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Top 100 Design Patterns C# Interview Questions and Answers

Top 100 Design Patterns C# Interview Questions and Answers

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1. What is a Design Pattern?

Answer: Design Patterns are proven solutions to common problems encountered in software design. They represent best practices to solve recurring design challenges, providing a template for writing quality code.


2. Explain the Singleton Pattern.

Answer: The Singleton Pattern ensures a class has only one instance and provides a global point of access to that instance. It is useful when exactly one object is needed to coordinate actions across the system.

public class Singleton
{
    private static Singleton instance;
    private Singleton() { }
    public static Singleton Instance => instance ??= new Singleton();
}

3. Describe the Factory Method Pattern.

Answer: The Factory Method Pattern defines an interface for creating an object, but lets subclasses alter the type of objects that will be created. It provides a way to delegate the instantiation logic to its subclasses.

public abstract class Product
{
    public abstract void Operation();
}

public class ConcreteProductA : Product
{
    public override void Operation()
    {
        Console.WriteLine("Product A operation");
    }
}

public class ConcreteProductB : Product
{
    public override void Operation()
    {
        Console.WriteLine("Product B operation");
    }
}

public abstract class Creator
{
    public abstract Product FactoryMethod();
}

public class ConcreteCreatorA : Creator
{
    public override Product FactoryMethod()
    {
        return new ConcreteProductA();
    }
}

public class ConcreteCreatorB : Creator
{
    public override Product FactoryMethod()
    {
        return new ConcreteProductB();
    }
}

4. What is the Observer Pattern?

Answer: The Observer Pattern defines a dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. It establishes a one-to-many relationship between a subject and its observers.

public class Subject
{
    private List<IObserver> observers = new List<IObserver>();
    public void Attach(IObserver observer) => observers.Add(observer);
    public void Detach(IObserver observer) => observers.Remove(observer);
    public void Notify() => observers.ForEach(o => o.Update());
}

public interface IObserver
{
    void Update();
}

public class ConcreteObserver : IObserver
{
    public void Update()
    {
        Console.WriteLine("Observer notified");
    }
}

5. Explain the Strategy Pattern.

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the client that uses it.

public interface IStrategy
{
    void Execute();
}

public class ConcreteStrategyA : IStrategy
{
    public void Execute()
    {
        Console.WriteLine("Strategy A executed");
    }
}

public class ConcreteStrategyB : IStrategy
{
    public void Execute()
    {
        Console.WriteLine("Strategy B executed");
    }
}

public class Context
{
    private IStrategy strategy;
    public Context(IStrategy strategy) => this.strategy = strategy;
    public void ExecuteStrategy() => strategy.Execute();
}

6. What is the Decorator Pattern?

Answer: The Decorator Pattern attaches additional responsibilities to an object dynamically. It provides a flexible alternative to subclassing for extending functionality.

public abstract class Component
{
    public abstract void Operation();
}

public class ConcreteComponent : Component
{
    public override void Operation()
    {
        Console.WriteLine("Base operation");
    }
}

public abstract class Decorator : Component
{
    protected Component component;
    public void SetComponent(Component component) => this.component = component;
    public override void Operation() => component?.Operation();
}

public class ConcreteDecoratorA : Decorator
{
    public override void Operation()
    {
        base.Operation();
        Console.WriteLine("Decorator A operation");
    }
}

public class ConcreteDecoratorB : Decorator
{
    public override void Operation()
    {
        base.Operation();
        Console.WriteLine("Decorator B operation");
    }
}

7. Explain the Builder Pattern.

Answer: The Builder Pattern separates the construction of a complex object from its representation. It allows the same construction process to create different representations.

public class Product
{
    private List<string> parts = new List<string>();
    public void AddPart(string part) => parts.Add(part);
    public void Show() => parts.ForEach(Console.WriteLine);
}

public abstract class Builder
{
    public abstract void BuildPartA();
    public abstract void BuildPartB();
    public abstract Product GetResult();
}

public class ConcreteBuilder1 : Builder
{
    private Product product = new Product();
    public override void BuildPartA() => product.AddPart("Part A1");
    public override void BuildPartB() => product.AddPart("Part B1");
    public override Product GetResult() => product;
}

public class ConcreteBuilder2 : Builder
{
    private Product product = new Product();
    public override void BuildPartA() => product.AddPart("Part A2");
    public override void BuildPartB() => product.AddPart("Part B2");
    public override Product GetResult() => product;
}

public class Director
{
    public void Construct(Builder builder)
    {
        builder.BuildPartA();
        builder.BuildPartB();
    }
}

