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

Feature	Abstract Class	Interface
Can be instantiated?	No	No
Can contain implementation?	Yes	Yes, modern C# supports default implementations
Can contain fields?	Yes	No instance fields
Can have constructors?	Yes	No instance constructors
Multiple inheritance	No	A class can implement multiple interfaces
Shared state	Yes	No instance state
Best for	Closely related types	Contracts/capabilities
Dependency Injection	Possible	Very common
Common implementation	Strong support	Limited/default implementations
Versioning considerations	Base-class changes can affect derived types	Interface evolution requires care

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10. Choosing Between an Abstract Class and an Interface

Here are some practical guidelines.

Use an abstract class when:

• The types have a strong "is-a" relationship.
• You need to share common implementation.
• You need common state or fields.
• You need constructors.
• You want to provide a common workflow.
• Derived classes are expected to follow a common architecture.

Example:

PaymentProcessor
       |
 ┌─────┴─────┐
CardPayment  BankPayment

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Use an interface when:

• You want to define a contract.
• Different and potentially unrelated classes need the same capability.
• You want multiple capabilities on a class.
• You are designing for dependency injection.
• You want to reduce coupling.
• You want to make implementations easier to replace or mock.

Example:

ICacheable
   ↑
 ┌─┴───────────────┐
Customer           Product

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11. Keep Interfaces Small

A useful modern design principle is to avoid creating very large interfaces.

Instead of:

public interface IUserService
{
    void Create();
    void Update();
    void Delete();
    void SendEmail();
    void GenerateReport();
    void Export();
}

consider separating responsibilities:

public interface IUserRepository
{
    Task<User?> GetAsync(int id);
}

public interface IUserNotificationService
{
    Task SendAsync(User user);
}

public interface IUserReportService
{
    Task GenerateAsync();
}

This follows the Interface Segregation Principle and generally produces easier-to-maintain code.

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12. Don't Automatically Choose an Interface

Interfaces are extremely common in modern .NET applications, particularly because of dependency injection. However, adding an interface to every class isn't automatically good design.

For example, creating:

IUserService
UserService

only because "every service needs an interface" can introduce unnecessary abstraction.

The better question is:

What design problem does this abstraction solve?

If the abstraction provides value through polymorphism, dependency inversion, testing, multiple implementations, or architectural boundaries, an interface can be useful.

Otherwise, a concrete class may be perfectly appropriate.

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13. A Simple Rule of Thumb

A practical way to remember the difference is:

Abstract class = shared foundation

Interface = shared contract or capability

For example:

Abstract Class

Animal
  |
  ├── Dog
  └── Cat

They share common characteristics and behavior.

Whereas:

Interface

IFlyable
  ↑
  ├── Bird
  ├── Airplane
  └── Drone

These types don't need to belong to the same class hierarchy, but they share a capability.

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Conclusion

Abstract classes and interfaces are both powerful mechanisms for abstraction and polymorphism in C#, but they should be used for different design purposes.

Use an abstract class when you need a common base, shared state, shared implementation, or a common workflow among closely related types.

Use an interface when you need to define a contract or capability that can be implemented by multiple, potentially unrelated types.

With modern C# and .NET, interfaces have become even more capable, while dependency injection, SOLID principles, composition, and loose coupling have made interfaces particularly important in application architecture.