Top 6 SOLID Principles C# Interview Questions with code
The SOLID principles are a set of design guidelines that help developers create more understandable, flexible, and maintainable software.
These principles are essential for writing clean code and are widely discussed in technical interviews for any software developers including C#.
In this blog, we'll explore the most commonly asked SOLID principles interview questions in 2025 and provide clear, concise answers to help you prepare.
Breakdown of what SOLID principle stand for
- S - Single Responsibility Principle
- O - Open/Closed Principle
- L - Liskov Substitution Principle
- I - Interface Segregation Principle
- D - Dependency Inversion Principle
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Page Contents
1. What are the SOLID principles?
2. Can you explain the Single Responsibility Principle (SRP)?
3. What is the Open/Closed Principle (OCP)?
4. Can you describe the Liskov Substitution Principle (LSP)?
5. What is the Interface Segregation Principle (ISP)?
6. Can you explain the Dependency Inversion Principle (DIP)?
1. What are the SOLID principles?
The SOLID principles are five design principles intended to make software designs more understandable, flexible, and maintainable. They are:
- Single Responsibility Principle (SRP)
- Open/Closed Principle (OCP)
- Liskov Substitution Principle (LSP)
- Interface Segregation Principle (ISP)
- Dependency Inversion Principle (DIP)
2. Can you explain the Single Responsibility Principle (SRP)?
The Single Responsibility Principle states that a class should have only one reason to change, meaning it should have only one job or responsibility.
This principle aims to reduce the complexity of code by ensuring that each class addresses a specific aspect of the program's functionality. For example:
public class Report
{
public string ReportType { get; set; }
public void GenerateReport()
{
/*Code to generate report*/
}
public void SaveReport()
{
/*Code to save report*/
}
}Here, Report class violates SRP as it handles both generating and saving the report.
To follow SRP, separate each responsibility into its own class.
public class ReportGenerator
{
public void GenerateReport()
{
/*Code to generate report*/
}
}
public class ReportSaver
{
public void SaveReport()
{
/*Code to save report*/
}
}3. What is the Open/Closed Principle (OCP)?
The Open/Closed Principle states that software entities (classes, modules, functions, etc.) should be open for extension but closed for modification.
This means you should be able to add new functionality without changing existing code. For example:
public abstract class Shape
{
public abstract double GetArea();
}
public class Circle : Shape
{
public double Radius { get; set; }
public override double GetArea() => Math.PI \* Radius \* Radius;
}
public class Rectangle : Shape
{
public double Width { get; set; }
public double Height { get; set; }
public override double GetArea() => Width \* Height;
}Here, you can add new shapes without modifying the existing code by extending the Shape class.
4. Can you describe the Liskov Substitution Principle (LSP)?
The Liskov Substitution Principle states that objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program.
This principle ensures that a subclass can stand in for its superclass. For example:
public class Bird
{
public virtual void Fly()
{
/*Flying logic*/
}
}
public class Eagle : Bird
{
public override void Fly()
{
/*Flying logic specific to Eagle */
}
}
public class Ostrich : Bird
{
public override void Fly()
{
throw new NotImplementedException();
}
}Here, Ostrich violates LSP because it can't fly.
Example Breakdown:
The Bird class is a superclass, and both Eagle and Ostrich are subclasses.
The Fly() method in the Bird class is intended for all birds, but the Ostrich subclass doesn’t logically support flying, so overriding Fly() in Ostrich with an exception (throw new NotImplementedException()) violates LSP.
This is because Ostrich is a type of Bird, but it doesn't fulfill the expected behavior of a bird that can fly. If the code expects all birds to be able to fly (as defined by the Fly() method), replacing a Bird with an Ostrich would break the program.
A better approach:
public abstract class Bird { }
public class FlyingBird : Bird
{
public virtual void Fly()
{
/*Flying logic */
}
}
public class Eagle : FlyingBird
{
public override void Fly()
{
/*Flying logic specific to Eagle */
}
}
public class Ostrich : Bird
{
/* Ostrich-specific logic */
}The problem is resolved by introducing an abstract class hierarchy:
The base class Bird is now abstract, without any implementation of Fly().
