Interface vs Abstract Class in C# — What’s the Difference and When Should You Use Each?

Interface vs Abstract Class in C# Choosing between an interface and an abstract class is one of the most fundamental design decisions in C#. It affects flexibility, testability, extensibility, and how your system evolves over time. This guide breaks down the differences with definitions, examples, real-world scenarios, and clear rules you can apply in interviews and production code. What Is an Interface? An interface defines a contract — a set of members a class must implement. It contains no implementation (until default interface methods were introduced in C# 8). Key characteristics Supports multiple inheritance Defines capabilities , not behavior Ideal for loose coupling Perfect for dependency injection Members are public by default Example public interface IEmailService { void Send ( string to , string subject , string body ); } Any class implementing this interface must provide the Send method. What Is an Abstract Class? An abstract class is a partially implemented class. It can c