persist_user_logic.cs

public interface IRepository<T> 
{
    Task Insert(T entity);
    Task<T> Get(T Id);
    Task Update(T entity);
    Task Delete(T entity);
}

public interface IUserRepository : IRepository<User> {
}

public class UserRepository : IRepository {

    public async Task Insert(User entity) {
        // logic
    }
    public async Task<User> Get(Guid Id){
        // logic
    }
    public async Task Update(User entity) {
        // logic
    }
    public async Task Delete(User entity) {
        // logic
    }
}

public class UserService {
    private IUserRepository _userRepository;

    // receives a IUserRepository by dependency injection
    public UserService(IUserRepository userRepository) {
        _userRepository = userRepository;
    }

    public async Task PersisteUser(User user) {

        // a lot of code
        // bla bla bla
        // everything that needs to be done

        // finally persist user
        _userRepository.Insert(user);
    }
}