A new subclass, FlyingBird, is created for birds that can actually fly. This allows birds like Eagle to inherit from FlyingBird and implement their flying logic.
Birds that can’t fly (like Ostrich) simply don’t have the Fly() method, ensuring no unexpected behaviors or exceptions.
This way, the Bird class can still be used in the general sense (for non-flying birds), while FlyingBird only contains the flying behavior, ensuring that subclasses either fly or don’t (without violating expectations).
5. What is the Interface Segregation Principle (ISP)?
The Interface Segregation Principle states that no client should be forced to depend on methods it does not use.
Instead of one fat interface, many small, specific interfaces are preferred. For example:
public interface IWorker
{
void Work();
void Eat();
}Here, the IWorker interface is too broad. To follow ISP:
public interface IWork
{
void Work();
}
public interface IEat
{
void Eat();
}
public class Worker : IWork, IEat
{
public void Work()
{
/*Work logic */
}
public void Eat()
{
/*Eat logic */
}
}Now, classes can implement only the interfaces they need.
6. Can you explain the Dependency Inversion Principle (DIP)?
The Dependency Inversion Principle states that high-level modules should not depend on low-level modules.
Both should depend on abstractions (e.g., interfaces or abstract classes).
Additionally, abstractions should not depend on details. Details should depend on abstractions.
For example:
public class LightBulb
{
public void TurnOn()
{
/*Turn on logic */
}
public void TurnOff()
{
/*Turn off logic */
}
}
public class Switch
{
private LightBulb _bulb;
public Switch(LightBulb bulb)
{
_bulb = bulb;
}
public void Operate()
{
/*Switch logic */
}
}Here, Switch depends on the LightBulb class, which violates DIP.
The Switch class is tightly coupled to the LightBulb class. This means that if you need to change the behavior of the light bulb (e.g., add a new kind of light bulb), you would have to modify the Switch class as well.
The Switch depends on the concrete LightBulb class, which violates DIP because the high-level module (Switch) depends directly on a low-level module (LightBulb).
To follow DIP:
public interface ILightBulb
{
void TurnOn();
void TurnOff();
}
public class LightBulb : ILightBulb
{
public void TurnOn()
{
/*Turn on logic */
}
public void TurnOff()
{
/*Turn off logic */
}
}
public class Switch
{
private ILightBulb _bulb;
public Switch(ILightBulb bulb)
{
_bulb = bulb;
}
public void Operate()
{
/*Switch logic */
}
}Now, Switch depends on the ILightBulb interface, not the concrete LightBulb class.
This means that Switch doesn’t need to know what specific type of light bulb is being used—just that it adheres to the ILightBulb interface.
ILightBulb acts as an abstraction, and now the high-level module (Switch) depends on the abstraction rather than a specific implementation.
The low-level module (LightBulb) depends on this abstraction as well.
You can easily introduce new types of light bulbs (e.g., SmartLightBulb, LEDLightBulb, etc.) without modifying the Switch class. All you need to do is ensure that the new bulb class implements the ILightBulb interface.
A SmartLightBulb might have additional features like being able to connect to a network or adjusting brightness.
public class SmartLightBulb : ILightBulb
{
public void TurnOn()
{
// Add logic for remote control or network connection
}
public void TurnOff()
{
// Add logic for disconnecting from the network or saving settings
}
}An LEDLightBulb would be a more energy-efficient bulb with simpler functionality.
public class LEDLightBulb : ILightBulb
{
public void TurnOn()
{
// Logic for LED light behavior
}
public void TurnOff()
{
// Logic for turning off the LED
}
}Conclusion
Understanding the SOLID principles is essential for writing clean, maintainable, and scalable code in any programming language including C#.
These principles help ensure that your code is robust and can adapt to changes over time.
By familiarizing yourself with these commonly asked interview questions and their answers, you'll be well-prepared to discuss the SOLID principles in your next .NET, C# or any other tech interview. Good luck!
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Checkout Blind 75 Leetcode problems, a curated list of the most essential coding problems to crack your next technical interviews.
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Additional Resources
Here are some great books and resources to deepen your understanding:
Clean Code: A Handbook of Agile Software Craftsmanship by Robert C. Martin
Head First Design Patterns: Building Extensible and Maintainable Object-Oriented Software
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