8. Describe the Prototype Pattern.

Answer: The Prototype Pattern creates new objects by copying an existing object, known as the prototype. This pattern is particularly useful when the cost of creating an instance is more expensive than copying an existing one.

public interface IPrototype<T>
{
    T Clone();
}

public class ConcretePrototype : IPrototype<ConcretePrototype>
{
    public int Id { get; set; }
    public ConcretePrototype Clone() => (ConcretePrototype)MemberwiseClone();
}

9. What is the Adapter Pattern?

Answer: The Adapter Pattern allows the interface of an existing class to be used as another interface. It is often used to make existing classes work with others without modifying their source code.

public class Adaptee
{
    public void SpecificRequest() => Console.WriteLine("Adaptee method called");
}

public interface ITarget
{
    void Request();
}

public class Adapter : ITarget
{
    private readonly Adaptee adaptee;
    public Adapter(Adaptee adaptee) => this.adaptee = adaptee;
    public void Request() => adaptee.SpecificRequest();
}

10. Explain the Composite Pattern.

Answer: The Composite Pattern lets clients treat individual objects and compositions of objects uniformly. It is used when clients should treat individual objects and compositions of objects uniformly.

public abstract class Component
{
    public string Name { get; set; }
    public abstract void Add(Component c);
    public abstract void Remove(Component c);
    public abstract void Display(int depth);
}

public class Leaf : Component
{
    public override void Add(Component c) => Console.WriteLine("Cannot add to a leaf");
    public override void Remove(Component c) => Console.WriteLine("Cannot remove from a leaf");
    public override void Display(int depth) => Console.WriteLine(new string('-', depth) + Name);
}

public class Composite : Component
{
    private List<Component> children = new List<Component>();
    public override void Add(Component c) => children.Add(c);
    public override void Remove(Component c) => children.Remove(c);
    public override void Display(int depth)
    {
        Console.WriteLine(new string('-', depth) + Name);
        foreach (Component component in children)
        {
            component.Display(depth + 2);
        }
    }
}

11. Explain the Decorator Pattern.

Answer: The Decorator Pattern allows behavior to be added to individual objects, either statically or dynamically, without affecting the behavior of other objects from the same class. It’s achieved by creating a set of decorator classes that are used to wrap concrete components.

public interface IComponent
{
    void Operation();
}

public class ConcreteComponent : IComponent
{
    public void Operation() => Console.WriteLine("ConcreteComponent operation");
}

public abstract class Decorator : IComponent
{
    protected IComponent component;
    public Decorator(IComponent component) => this.component = component;
    public virtual void Operation() => component.Operation();
}

public class ConcreteDecoratorA : Decorator
{
    public ConcreteDecoratorA(IComponent component) : base(component) { }
    public override void Operation()
    {
        base.Operation();
        Console.WriteLine("ConcreteDecoratorA operation");
    }
}

public class ConcreteDecoratorB : Decorator
{
    public ConcreteDecoratorB(IComponent component) : base(component) { }
    public override void Operation()
    {
        base.Operation();
        Console.WriteLine("ConcreteDecoratorB operation");
    }
}

12. Describe the Proxy Pattern.

Answer: The Proxy Pattern provides a surrogate or placeholder for another object to control access to it. This can be useful for controlling access, managing resource usage, or providing a simplified interface.

public interface ISubject
{
    void Request();
}

public class RealSubject : ISubject
{
    public void Request() => Console.WriteLine("RealSubject request");
}

public class Proxy : ISubject
{
    private readonly RealSubject realSubject;
    public Proxy() => realSubject = new RealSubject();
    public void Request()
    {
        Console.WriteLine("Proxy request");
        realSubject.Request();
    }
}

13. What is the State Pattern?

Answer: The State Pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

public interface IState
{
    void Handle(Context context);
}

public class ConcreteStateA : IState
{
    public void Handle(Context context)
    {
        context.State = new ConcreteStateB();
        Console.WriteLine("Switching to State B");
    }
}

public class ConcreteStateB : IState
{
    public void Handle(Context context)
    {
        context.State = new ConcreteStateA();
        Console.WriteLine("Switching to State A");
    }
}

public class Context
{
    public IState State { get; set; }
    public Context(IState state) => State = state;
    public void Request() => State.Handle(this);
}

