional instances through methods like reflection or serialization.
• Violates Single Responsibility Principle: The Singleton pattern violates the SRP rule, which says that a class should perform only one specific job. Still, here, Singleton allows the creation of an instance of the class and makes that instance available for the whole application to use.
• Global Dependency: Overusing singletons can lead to a global dependency problem, where multiple parts of the application depend on a shared instance, making it harder to manage and reason about.
• Global Access Abuse: The ease of global access in a singleton can lead to abuse, with developers using it as a convenient way to avoid proper dependency injection and promote poor coding practices.

Example: Thread Safe Singleton Class in C#

using System;
public sealed class Singleton
{
    private static readonly Singleton instance = new Singleton();
    private static object syncRoot = new object();
    private static Lazy<Singleton> lazy = new Lazy<Singleton>(() => new Singleton());
    private static volatile Singleton vInstance;

    private Singleton() { }

    public static Singleton Instance
    {
        get
        {
            return instance;
        }
    }

    public static Singleton Instance_v1
    {
        get
        {
            if (vInstance == null)
            {
                lock (syncRoot)
                {
                    if (vInstance == null)
                        vInstance = new Singleton();
                }
            }
            return vInstance;
        }
    }

    public static Singleton Instance_v2
    {
        get
        {
            return lazy.Value;
        }
    }

    public void Print()
    {
        Console.WriteLine(" Print from Singleton\n");
    }
}
public class Program
{
    public static void Main()
    {
      Singleton Instance =  Singleton.Instance;
      Singleton Instance_v1 = Singleton.Instance_v1;
      Singleton Instance_v2 = Singleton.Instance_v2;

        Console.WriteLine(" Instance using Eager Initialization method:");
        Instance.Print();
        Console.WriteLine(" Instance_v1 using Double Check Locking method:");
        Instance_v1.Print();
        Console.WriteLine(" Instance_v2 using Lazy Initialization method:");
        Instance_v2.Print();

        Console.ReadKey();
    }
}

Output:

Code Explanation:

The above code is a complete working code you can use in your application.

It has the following three different methods to create a singleton class.

• The first Singleton Instance uses the Eager Initialization method, which is created as soon as the class is loaded.
•  The second one is Instance_v1, which uses a double-check locking mechanism to ensure that only one instance of the class is created, even in a multi-threaded environment.
•  The third one is Instance_v2 uses the Lazy Initialization method, which creates the instance only when it is first requested.

Implementing the Singleton Design Pattern in C#

The following are three ways to implement the Singleton design pattern in C#:

01. Singleton Class with Lazy Initialization

The lazy initialization technique delays the creation of a class instance until it is needed. It is helpful in situations where the singleton object requires a lot of resources and is not always necessary to create it.

Example:

// sealed class cannot be inherited
public sealed class LazySingleton
{
    // Static Lazy<T> instance variable used for Lazy initialization of the instance
    private static Lazy<LazySingleton> lazy = new Lazy<LazySingleton>(() => new LazySingleton());

    // Public property provides global access to the instance
    public static LazySingleton Instance { get { return lazy.Value; } }

    // Private constructor prevents instantiation of the class from outside the class
    private LazySingleton()
    {
    }

    //Method that can be used to perform some action on the singleton instance
    public void Print()
    {
        Console.WriteLine("Print from LazySingleton");
    }
}

The Lazy singleton design pattern utilizes the Lazy Initialization technique to only create the class instance when a user or program initially requests it. This approach ensures thread safety in the creation of the singleton pattern.

The Lazy<T> object of the singleton class is employed to postpone the creation of the object until it is required for the first time.

02. Singleton Class With Eager Initialization

The Eager initialization method creates an instance of a class as soon as the class is loaded in memory. This approach is practical when the singleton object is lightweight and must be created regardless of its use.

