What is the difference between an abstract class and an interface in C#?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Decide: share code or declare capability

   If closely related types need shared state and logic, reach for an abstract class; if unrelated types just need to promise behaviour, use an interface.

2. Step 2

   Define the abstract class

   Mark the class abstract, add fields, a constructor, concrete helpers, and abstract members that derived classes must override.

3. Step 3

   Define the interface

   Declare method, property, and event signatures the implementer must provide; optionally add default implementations in C# 8+.

4. Step 4

   Implement in a concrete type

   A class inherits one abstract base with a colon and implements any number of interfaces in the same base list.

5. Step 5

   Consume via the abstraction

   Program against the abstract type or interface so callers stay decoupled from concrete implementations.

Code Example

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

public abstract class ShapeBase
{
    protected ShapeBase(string name) => Name = name;
    public string Name { get; }
    public abstract double Area();      // must override
    public void Describe() =>            // shared logic
        Console.WriteLine($"{Name} area = {Area()}");
}

public class Circle : ShapeBase, ILogger
{
    private readonly double _r;
    public Circle(double r) : base("Circle") => _r = r;
    public override double Area() => Math.PI * _r * _r;
    public void Log(string message) => Console.WriteLine(message);
}

Follow-up Questions

• Can an abstract class implement an interface?
• What changed with default interface methods in C# 8?
• When would you prefer composition over an abstract base class?
• Can you instantiate an abstract class directly?
• How do explicit interface implementations resolve name clashes?