14. Explain the Strategy Pattern.

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the context that uses it.

public interface IStrategy
{
    void Execute();
}

public class ConcreteStrategyA : IStrategy
{
    public void Execute() => Console.WriteLine("Executing Strategy A");
}

public class ConcreteStrategyB : IStrategy
{
    public void Execute() => Console.WriteLine("Executing Strategy B");
}

public class Context
{
    private IStrategy strategy;
    public Context(IStrategy strategy) => this.strategy = strategy;
    public void SetStrategy(IStrategy strategy) => this.strategy = strategy;
    public void ExecuteStrategy() => strategy.Execute();
}

15. Explain the Observer Pattern.

Answer: The Observer Pattern defines a dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. It’s useful for implementing distributed event handling systems.

public interface IObserver
{
    void Update(string message);
}

public class ConcreteObserver : IObserver
{
    public string Name { get; }
    public ConcreteObserver(string name) => Name = name;
    public void Update(string message) => Console.WriteLine($"{Name} received message: {message}");
}

public class Subject
{
    private List<IObserver> observers = new List<IObserver>();
    public void Attach(IObserver observer) => observers.Add(observer);
    public void Detach(IObserver observer) => observers.Remove(observer);
    public void Notify(string message) => observers.ForEach(observer => observer.Update(message));
}

16. Describe the Memento Pattern.

Answer: The Memento Pattern provides the ability to restore an object to its previous state, typically used for implementing undo functionality. It involves three roles: the Originator (the object to be restored), the Memento (stores the state), and the Caretaker (holds and manages the mementos).

public class Memento
{
    public string State { get; }
    public Memento(string state) => State = state;
}

public class Originator
{
    public string State { get; set; }
    public Memento CreateMemento() => new Memento(State);
    public void SetMemento(Memento memento) => State = memento.State;
}

public class Caretaker
{
    public Memento Memento { get; set; }
}

17. Explain the Visitor Pattern.

Answer: The Visitor Pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

public interface IElement
{
    void Accept(IVisitor visitor);
}

public class ConcreteElementA : IElement
{
    public void Accept(IVisitor visitor) => visitor.Visit(this);
    public void OperationA() => Console.WriteLine("ConcreteElementA operation");
}

public class ConcreteElementB : IElement
{
    public void Accept(IVisitor visitor) => visitor.Visit(this);
    public void OperationB() => Console.WriteLine("ConcreteElementB operation");
}

public interface IVisitor
{
    void Visit(ConcreteElementA element);
    void Visit(ConcreteElementB element);
}

public class ConcreteVisitor : IVisitor
{
    public void Visit(ConcreteElementA element) => element.OperationA();
    public void Visit(ConcreteElementB element) => element.OperationB();
}

18. Describe the Chain of Responsibility Pattern.

Answer: The Chain of Responsibility Pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along to the next handler in the chain. It’s used for decoupling the sender and receiver of a request.

public abstract class Handler
{
    protected Handler successor;
    public void SetSuccessor(Handler successor) => this.successor = successor;
    public abstract void HandleRequest(int request);
}

public class ConcreteHandler1 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request >= 0 && request < 10)
            Console.WriteLine($"{this.GetType().Name} handled request {request}");
        else if (successor != null)
            successor.HandleRequest(request);
    }
}

public class ConcreteHandler2 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request >= 10 && request < 20)
            Console.WriteLine($"{this.GetType().Name} handled request {request}");
        else if (successor != null)
            successor.HandleRequest(request);
    }
}

19. Explain the State Pattern.

Answer: The State Pattern allows an object to change its behavior when its internal state changes. This pattern is particularly useful when an object needs to switch between different behaviors based on its internal state.

public interface IState
{
    void Handle(Context context);
}

public class ConcreteStateA : IState
{
    public void Handle(Context context)
    {
        context.State = new ConcreteStateB();
    }
}

public class ConcreteStateB : IState
{
    public void Handle(Context context)
    {
        context.State = new ConcreteStateA();
    }
}

public class Context
{
    public IState State { get; set; }
    public void Request() => State.Handle(this);
}

20. Describe the Strategy Pattern.

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the context that uses it.

