Overview

In .NET, understanding the differences between abstract classes and interfaces is essential for designing and implementing flexible, modular, and testable software. Both are used to achieve abstraction in C#, allowing for defining capabilities (methods, properties) without specifying the implementation. This topic holds significance as it directly impacts class design and architecture, influencing how components interact and evolve in an application.

Key Concepts

1. Abstraction and Encapsulation: Both abstract classes and interfaces promote these principles by defining a contract for what a class does, without specifying how.
2. Inheritance and Implementation: Abstract classes are extended using inheritance, while interfaces are implemented. This affects how they are used to compose objects.
3. Usage Scenarios: Understanding when to use an abstract class versus an interface is crucial for creating a maintainable and scalable application architecture.

Common Interview Questions

Basic Level

1. What are the basic differences between abstract classes and interfaces in C#?
2. Can you write a simple interface and an abstract class in C#?

Intermediate Level

3. How does inheritance work with abstract classes and interfaces in C#?

Advanced Level

4. Can you discuss a scenario where choosing between an abstract class and an interface could significantly impact the design or performance of a .NET application?

Detailed Answers

1. What are the basic differences between abstract classes and interfaces in C#?

Answer: Abstract classes and interfaces in C# are both types that allow you to define methods and properties without implementations, serving as a blueprint for other classes. However, they serve different purposes and have different rules:
- Abstract Classes can have a mix of methods with and without implementations. They can have fields, constructors, and can inherit from another class.
- Interfaces cannot have any implementation at all until C# 8.0, where default implementations were introduced. They can declare properties and methods but cannot have fields or constructors. A class can implement multiple interfaces, facilitating multiple inheritance of sorts.

Key Points:
- Abstract classes are suitable when classes share a common base of operations and fields.
- Interfaces are ideal for defining capabilities that can be added to a variety of classes.
- C# does not support multiple inheritance for classes but allows a class to implement multiple interfaces.

Example:

public abstract class Vehicle // Abstract class
{
    public abstract void Drive(); // Abstract method

    public void Start() // Non-abstract method
    {
        Console.WriteLine("Vehicle started");
    }
}

public interface IVehicle // Interface
{
    void Drive(); // Method declaration
}

2. Can you write a simple interface and an abstract class in C#?

Answer: Yes, here's a simple example demonstrating both an interface and an abstract class:

Key Points:
- An interface IVehicle declares a Drive method.
- An abstract class Vehicle implements IVehicle and provides an implementation for Start.

Example:

public interface IVehicle
{
    void Drive();
}

public abstract class Vehicle : IVehicle
{
    public abstract void Drive();

    public void Start()
    {
        Console.WriteLine("Starting the vehicle.");
    }
}

3. How does inheritance work with abstract classes and interfaces in C#?

Answer: In C#, abstract classes are used for base class inheritance where a derived class inherits from a single base class. Interfaces are used for implementing functionality that can come from various sources, thus allowing a form of multiple inheritance.

Key Points:
- A class can inherit only one abstract class but can implement multiple interfaces.
- Abstract classes can have constructors, which can be called using the base keyword in derived classes.
- Interfaces cannot have constructors. When a class implements an interface, it must provide implementations for all of its methods.

Example:

public interface IVehicle
{
    void Drive();
}

public abstract class Vehicle
{
    public abstract void Start();
}

public class Car : Vehicle, IVehicle // Inherits from Vehicle and implements IVehicle
{
    public override void Start()
    {
        Console.WriteLine("Car starting.");
    }

    public void Drive()
    {
        Console.WriteLine("Car driving.");
    }
}

4. Can you discuss a scenario where choosing between an abstract class and an interface could significantly impact the design or performance of a .NET application?

Answer: Consider a scenario where you're designing a plugin system for a large application. Choosing an interface for plugins can significantly impact the design and flexibility of the system. By using interfaces, you can define a set of functionalities that all plugins must implement without dictating the inheritance chain. This allows plugin developers to inherit from their preferred classes while still adhering to the required plugin interface, fostering a more diverse and flexible ecosystem.

Key Points:
- Interfaces offer greater flexibility in scenarios requiring multiple inheritance or adherence to a specific contract without enforcing a common ancestor.
- Abstract classes are more suited for situations where a base implementation or shared state needs to be provided to derived classes.
- Performance differences are generally negligible but choosing an interface can reduce coupling, making the system more modular and easier to test and maintain.

Example:

public interface IPlugin
{
    void Execute();
}

// Plugin developers can implement this interface in any way they see fit,
// without being forced into a specific inheritance chain.
public class MyPlugin : IPlugin
{
    public void Execute()
    {
        Console.WriteLine("Plugin executed.");
    }
}

This design allows for a high degree of flexibility and scalability in the application architecture, enabling developers to easily add new functionalities without impacting existing code.