Overview
In C#, both abstract classes and interfaces are foundational concepts used for abstraction, allowing the definition of required structure and behavior without implementing the details. Understanding their differences is crucial for designing flexible and maintainable object-oriented software.
Key Concepts
- Purpose and Usage: Abstract classes serve as a partial base class implementation, while interfaces define a contract without any implementation.
- Inheritance vs. Implementation: A class can inherit from only one abstract class but can implement multiple interfaces.
- Members: Abstract classes can have implemented methods, properties, or fields, whereas interfaces can only declare methods or properties without fields.
Common Interview Questions
Basic Level
- What is the main difference between an abstract class and an interface in C#?
- Can you provide a simple example showing an interface and an abstract class in C#?
Intermediate Level
- How does C# 8.0's introduction of default interface methods affect the difference between interfaces and abstract classes?
Advanced Level
- In what scenarios would you choose an abstract class over an interface in designing a software component?
Detailed Answers
1. What is the main difference between an abstract class and an interface in C#?
Answer: The main difference lies in their intended use and capabilities. Abstract classes are used as a base class for other classes to provide common functionality, allowing for both abstract and concrete method implementations. Interfaces, on the other hand, define a contract that implementing classes must follow, containing only method and property declarations without any implementation.
Key Points:
- Abstract classes can contain constructors, fields, and implemented methods.
- Interfaces cannot contain fields and are entirely declaration-based.
- A class can inherit multiple interfaces but only one abstract class.
2. Can you provide a simple example showing an interface and an abstract class in C#?
Answer: Sure, below is an example showcasing both concepts:
Example:
public interface IVehicle
{
void Drive();
}
public abstract class VehicleBase
{
public string LicensePlate { get; set; }
// Abstract method
public abstract void StartEngine();
// Concrete method
public void StopEngine()
{
Console.WriteLine("Engine stopped.");
}
}
public class Car : VehicleBase, IVehicle
{
public override void StartEngine()
{
Console.WriteLine("Engine started.");
}
public void Drive()
{
Console.WriteLine("Car is driving.");
}
}
3. How does C# 8.0's introduction of default interface methods affect the difference between interfaces and abstract classes?
Answer: With C# 8.0, interfaces can now have default implementations for methods, reducing the gap between interfaces and abstract classes. This allows developers to add new methods to an interface without breaking existing implementations. However, abstract classes still provide more flexibility by allowing fields and constructors, which interfaces cannot have.
Key Points:
- Default interface methods maintain backward compatibility.
- Abstract classes can still hold state through fields, unlike interfaces.
- Choosing between an interface with default methods and an abstract class may depend on whether instance state and constructors are needed.
4. In what scenarios would you choose an abstract class over an interface in designing a software component?
Answer: You might choose an abstract class over an interface when:
- You need to share code among several closely related classes.
- Your classes require access to shared state or fields.
- The functionality evolves frequently, and you wish to provide a base implementation that subclasses can automatically inherit.
Key Points:
- Abstract classes are ideal for situations where classes share a common implementation.
- Interfaces are better suited for defining a common contract for unrelated classes.
- With the introduction of default interface methods, the decision may also consider versioning and backward compatibility concerns.