public interface IStrategy
{
    void Execute();
}

public class ConcreteStrategyA : IStrategy
{
    public void Execute() => Console.WriteLine("Executing strategy A");
}

public class ConcreteStrategyB : IStrategy
{
    public void Execute() => Console.WriteLine("Executing strategy B");
}

public class Context
{
    private IStrategy strategy;
    public Context(IStrategy strategy) => this.strategy = strategy;
    public void SetStrategy(IStrategy strategy) => this.strategy = strategy;
    public void ExecuteStrategy() => strategy.Execute();
}

21. Explain the Command Pattern.

Answer: The Command Pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations and transactional behavior.

public interface ICommand
{
    void Execute();
}

public class ConcreteCommand : ICommand
{
    private Receiver receiver;
    public ConcreteCommand(Receiver receiver) => this.receiver = receiver;
    public void Execute() => receiver.Action();
}

public class Receiver
{
    public void Action() => Console.WriteLine("Receiver is performing an action");
}

public class Invoker
{
    private ICommand command;
    public void SetCommand(ICommand command) => this.command = command;
    public void ExecuteCommand() => command.Execute();
}

22. Elaborate on the Observer Pattern.

Answer: The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. It’s useful for establishing relationships between objects where changes in one object need to trigger actions in others.

public interface IObserver
{
    void Update(string message);
}

public class ConcreteObserver : IObserver
{
    private string name;
    public ConcreteObserver(string name) => this.name = name;
    public void Update(string message) => Console.WriteLine($"{name} received message: {message}");
}

public class Subject
{
    private List<IObserver> observers = new List<IObserver>();
    public void AddObserver(IObserver observer) => observers.Add(observer);
    public void RemoveObserver(IObserver observer) => observers.Remove(observer);
    public void NotifyObservers(string message) => observers.ForEach(o => o.Update(message));
}

23. Discuss the Chain of Responsibility Pattern.

Answer: The Chain of Responsibility Pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process it or to pass it along to the next handler in the chain. It allows for more than one object to handle a request.

public abstract class Handler
{
    protected Handler successor;
    public void SetSuccessor(Handler successor) => this.successor = successor;
    public abstract void HandleRequest(int request);
}

public class ConcreteHandler1 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request <= 10)
            Console.WriteLine($"Request {request} handled by Handler1");
        else if (successor != null)
            successor.HandleRequest(request);
    }
}

public class ConcreteHandler2 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request > 10 && request <= 20)
            Console.WriteLine($"Request {request} handled by Handler2");
        else if (successor != null)
            successor.HandleRequest(request);
    }
}

24. Explain the Interpreter Pattern.

Answer: The Interpreter Pattern provides a way to evaluate language grammar or expressions. It’s useful when we have a language to interpret and need to build an interpreter that interprets sentences of the language.

public interface IExpression
{
    int Interpret(Context context);
}

public class TerminalExpression : IExpression
{
    public int Interpret(Context context) => context.GetValue();
}

public class NonTerminalExpression : IExpression
{
    private IExpression expression1;
    private IExpression expression2;

    public NonTerminalExpression(IExpression expression1, IExpression expression2)
    {
        this.expression1 = expression1;
        this.expression2 = expression2;
    }

    public int Interpret(Context context) => expression1.Interpret(context) + expression2.Interpret(context);
}

public class Context
{
    private int value;
    public Context(int value) => this.value = value;
    public int GetValue() => value;
}

25. Describe the Memento Pattern.

Answer: The Memento Pattern captures an object’s internal state so that it can be restored to that state at a later time. It’s useful when we want to implement undo functionality or provide a snapshot of an object’s state.

public class Memento
{
    private string state;
    public Memento(string state) => this.state = state;
    public string GetState() => state;
}

public class Originator
{
    private string state;
    public void SetState(string state) => this.state = state;
    public Memento SaveStateToMemento() => new Memento(state);
    public void GetStateFromMemento(Memento memento) => state = memento.GetState();
}

26. Explain the State Pattern.

Answer: The State Pattern allows an object to change its behavior when its internal state changes. It’s useful when an object has multiple states and different actions need to be taken based on the current state.

public interface IState
{
    void Handle(Context context);
}

public class ConcreteStateA : IState
{
    public void Handle(Context context) => Console.WriteLine("Handling State A");
}

public class ConcreteStateB : IState
{
    public void Handle(Context context) => Console.WriteLine("Handling State B");
}

public class Context
{
    private IState state;
    public void SetState(IState state) => this.state = state;
    public void Request() => state.Handle(this);
}

