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Interface vs abstract class in C#

C# 2 min read
Short answer

Use an interface to describe a capability that unrelated types can share. Use an abstract class when the types are genuinely related and you want to share implementation.

csharp
public interface IExportable
{
    string ToCsv();
}

public abstract class Lesson
{
    public string Title { get; init; } = "";
    public abstract int DurationSeconds { get; }

    public string Formatted() =>
        $"{DurationSeconds / 60}:{DurationSeconds % 60:D2}";   // shared logic
}

#The decisive difference

A class can implement many interfaces but inherit from one base class. So an interface is the right choice for a capability that cuts across unrelated types — IDisposable, IComparable, IEnumerable.

#Ask "is-a" or "can-do"

  • A VideoLesson is a Lesson → abstract class.
  • A VideoLesson can be exported → interface.

#Interfaces can have default implementations now

Since C# 8:

csharp
public interface ILogger
{
    void Log(string message);
    void LogError(string message) => Log($"ERROR: {message}");
}

This lets you add a method to a published interface without breaking implementers. It does not make interfaces a replacement for abstract classes — interfaces still cannot hold state.

#Prefer interfaces for testability

csharp
public class VideoService
{
    private readonly IRepository _repo;
    public VideoService(IRepository repo) => _repo = repo;
}

Depending on an interface means a test can pass a fake. Depending on a concrete class means the test needs a real database.

#Do not over-abstract

An interface with exactly one implementation, created "for flexibility", is usually just an extra file. Add it when you have a second implementation or a test that needs one.

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