15 SOLID Principles C# Interview Questions and Answers

The SOLID principles are a set of design guidelines in object-oriented programming that aim to make software designs more understandable, flexible, and maintainable. These principles are particularly relevant in C# development, where they help developers create robust and scalable applications. By adhering to SOLID principles, developers can reduce the risk of code rot and technical debt, ensuring that their codebase remains clean and adaptable to change.

This article provides a curated selection of interview questions focused on SOLID principles in the context of C#. Reviewing these questions will help you deepen your understanding of these essential design principles and prepare you to discuss them confidently in your next technical interview.

SOLID Principles C# Interview Questions and Answers

1. What is the Single Responsibility Principle (SRP) and why is it important?

The Single Responsibility Principle (SRP) is a key concept in object-oriented design, emphasizing that a class should have only one reason to change. This promotes high cohesion and low coupling, making the codebase easier to understand, maintain, and extend. By adhering to SRP, you can achieve improved readability, enhanced maintainability, and better testability.

Example in C#:

public class ReportGenerator
{
    public string GenerateReport()
    {
        return "Report content";
    }
}

public class ReportPrinter
{
    public void PrintReport(string reportContent)
    {
        Console.WriteLine(reportContent);
    }
}

// Usage
var reportGenerator = new ReportGenerator();
var reportContent = reportGenerator.GenerateReport();

var reportPrinter = new ReportPrinter();
reportPrinter.PrintReport(reportContent);

In this example, ReportGenerator is responsible for generating the report, while ReportPrinter handles printing, each with a single responsibility.

2. Can you provide an example of how to implement the Interface Segregation Principle (ISP) in C#?

The Interface Segregation Principle (ISP) states that no client should be forced to depend on methods it does not use. Larger interfaces should be split into smaller, more specific ones.

Example in C#:

// Segregated interfaces
public interface IWorkable
{
    void Work();
}

public interface IFeedable
{
    void Eat();
}

// Classes implementing specific interfaces
public class Worker : IWorkable, IFeedable
{
    public void Work() { }

    public void Eat() { }
}

public class Robot : IWorkable
{
    public void Work() { }
}

Here, IWorker is split into IWorkable and IFeedable. Worker implements both, while Robot only implements IWorkable.

3. Explain how the Open/Closed Principle can improve software maintainability.

The Open/Closed Principle (OCP) suggests that a class should be open for extension but closed for modification, minimizing the risk of introducing new bugs. In C#, this can be achieved through interfaces and abstract classes.

Example:

public interface IShape
{
    double Area();
}

public class Circle : IShape
{
    private double radius;

    public Circle(double radius)
    {
        this.radius = radius;
    }

    public double Area()
    {
        return Math.PI * radius * radius;
    }
}

public class Rectangle : IShape
{
    private double width;
    private double height;

    public Rectangle(double width, double height)
    {
        this.width = width;
        this.height = height;
    }

    public double Area()
    {
        return width * height;
    }
}

IShape defines a contract for shapes. New shapes can be added by implementing IShape, without modifying existing classes.

4. How do you ensure that your classes adhere to the Liskov Substitution Principle?

The Liskov Substitution Principle (LSP) ensures that a subclass can be substituted for its superclass without altering program correctness. To adhere to LSP, ensure method signatures match, maintain behavioral consistency, and honor postconditions and invariants.

Example in C#:

public class Bird
{
    public virtual void Fly()
    {
        Console.WriteLine("Flying");
    }
}

public class Sparrow : Bird
{
    public override void Fly()
    {
        Console.WriteLine("Sparrow flying");
    }
}

public class Ostrich : Bird
{
    public override void Fly()
    {
        throw new NotSupportedException("Ostriches can't fly");
    }
}

Ostrich violates LSP by changing the expected behavior of Fly. Refactor to avoid this issue.

5. What role does the Dependency Inversion Principle play in dependency injection frameworks?

The Dependency Inversion Principle (DIP) ensures high-level modules depend on abstractions rather than concrete implementations, allowing for more flexible and maintainable code. In dependency injection frameworks, DIP is implemented by injecting dependencies through constructors, properties, or methods.

Example:

// Abstraction
public interface ILogger
{
    void Log(string message);
}

// Concrete Implementation
public class ConsoleLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

// High-level Module
public class UserService
{
    private readonly ILogger _logger;

    public UserService(ILogger logger)
    {
        _logger = logger;
    }

    public void CreateUser(string username)
    {
        _logger.Log("User created: " + username);
    }
}

// Dependency Injection Setup
var services = new ServiceCollection();
services.AddTransient<ILogger, ConsoleLogger>();
services.AddTransient<UserService>();

var serviceProvider = services.BuildServiceProvider();
var userService = serviceProvider.GetService<UserService>();
userService.CreateUser("JohnDoe");

UserService depends on ILogger rather than a concrete implementation, managed by the framework.

6. How can adhering to the Single Responsibility Principle lead to better code reuse?

Adhering to the Single Responsibility Principle (SRP) leads to better code reuse by promoting modularity, maintainability, decoupling, and testability. When classes are designed with a single responsibility, they become more modular, allowing for easy extraction and reuse in different parts of the application or projects.

7. Explain the relationship between SOLID principles and design patterns.

SOLID principles and design patterns both aim to improve software design and architecture. SOLID principles provide a foundation for creating maintainable and extendable software. Design patterns often embody one or more of these principles.

For example:

– The Strategy Pattern adheres to the Open/Closed Principle by allowing algorithms to be selected at runtime without modifying the context class.
– The Observer Pattern follows the Dependency Inversion Principle by decoupling the subject from its observers.
– The Decorator Pattern supports the Single Responsibility Principle by allowing behavior to be added to individual objects without affecting others.