27. Elaborate on the Strategy Pattern.

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the context that uses it.

public interface IStrategy
{
    void Execute();
}

public class ConcreteStrategy1 : IStrategy
{
    public void Execute() => Console.WriteLine("Executing Strategy 1");
}

public class ConcreteStrategy2 : IStrategy
{
    public void Execute() => Console.WriteLine("Executing Strategy 2");
}

public class Context
{
    private IStrategy strategy;
    public void SetStrategy(IStrategy strategy) => this.strategy = strategy;
    public void ExecuteStrategy() => strategy.Execute();
}

28. Explain the Chain of Responsibility Pattern.

Answer: The Chain of Responsibility Pattern allows multiple objects to handle a request without the sender needing to know which object will ultimately process it. Each handler in the chain either processes the request or passes it along to the next handler.

public abstract class Handler
{
    protected Handler successor;
    public void SetSuccessor(Handler successor) => this.successor = successor;
    public abstract void HandleRequest(int request);
}

public class ConcreteHandler1 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request <= 10)
            Console.WriteLine("Handled by Handler 1");
        else
            successor?.HandleRequest(request);
    }
}

public class ConcreteHandler2 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request > 10 && request <= 20)
            Console.WriteLine("Handled by Handler 2");
        else
            successor?.HandleRequest(request);
    }
}

29. Describe the Interpreter Pattern.

Answer: The Interpreter Pattern is used to define a grammar for interpreting a language and provides an interpreter that interprets sentences in that language. It’s suitable for scenarios where we need to evaluate expressions.

public abstract class Expression
{
    public abstract bool Interpret(string context);
}

public class TerminalExpression : Expression
{
    private readonly string data;
    public TerminalExpression(string data) => this.data = data;
    public override bool Interpret(string context) => context.Contains(data);
}

public class OrExpression : Expression
{
    private readonly Expression expr1;
    private readonly Expression expr2;
    public OrExpression(Expression expr1, Expression expr2)
    {
        this.expr1 = expr1;
        this.expr2 = expr2;
    }
    public override bool Interpret(string context) => expr1.Interpret(context) || expr2.Interpret(context);
}

30. Elaborate on the Visitor Pattern.

Answer: The Visitor Pattern represents an operation to be performed on elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.

public interface IVisitor
{
    void Visit(Element element);
}

public class ConcreteVisitor : IVisitor
{
    public void Visit(Element element) => Console.WriteLine("Visiting element");
}

public abstract class Element
{
    public abstract void Accept(IVisitor visitor);
}

public class ConcreteElement : Element
{
    public override void Accept(IVisitor visitor) => visitor.Visit(this);
}

31. Explain the Memento Pattern.

Answer: The Memento Pattern allows an object’s state to be captured and restored later. This is useful for implementing features like undo functionality.

public class Memento
{
    public string State { get; }
    public Memento(string state) => State = state;
}

public class Originator
{
    public string State { get; set; }
    public Memento CreateMemento() => new Memento(State);
    public void SetMemento(Memento memento) => State = memento.State;
}

32. Describe the State Pattern.

Answer: The State Pattern allows an object to alter its behavior when its internal state changes. It achieves this by representing different states as objects and delegating the state-specific behavior to these objects.

public abstract class State
{
    public abstract void Handle(Context context);
}

public class ConcreteStateA : State
{
    public override void Handle(Context context) => Console.WriteLine("Handling state A");
}

public class ConcreteStateB : State
{
    public override void Handle(Context context) => Console.WriteLine("Handling state B");
}

public class Context
{
    public State State { get; set; }
    public void Request() => State.Handle(this);
}

33. Elaborate on the Strategy Pattern.

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the context that uses it.

public interface IStrategy
{
    void Execute();
}

public class ConcreteStrategy1 : IStrategy
{
    public void Execute() => Console.WriteLine("Executing strategy 1");
}

public class ConcreteStrategy2 : IStrategy
{
    public void Execute() => Console.WriteLine("Executing strategy 2");
}

public class Context
{
    private IStrategy strategy;
    public Context(IStrategy strategy) => this.strategy = strategy;
    public void SetStrategy(IStrategy strategy) => this.strategy = strategy;
    public void ExecuteStrategy() => strategy.Execute();
}

These are some advanced design patterns in C#. If you’d like to explore more or have any other questions, feel free to ask!


