Interfaces vs Abstract Classes in C

Two ways to define a contract for your code — and how to pick the right one without overthinking it

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These are the most confusing topics for every beginner, aren’t they?

This article will help you understand the basics of them.

The Core Idea

Interface — A contract

Abstract Classes — A partial blueprint

An interface says, “any class that implements me MUST have these methods.” No implementation, no logic, just the promise.

An abstract class can have some finished code AND some unfinished code that child classes must complete.

A Simple Example

Interface Example:

public interface IAnimal
{
    void MakeSound(); // no implementation, just a promise
}

public class Dog : IAnimal
{
    public void MakeSound()
    {
        Console.WriteLine("Woof!");
    }
}

public class Cat : IAnimal
{
    public void MakeSound()
    {
        Console.WriteLine("Meow!");
    }
}

Every class that implements IAnimal must write its own MakeSound().

The interface is ruthless and offers no help — it just demands that the method exist.

Abstract Example:

public abstract class Animal
{
    public void Eat()
    {
        Console.WriteLine("Eating food...");
    }

    public abstract void MakeSound();
}

public class Dog : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Woof!");
    }
}

Here, Animal already has a shared implementationEat().

Only MakeSound() is left unfinished, which has no code body — that's the part each animal must define for itself — And that’s important.

It’s okay if we don’t defineEat, but that’s not the case for MakeSound().

Key Differences

Interface

• Defines a contract (methods without implementation).
• A class that implements an interface must provide all method definitions.
• Supports multiple inheritance (a class can implement many interfaces).
• Cannot have instance variables (only constants).
• Methods are implicitly public and abstract.

Abstract Class

• Serves as a base class with partial implementation.
• Can have both abstract methods (no body) and concrete methods (with body).
• Supports single inheritance (a class can extend only one abstract class).
• Can have instance variables and constructors.
• Useful when classes share common behavior but also need to enforce certain methods.

Important:

Interfaces started having body implementations in C# with the release of C# 8.0 in September 2019.

This feature is officially known as Default Interface Methods.

I have not used the Default Interface Method because I aimed to explain the difference between the two.

The deciding factor:

• Interface — Like a “blueprint” (what must be done).
• Abstract Class — Like a “template” (what is partly done + what must be done).

When to Use Which

If I summarize you the above discussion, use:

Interfaces:

• Use this when you have a shared contract.
• For dependency injection and loose coupling.
• When you need multiple inheritance of behavior.

Abstract Classes :

• Use this when you have a shared behavior
• When you want to define default behavior but still require subclasses to override specific parts.
• When you need to maintain common fields or properties across subclasses.

The 3-Minute Takeaway

If you’re just defining what something should do — interface.

If you’re defining what something should do and giving it some shared code to start with — abstract class.

That’s really it.

Most real-world C# codebases use both together — interfaces for flexibility, abstract classes for shared logic — and once you’ve written a few of each, the choice starts to feel obvious rather than confusing.

Thank you for reading 😊

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