将数据访问代码分解到单独的项目中

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

public class Repository<T> : IRepository<T> where T : class
{
    private readonly ApplicationDbContext _context;

    public Repository(ApplicationDbContext context)
    {
        _context = context;
    }

    public IEnumerable<T> GetAll()
    {
        return _context.Set<T>();
    }

    public T GetById(int id)
    {
        return _context.Set<T>().Find(id);
    }

    public void Add(T entity)
    {
        _context.Set<T>().Add(entity);
        _context.SaveChanges();
    }

    public void Update(T entity)
    {
        _context.Set<T>().Update(entity);
        _context.SaveChanges();
    }

    public void Delete(T entity)
    {
        _context.Set<T>().Remove(entity);
        _context.SaveChanges();
    }
}

public class UserService
{
    private readonly IRepository<User> _userRepository;

    public UserService(IRepository<User> userRepository)
    {
        _userRepository = userRepository;
    }

    public User GetUserById(int id)
    {
        return _userRepository.GetById(id);
    }

    public void CreateUser(User user)
    {
        _userRepository.Add(user);
    }
}