CategoryController.cs

using Models;
using Providers;

public class CategoryController : ApiController
{
    [ResponseType(typeof(List<Category>))]
    public IHttpActionResult Get() => Ok(CategoryProvider.Categories);

    [ResponseType(typeof(Category))]
    public IHttpActionResult Get(string id) => Ok(CategoryProvider.Categories.FirstOrDefault(x => x.Id == id));
}