8. Describe how the Dependency Inversion Principle can facilitate easier testing and mocking.

The Dependency Inversion Principle (DIP) helps create a decoupled architecture by ensuring high-level modules are not tightly coupled to low-level modules. This decoupling allows for easier substitution of real implementations with mock objects, useful in unit testing.

Example in C#:

public interface ILogger
{
    void Log(string message);
}

public class ConsoleLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

public class Application
{
    private readonly ILogger _logger;

    public Application(ILogger logger)
    {
        _logger = logger;
    }

    public void Run()
    {
        _logger.Log("Application is running");
    }
}

In this example, Application depends on ILogger, allowing easy substitution with a mock during testing.

9. Can you provide an example of a violation of the Single Responsibility Principle in a real-world project and how you resolved it?

The Single Responsibility Principle (SRP) states that a class should have only one reason to change. Violating this principle often leads to classes that are difficult to maintain and understand.

Example of SRP Violation:

public class UserManager
{
    public void CreateUser(string username, string password)
    {
        SaveUserToDatabase(username, password);
    }

    private void SaveUserToDatabase(string username, string password)
    {
        // Code to save user to the database
    }
}

UserManager handles both business logic and data access, violating SRP. Refactor by separating responsibilities:

public class UserManager
{
    private readonly IUserRepository _userRepository;

    public UserManager(IUserRepository userRepository)
    {
        _userRepository = userRepository;
    }

    public void CreateUser(string username, string password)
    {
        _userRepository.SaveUser(username, password);
    }
}

public interface IUserRepository
{
    void SaveUser(string username, string password);
}

public class UserRepository : IUserRepository
{
    public void SaveUser(string username, string password)
    {
        // Code to save user to the database
    }
}

UserManager now focuses on business logic, while UserRepository handles data access.

10. How can the Open/Closed Principle be applied in a plugin architecture?

The Open/Closed Principle can be applied in a plugin architecture by designing the core system to be extendable through interfaces or abstract classes. This allows new plugins to be added without modifying the existing codebase.

Example:

public interface IPlugin
{
    void Execute();
}

public class PluginManager
{
    private List<IPlugin> plugins = new List<IPlugin>();

    public void LoadPlugin(IPlugin plugin)
    {
        plugins.Add(plugin);
    }

    public void ExecutePlugins()
    {
        foreach (var plugin in plugins)
        {
            plugin.Execute();
        }
    }
}

public class SpellCheckPlugin : IPlugin
{
    public void Execute()
    {
        // Spell check logic
    }
}

public class GrammarCheckPlugin : IPlugin
{
    public void Execute()
    {
        // Grammar check logic
    }
}

The core system remains unchanged, and new functionality can be added by creating new plugin classes.

11. What are the consequences of violating the Liskov Substitution Principle in a large codebase?

Violating the Liskov Substitution Principle in a large codebase can lead to code fragility, reduced reusability, increased maintenance costs, poor readability, and testing challenges. When subclasses do not adhere to the behavior expected by their superclass, it can lead to unexpected behavior and bugs, making the codebase fragile and prone to errors.

12. How would you design an e-commerce checkout system using the Interface Segregation Principle?

The Interface Segregation Principle (ISP) states that a client should not be forced to implement an interface it does not use. In an e-commerce checkout system, this means designing interfaces that are specific to the needs of different clients.

Example:

public interface IPaymentProcessor
{
    void ProcessPayment(Order order);
}

public interface IOrderValidator
{
    bool ValidateOrder(Order order);
}

public interface IInventoryManager
{
    void UpdateInventory(Order order);
}

By segregating the interfaces, each client only depends on the methods it actually uses.

13. How can the Interface Segregation Principle improve API design?

The Interface Segregation Principle (ISP) improves API design by promoting the creation of smaller, more specific interfaces. This ensures that clients only need to implement the methods that are relevant to them, reducing the risk of implementing unnecessary methods.

Example:

public interface IPrinter
{
    void Print();
}

public interface IScanner
{
    void Scan();
}

public interface IFax
{
    void Fax();
}

A class that only needs to print documents can implement just the IPrinter interface.

14. Describe a situation where following the Dependency Inversion Principle might introduce complexity.

The Dependency Inversion Principle (DIP) states that high-level modules should not depend on low-level modules. This principle aims to reduce the coupling between different parts of a system, making it more modular and easier to maintain.

However, following DIP can sometimes introduce complexity, especially in smaller projects. Introducing interfaces and abstract classes to decouple modules can lead to an increase in the number of classes and interfaces, making the codebase harder to navigate.

15. How would you implement a feature toggle system that follows the Open/Closed Principle?

To implement a feature toggle system that follows the Open/Closed Principle in C#, use interfaces and dependency injection. This allows you to add new features or toggle existing ones without modifying the existing codebase.

Example:

public interface IFeature
{
    void Execute();
}

public class FeatureA : IFeature
{
    public void Execute()
    {
        Console.WriteLine("Feature A is executed.");
    }
}

public class FeatureB : IFeature
{
    public void Execute()
    {
        Console.WriteLine("Feature B is executed.");
    }
}

public class FeatureToggle
{
    private readonly IFeature _feature;

    public FeatureToggle(IFeature feature)
    {
        _feature = feature;
    }

    public void ExecuteFeature()
    {
        _feature.Execute();
    }
}

class Program
{
    static void Main(string[] args)
    {
        IFeature feature = new FeatureA(); // Toggle to new FeatureB() as needed
        FeatureToggle featureToggle = new FeatureToggle(feature);
        featureToggle.ExecuteFeature();
    }
}