34. Explain the Chain of Responsibility Pattern.

Answer: The Chain of Responsibility Pattern allows multiple objects to handle a request without the sender needing to know which object will ultimately process it.


35. Describe the Interpreter Pattern.

Answer: The Interpreter Pattern is used to define a grammar for interpreting a language and provides an interpreter to evaluate sentences in that language.


36. Elaborate on the Visitor Pattern.

Answer: The Visitor Pattern represents an operation to be performed on the elements of an object structure. It allows you to define a new operation without changing the classes of the elements on which it operates.


37. Explain the Flyweight Pattern.

Answer: The Flyweight Pattern is used to minimize memory usage and improve performance by sharing as much as possible with related objects. It is especially useful when dealing with a large number of similar objects.


38. What is the Memento Pattern and when is it used?

Answer: The Memento Pattern is used to capture an object’s internal state so that it can be restored to that state at a later time. It’s useful when you need to implement an undo feature or maintain a history of an object’s state.


39. Describe the State Pattern.

Answer: The State Pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.


40. Elaborate on the Null Object Pattern.

Answer: The Null Object Pattern is used to provide an object as a surrogate for the lack of an object of a given value. It helps to avoid null checks and simplify code.


41. What is the Strategy Pattern?

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the context that uses it.


42. Explain the Template Method Pattern.

Answer: The Template Method Pattern defines the skeleton of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.


43. Describe the Mediator Pattern.

Answer: The Mediator Pattern defines an object that centralizes communication between objects in a system. It promotes loose coupling by keeping objects from referring to each other explicitly.


44. What is the Observer Pattern?

Answer: The Observer Pattern is used to define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.


45. Elaborate on the Command Pattern.

Answer: The Command Pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.


37. Explain the Flyweight Pattern.

Answer: The Flyweight Pattern is used to minimize memory usage and improve performance by sharing as much as possible with related objects. It is especially useful when dealing with a large number of similar objects.


38. What is the Memento Pattern and when is it used?

Answer: The Memento Pattern is used to capture an object’s internal state so that it can be restored to that state at a later time. It’s useful when you need to implement an undo feature or maintain a history of an object’s state.


39. Describe the State Pattern.

Answer: The State Pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.


40. Elaborate on the Null Object Pattern.

Answer: The Null Object Pattern is used to provide an object as a surrogate for the lack of an object of a given value. It helps to avoid null checks and simplify code.


41. What is the Strategy Pattern?

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the context that uses it.


42. Explain the Template Method Pattern.

Answer: The Template Method Pattern defines the skeleton of an algorithm in the superclass but lets subclasses override specific steps of the algorithm without changing its structure.


43. Describe the Mediator Pattern.

Answer: The Mediator Pattern defines an object that centralizes communication between objects in a system. It promotes loose coupling by keeping objects from referring to each other explicitly.


44. What is the Observer Pattern?

Answer: The Observer Pattern is used to define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.


45. Elaborate on the Command Pattern.

Answer: The Command Pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for the support of undoable operations.


46. Explain the Chain of Responsibility Pattern.

Answer: The Chain of Responsibility Pattern passes a request along a chain of handlers. Upon receiving a request, each handler decides either to process the request or to pass it along to the next handler in the chain.


47. Describe the Interpreter Pattern.

Answer: The Interpreter Pattern is used to evaluate language grammar or expressions. It provides a way to evaluate language grammar or expressions in a language.


48. What is the Visitor Pattern and when is it used?

Answer: The Visitor Pattern represents an operation to be performed on the elements of an object structure. It’s used when you want to define a new operation without changing the classes of the elements.


49. Elaborate on the Iterator Pattern.

Answer: The Iterator Pattern provides a way to access elements of a collection sequentially without exposing its underlying representation. It decouples the client code from the internal structure of the collection.


50. What is the Composite Pattern and when is it applicable?

Answer: The Composite Pattern is used when you want clients to treat individual objects and compositions of objects uniformly. It composes objects into tree structures to represent part-whole hierarchies.


51. Explain the Proxy Pattern.

Answer: The Proxy Pattern provides a surrogate or placeholder for another object to control access to it. It’s useful for scenarios like controlling access, caching, or lazy loading.