Example:

/* EagerSingleton class is a singleton class that creates an instance 
 of itself at the time of class loading
 and provides a global point of access to that instance */
using System;
public sealed class EagerSingleton
{
    // private static readonly field that holds the instance of the class
    private static readonly EagerSingleton instance = new EagerSingleton();

    // Static property that returns the instance of the class
    public static EagerSingleton Instance { get { return instance; } }

    // private constructor to prevent instantiation of the class from outside
    private EagerSingleton()
    {
    }

    // Method to demonstrate the functionality of the singleton class
    public void Print()
    {
        Console.WriteLine("Print from Eager Singleton");
    }
}

03. Singleton Class With Double Check Locking

The double-check locking method is a technique that is used to ensure that only one instance of a class is created, even in a multi-threaded environment, by adding a check when multiple threads try to access the object simultaneously. It helps save resources and maintain performance.

Example:

public sealed class DoubleCheckSingleton
{
    //volatile keyword ensures that the instance is created only once even in a multi-threaded environment
    private static volatile DoubleCheckSingleton instance;

    //object used for thread-safe initialization of the instance
    private static object syncRoot = new object();

    // Public property provides global access to the instance
    public static DoubleCheckSingleton Instance
    {
        get
        {
            //Double-checked locking mechanism to ensure that only one instance of the class is created
            if (instance == null)
            {
                lock (syncRoot)
                {
                    if (instance == null)
                        instance = new DoubleCheckSingleton();
                }
            }
            return instance;
        }
    }

    //Private constructor prevents instantiation of the class from outside the class
    private DoubleCheckSingleton()
    {
    }

    //Method that can be used to perform some action on the singleton instance
    public void Print()
    {
        Console.WriteLine("Print from DoubleCheckSingleton");
    }
}

This Double Check Locking Singleton code uses the Double Check Locking method to ensure that only one class instance is created, even in a multi-threaded environment.

The volatile keyword ensures that the instance is created only once, and the lock statement provides thread safety.

It’s a best practice when working with multi-threading to use thread-safe techniques to prevent race conditions.

C# Singleton class vs. Static methods

Here’s a comparison between a Singleton class and Static methods in C#:

• Instance Management:
  • Singleton Class Involves the creation of a single instance of the class, ensuring global access to that instance.
  • Static Methods do not involve the concept of instances, as static methods are associated directly with the class itself.
• Initialization Control:
  • Singleton Class Provides a controlled way to initialize and manage a single instance, often allowing lazy loading or on-demand instantiation.
  • Static Methods Initialization is not explicitly controlled, and methods are available once the class is loaded in the memory.
• State Management:
  • Singleton Class can maintain state across multiple method calls by encapsulating data within a single instance.
  • Static Methods are Generally stateless, as they don’t have access to instance-specific data.
• Inheritance and Polymorphism:
  • Singleton Class: Can be extended through inheritance, allowing for polymorphic behavior.
  • Static Methods Cannot be inherited, limiting the potential for polymorphism.
• Interface Implementation:
  • Singleton Class can implement interfaces, enabling adherence to a contract.
  • Static Methods Cannot implement interfaces directly.
• Mocking and Unit Testing:
  • Singleton Class may require additional effort for mocking and unit testing due to a single instance and potential global state.
  • Static Methods are easier to mock and test as they are stateless and don’t rely on instance-specific data.

Singleton Design Pattern Uses:

The following are the few uses of the Singleton design pattern.

It’s important to note that using the Singleton pattern may only sometimes be the best choice as it can result in the tight coupling, making it difficult to test and maintain the code. Therefore, we should use it with caution.

Conclusion

The Singleton design pattern is powerful and widely used in software development. It ensures that a class can have only one instance, which we can access throughout the application. In this blog post, we have discussed the use cases of the Singleton design pattern and the three different ways to implement it in our C# code: 01. Lazy Initialization, 02. Eager Initialization, and 03. Double Check Locking. It is important to note that when we implement a singleton pattern in a multi-threaded environment, we must use thread-safe techniques to prevent race conditions.

FAQs

The following is the list of some frequently asked questions and answers about the Singleton design pattern:

References: CSharpindepth- Singleton pattern in C#

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Shekh Ali

Meet Sheikh Ali, a skilled Software engineer and passionate a blogger. He enjoys sharing his expertise in C#, Java, SQL, Design Patterns, and other related topics through his writing. When he's not coding, Sheikh Ali can often indulge in his passions for reading books and fitness training.

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