Abstract Classes vs Interfaces in C

In C#, both abstract classes and interfaces are fundamental tools used to achieve abstraction and polymorphism, yet they serve distinct architectural purposes. This article explores the core definitions of abstract classes and interfaces, contrasts their key technical differences—including inheritance capabilities, state management, and member implementations—and provides clear guidelines on when to use each in your application design.

Core Definitions

An abstract class is an incomplete class marked with the abstract keyword that cannot be instantiated directly. It serves as a blueprint for other classes, allowing you to define a common identity, maintain internal state, and provide both complete and incomplete (abstract) implementations of methods, properties, and events.

An interface is a contractual specification defined with the interface keyword. It specifies a set of members that any implementing class or struct must provide. Rather than defining an object's identity, an interface defines a capability or behavior.

Key Differences

1. Inheritance

• Abstract Classes: C# does not support multiple class inheritance. A class can inherit from only one abstract class.
• Interfaces: A class or struct can implement multiple interfaces, allowing for flexible, decoupled system designs.

2. State and Fields

• Abstract Classes: Can declare instance fields, constants, and constructors to initialize shared state across derived types.
• Interfaces: Cannot declare instance fields or constructors. They define behaviors and contracts, not internal object state.

3. Implementation and Modern C# Features

• Abstract Classes: Can contain both fully implemented concrete methods and abstract methods that force derived classes to provide their own implementation.
• Interfaces: Traditionally contain only method signatures. Since C# 8.0, interfaces support Default Interface Methods (DIM), allowing default behavior without breaking existing implementations. However, interfaces still cannot maintain instance state to support these implementations.

4. Access Modifiers

• Abstract Classes: Support all standard access modifiers (public, protected, internal, private) for methods, properties, and fields.
• Interfaces: Members are public by default. While modern C# allows explicit access modifiers for default implementations, public API contracts remain the standard usage.

5. Design Philosophy: "Is-A" vs. "Can-Do"

• Abstract Classes represent an "is-a" relationship: For example, a Dog is a Mammal. Mammals share common physiological mechanisms, internal state, and behaviors.
• Interfaces represent a "can-do" capability: For example, both a Document and a Bitmap can implement IPrintable. These two types share no common lineage, only the shared ability to be printed.

Summary of Differences

When to Choose Which

Use an Abstract Class when:

• You are building a closely related hierarchy of types sharing common logic and state.
• You need to provide protected internal helper methods to derived classes.
• You need constructors to enforce proper initialization of base state.
• You expect to evolve the base functionality over time within a self-contained library without breaking derived classes.

Use an Interface when:

• You want to define a specific role or behavior for disparate, unrelated classes.
• You need to enable multiple inheritance of type definitions.
• You are building loosely coupled architectures, such as dependency injection in ASP.NET Core.
• You are defining contracts for structs, which cannot inherit from classes.