52. Describe the Decorator Pattern.

Answer: The Decorator Pattern attaches additional responsibilities to an object dynamically. It provides a flexible alternative to subclassing for extending functionality.


53. What is the Adapter Pattern and when would you use it?

Answer: The Adapter Pattern allows the interface of an existing class to be used as another interface. It’s used when you want to make existing classes work with others without modifying their source code.


54. Elaborate on the Bridge Pattern.

Answer: The Bridge Pattern decouples an abstraction from its implementation, allowing both to vary independently. It involves an interface or abstract class that acts as a bridge that makes the functionality of concrete classes independent from the client code.


55. Explain the Flyweight Pattern.

Answer: The Flyweight Pattern is used when you need to support a large number of fine-grained objects efficiently. It does this by sharing common state between multiple objects instead of keeping it in each object.


56. Describe the Memento Pattern.

Answer: The Memento Pattern provides the ability to restore an object to its previous state. It involves three roles: the originator (object to be restored), the memento (stores the state), and the caretaker (keeps track of multiple mementos).


57. What is the State Pattern and when is it used?

Answer: The State Pattern allows an object to alter its behavior when its internal state changes. It’s used when the behavior of an object needs to change based on its state.


58. Elaborate on the Null Object Pattern.

Answer: The Null Object Pattern provides an object as a surrogate for the lack of an object of a given value. It can help to avoid null checks and handle cases where an object might not exist.


59. What is the Observer Pattern and why is it useful?

Answer: The Observer Pattern defines a dependency between objects so that when one object changes state, all its dependents are notified. It’s useful for decoupling the sender and receiver of information.


60. Explain the Mediator Pattern.

Answer: The Mediator Pattern defines an object that centralizes communication between objects in a system. It promotes loose coupling between objects by preventing direct connections between them.


61. Describe the Command Pattern.

Answer: The Command Pattern encapsulates a request as an object, thereby parameterizing clients with queues, requests, and operations. It allows for decoupling the sender and receiver of a request.


62. What is the Template Method Pattern and when is it applicable?

Answer: The Template Method Pattern defines the skeleton of an algorithm but lets subclasses override specific steps. It’s used when you want to define the structure of an algorithm but allow variations in specific steps.


63. Elaborate on the Strategy Pattern.

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the context that uses it.


64. What is the Chain of Responsibility Pattern and how does it work?

Answer: The Chain of Responsibility Pattern passes a request along a chain of handlers. Each handler decides either to process the request or pass it along to the next handler in the chain. It decouples senders and receivers.


65. Describe the Interpreter Pattern.

Answer: The Interpreter Pattern is used to define a grammar for interpreting language syntax and provides an interpreter to evaluate sentences in that language. It involves defining grammatical rules and providing an interpreter to execute them.


66. Explain the Visitor Pattern.

Answer: The Visitor Pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.


67. What is the Prototype Pattern and when is it useful?

Answer: The Prototype Pattern creates new objects by copying an existing object, known as the prototype. It’s useful when the cost of creating an object is more expensive than copying an existing one.


68. Elaborate on the Object Pool Pattern.

Answer: The Object Pool Pattern manages a set of objects to be reused. It can be beneficial when the cost of initializing an object is high and the number of instantiations in use at any one time is low.


69. Describe the Lazy Initialization Pattern.

Answer: The Lazy Initialization Pattern delays the initialization of an object until it’s actually needed. It can improve performance by only creating resources when they’re first required.


70. What is the Double Checked Locking Pattern?

Answer: The Double Checked Locking Pattern is used to reduce the overhead of acquiring a lock by first testing the locking criterion without actually acquiring the lock.


71. Explain the Eager Initialization Pattern.

Answer: The Eager Initialization Pattern initializes an object as soon as it is loaded into memory. This ensures that the object is always available when requested.


72. Describe the Lazy Loading Pattern.

Answer: The Lazy Loading Pattern defers the loading of an object until the point at which it is needed. This can save resources and improve performance by only loading objects when they’re required.


73. What is the Value Object Pattern?

Answer: The Value Object Pattern represents a simple entity whose equality isn’t based on identity. It’s defined by its attributes rather than a unique identifier.


74. Elaborate on the Data Access Object Pattern.

Answer: The Data Access Object Pattern provides an abstract interface to some type of database or other persistence mechanism. It separates low-level data accessing operations from high-level business services.


