class CategoryModel
{
    public int category_id { get; private set; }
    public string category_title { get; private set; }

    //Constructor
    public CategoryModel(int id, string title)
    {
        category_id = id;
        category_title = title;
    }

static List<CategoryModel> CategoryList = new List<CategoryModel>();

public static string GetCategoryName(int categoryNumber)
    {
        IEnumerable<string> title = from c in CategoryList
                    where c.category_id.Equals(categoryNumber)
                    select c.category_title;
        return title.ToString();
    }