ExampleService.cs

using System;
using System.Collections.Generic;

public class ExampleService
{
    private readonly IRepository _repository;

    public ExampleService(IRepository repository)
    {
        _repository = repository;
    }

    public List<string> GetItems()
    {
        return _repository.GetAllItems();
    }

    public string GetItemById(int id)
    {
        var item = _repository.GetItem(id);
        if (item == null)
        {
            throw new ArgumentException("Item not found");
        }
        return item;
    }

    public void AddItem(string item)
    {
        if (string.IsNullOrWhiteSpace(item))
        {
            throw new ArgumentException("Item cannot be null or empty");
        }

        _repository.AddItem(item);
    }

    public void DeleteItem(int id)
    {
        var item = _repository.GetItem(id);
        if (item == null)
        {
            throw new ArgumentException("Item not found");
        }

        _repository.DeleteItem(id);
    }
}

IRepository.cs

public interface IRepository
{
    List<string> GetAllItems();
    string GetItem(int id);
    void AddItem(string item);
    void DeleteItem(int id);
}