75. Explain the Unit of Work Pattern.

Answer: The Unit of Work Pattern maintains a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems.


76. What is the Identity Map Pattern?

Answer: The Identity Map Pattern ensures that each object gets loaded only once by keeping every loaded object in a map. When the application asks for an object, the Identity Map checks the map first.


77. Describe the Command Pattern.

Answer: The Command Pattern turns a request into a standalone object that contains all the information about the request. This decouples sender and receiver, allowing for more complex handling of the request.


78. Explain the Null Object Pattern.

Answer: The Null Object Pattern provides an object as a surrogate for the lack of an object of a given value. It can be used to avoid null checks and improve program stability.


79. What is the Extension Object Pattern?

Answer: The Extension Object Pattern adds new methods to an existing class without modifying its source code. It allows for the dynamic addition of behavior to individual objects.


80. Elaborate on the Module Pattern.

Answer: The Module Pattern is a design pattern used for creating loosely coupled, reusable code. It groups related functionality into a single object, keeping the details private.


81. Describe the Singleton Pattern.

Answer: The Singleton Pattern restricts the instantiation of a class to one “single” instance. This is useful when exactly one object is needed to coordinate actions across the system.


82. Explain the Multiton Pattern.

Answer: The Multiton Pattern extends the Singleton Pattern to manage a map of named instances as key-value pairs. Each instance is stored by a unique key.


83. What is the Monostate Pattern?

Answer: The Monostate Pattern ensures that all instances of a class share the same state, essentially making all objects behave as if they were a singleton.


84. Elaborate on the Flyweight Pattern.

Answer: The Flyweight Pattern minimizes memory usage by sharing as much as possible with related objects. It’s particularly useful when a large number of similar objects need to be created.


85. Describe the Adapter Pattern.

Answer: The Adapter Pattern allows the interface of an existing class to be used as another interface. It’s often used to make existing classes work with others without modifying their source code.


86. Explain the Facade Pattern.

Answer: The Facade Pattern provides a unified interface to a set of interfaces in a subsystem. It defines a higher-level interface that makes the subsystem easier to use.


87. What is the Observer Pattern?

Answer: The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.


88. Describe the Chain of Responsibility Pattern.

Answer: The Chain of Responsibility Pattern passes a request along a chain of handlers. Each handler decides either to process the request or to pass it along the chain.


89. Explain the Interpreter Pattern.

Answer: The Interpreter Pattern is used to define a grammar for interpreting language syntax and provides an interpreter to evaluate sentences in the language.


90. What is the Mediator Pattern?

Answer: The Mediator Pattern defines an object that centralizes communication between objects in a system. It promotes loose coupling by keeping objects from referring to each other explicitly.


91. Describe the Memento Pattern.

Answer: The Memento Pattern provides the ability to restore an object to its previous state. It involves capturing the internal state of an object and restoring it at a later time.


92. Explain the State Pattern.

Answer: The State Pattern allows an object to alter its behavior when its internal state changes. It appears as if the object changed its class.


93. What is the Strategy Pattern?

Answer: The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It allows the algorithm to vary independently from the client that uses it.


94. Elaborate on the Template Method Pattern.

Answer: The Template Method Pattern defines the program skeleton in a method but delays some steps to subclasses. It lets subclasses redefine certain steps without changing the algorithm’s structure.


95. Describe the Visitor Pattern.

Answer: The Visitor Pattern represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.


96. Explain the Composite Pattern.

Answer: The Composite Pattern composes objects into tree structures to represent part-whole hierarchies. It allows clients to treat individual objects and compositions of objects uniformly.


97. What is the Proxy Pattern?

Answer: The Proxy Pattern provides a surrogate or placeholder for another object to control access to it. It can be used for various purposes like security, caching, etc.


98. Elaborate on the Delegation Pattern.

Answer: The Delegation Pattern is a way to make composition as powerful for reuse as inheritance. It involves one object delegating a task to another object.


99. Describe the Lazy Initialization Pattern.

Answer: The Lazy Initialization Pattern defers the creation of an object or some expensive process until it’s first needed. This can improve performance by avoiding unnecessary work.


100. What is the Double Checked Locking Pattern?

Answer: The Double Checked Locking Pattern is used to reduce the overhead of acquiring a lock by first testing the locking criterion (the “lock hint”) without actually acquiring the lock.