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Top 10 Questions and Answers on Static vs Singleton in C#

 

In C#, both static classes and singleton patterns are used to ensure a single instance of a class is used throughout an application. However, they have different use cases, benefits, and limitations. Understanding the differences between them is crucial for making the right design decisions.
Top 10 Questions and Answers on the Liskov Substitution Principle in C#

1. What is a static class in C#?

A static class in C# is a class that cannot be instantiated. It contains only static members, meaning all methods, properties, and fields belong to the class itself rather than any object.

public static class Utility
{
    public static void PrintMessage(string message)
    {
        Console.WriteLine(message);
    }
}

// Usage
Utility.PrintMessage("Hello, Static Class!");

2. What is a singleton in C#?

A singleton is a design pattern that restricts the instantiation of a class to a single instance. This is typically achieved by making the constructor private and providing a static method to get the instance.

public class Singleton
{
    private static Singleton _instance;
    private static readonly object _lock = new object();

    private Singleton() { }

    public static Singleton Instance
    {
        get
        {
            lock (_lock)
            {
                if (_instance == null)
                {
                    _instance = new Singleton();
                }
                return _instance;
            }
        }
    }

    public void PrintMessage(string message)
    {
        Console.WriteLine(message);
    }
}

// Usage
Singleton.Instance.PrintMessage("Hello, Singleton!");

3. How do static and singleton classes differ in terms of instantiation?

  • Static Class: Cannot be instantiated; all members are accessed directly through the class.
  • Singleton Class: Can be instantiated, but only once; instance is accessed through a static property or method.

4. What are the use cases for a static class?

Static classes are used when you need a collection of utility or helper methods that do not require state. They are ideal for grouping methods that operate on input parameters without maintaining any instance-specific data.

public static class MathHelper
{
    public static int Add(int a, int b) => a + b;
    public static int Multiply(int a, int b) => a * b;
}

5. What are the use cases for a singleton class?

Singleton classes are used when you need to ensure a single instance of a class is used throughout the application, typically when managing shared resources like configuration settings, logging, or database connections.

public class Configuration
{
    private static Configuration _instance;
    private static readonly object _lock = new object();

    public string ConnectionString { get; set; }

    private Configuration() { }

    public static Configuration Instance
    {
        get
        {
            lock (_lock)
            {
                if (_instance == null)
                {
                    _instance = new Configuration();
                }
                return _instance;
            }
        }
    }
}

6. How do you handle thread safety in a singleton?

Thread safety in a singleton is typically handled using a lock mechanism to ensure that only one thread can create the instance at a time.

public class ThreadSafeSingleton
{
    private static ThreadSafeSingleton _instance;
    private static readonly object _lock = new object();

    private ThreadSafeSingleton() { }

    public static ThreadSafeSingleton Instance
    {
        get
        {
            lock (_lock)
            {
                if (_instance == null)
                {
                    _instance = new ThreadSafeSingleton();
                }
                return _instance;
            }
        }
    }
}

7. Can a static class implement interfaces or inherit from other classes?

No, a static class cannot implement interfaces or inherit from other classes because it cannot be instantiated or participate in inheritance hierarchies.

8. Can a singleton class be inherited?

Yes, a singleton class can be inherited. However, the singleton pattern is typically designed to prevent subclassing to ensure the single instance behavior is not compromised.

public class BaseSingleton
{
    private static BaseSingleton _instance;
    private static readonly object _lock = new object();

    protected BaseSingleton() { }

    public static BaseSingleton Instance
    {
        get
        {
            lock (_lock)
            {
                if (_instance == null)
                {
                    _instance = new BaseSingleton();
                }
                return _instance;
            }
        }
    }
}

public class DerivedSingleton : BaseSingleton { }

9. What are the limitations of using static classes?

  • Cannot be instantiated.
  • Cannot inherit or be inherited.
  • Not suitable for maintaining stateful information across method calls.

10. What are the advantages of using singletons over static classes?

  • Instance Control: Singleton provides control over the instance creation, allowing lazy instantiation and destruction.
  • State Management: Singleton can maintain state across method calls.
  • Inheritance: Singleton can be inherited, allowing for more flexible designs.

Conclusion

Understanding the differences between static classes and singleton patterns is crucial for making the right architectural decisions in C#. Static classes are ideal for stateless utility methods, while singletons are best for managing shared, stateful resources. Properly applying these concepts can lead to more maintainable and robust software designs.

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