C# Interfaces vs Abstract Classes: Complete Guide

What Is an Interface?

An interface defines a contract that implementing types agree to provide.

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

What Is an Abstract Class?

An abstract class provides a base type that can contain both implemented behavior and abstract members.

public abstract class Employee
{
    public string Name { get; set; }
public abstract decimal CalculateSalary();
}

Key Differences

Interface	Abstract Class
Defines a contract	Can define shared implementation
A class can implement multiple interfaces	A class has one direct base class
Ideal for capabilities	Ideal for a shared base identity

When Should You Use an Interface?

Use an interface when unrelated classes need to support the same capability or when you want loose coupling and easy dependency injection.

When Should You Use an Abstract Class?

Use an abstract class when related types share state, implementation, or a common base identity.

Conclusion

Choose interfaces for contracts and capabilities, and abstract classes when you need a shared base implementation or state. In larger applications, the two can also be used together.