interface IRepository<T>

Context: The Repository pattern is very common for data access. A generic IRepository<T> interface defines CRUD operations (Create, Read, Update, Delete) without depending on a specific entity type. Concrete implementations provide the code for a particular data source (database, API, file).

Usage Example

using System;
using System.Collections.Generic;
using System.Linq;

public interface IRepository<T>
{
    T GetById(int id);
    IEnumerable<T> GetAll();
    void Add(T entity);
    void Update(T entity);
    void Delete(int id);
}

// In-memory implementation (for testing)
public class InMemoryRepository<T> : IRepository<T> where T : class, IEntity
{
    private readonly List<T> _items = new List<T>();
    private int _nextId = 1;

    public T GetById(int id) => _items.FirstOrDefault(x => x.Id == id);

    public IEnumerable<T> GetAll() => _items.ToList();

    public void Add(T entity)
    {
        entity.Id = _nextId++;
        _items.Add(entity);
    }

    public void Update(T entity)
    {
        var existing = GetById(entity.Id);
        if (existing != null)
        {
            _items.Remove(existing);
            _items.Add(entity);
        }
    }

    public void Delete(int id)
    {
        var item = GetById(id);
        if (item != null) _items.Remove(item);
    }
}

public interface IEntity
{
    int Id { get; set; }
}

public class User : IEntity
{
    public int Id { get; set; }
    public string Name { get; set; }
}

class Program
{
    static void Main()
    {
        IRepository<User> userRepo = new InMemoryRepository<User>();
        userRepo.Add(new User { Name = "Alice" });
        userRepo.Add(new User { Name = "Bob" });

        foreach (var user in userRepo.GetAll())
        {
            Console.WriteLine($"{user.Id}: {user.Name}");
        }
    }
}

Output console

dotnet run
1: Alice
2: Bob

Important notes

• IRepository<T> is independent of the persistence mechanism.
• The pattern facilitates unit testing (mocking).

Real-world usage example

Entity Framework Core – DbSet<T> acts as a repository for each entity.
See docs on Repository pattern.

Related sections

• 11.3 Generic Interfaces
• 11.3.2 Covariance / Contravariance