Lesson 7 of 40 OOP Intermediate ⏱ 35 min

Interfaces & Abstract Classes

Design flexible systems with interfaces, default interface methods, explicit implementation, and understand when to choose interface vs abstract class.

Part 1: Defining & Implementing Interfaces

public interface IRepository<T>
{
    Task<T?> GetByIdAsync(int id);
    Task<IEnumerable<T>> GetAllAsync();
    Task SaveAsync(T entity);
    Task DeleteAsync(int id);
}

Any class that implements IRepository<T> must provide all four methods, enabling DI and testability.

Part 2: Default Interface Methods

public interface ILogger
{
    void Log(string message);
    // Default implementation — no need to override
    void LogError(string msg) => Log($"[ERROR] {msg}");
    void LogInfo(string msg) => Log($"[INFO] {msg}");
}

Part 3: Explicit Interface Implementation

public class DualLogger : IConsoleLogger, IFileLogger
{
    // Resolves name clash
    void IConsoleLogger.Write(string s) => Console.WriteLine(s);
    void IFileLogger.Write(string s) => File.AppendAllText("log.txt", s);
}

Part 4: Interface vs Abstract Class

	Interface	Abstract Class
Multiple inheritance	✅ Many interfaces	❌ One only
Constructors	❌ None	✅ Yes
Fields	❌ None	✅ Yes
Default impl	✅ Optional	✅ Yes
Use when	Capability contract	Shared base behavior

Interfaces & Abstract Classes

Part 1: Defining & Implementing Interfaces

Part 2: Default Interface Methods

Part 3: Explicit Interface Implementation

Part 4: Interface vs Abstract Class

C# in Visual Studio 2026