What is it different between (private vs public ) in this code (new lis<cartline> )

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ASP.NET

C#7.0

ASP.NET-Core

HI
I Use Asp.net core MVC 2 PDF for Learn asp.net core 2

in this book Used Session for store List Of product in the Card. Now i have two codes with the following :

first code :

private List lineCollection = new List();

second :

Public List lineCollection = new List();

when i Used Firs Code (private) all of Things work Correctly ... But I don't know what happened when i Use Second Code (public) List not work correctly .. when i click Button "add product to cart" then one product is added to list correct . but when i want to see List of CartLine in the index of CartController i see two product in the list but i add just one product no two product or when i want to remove that product from list it does not Removed from List ...

the complete Class Cart :

public class Cart
    {
        
       // public List lineCollection = new List();
        private List lineCollection = new List();
        public virtual void AddItem(Product product, int quantity)
        {
            CartLine line = lineCollection
            .Where(p => p.Product.ProductID == product.ProductID)
            .FirstOrDefault();
            if (line == null)
            {
                lineCollection.Add(new CartLine
                {
                    Product = product,
                    Quantity = quantity
                });
            }
            else
            {
                line.Quantity += quantity;
            }
        }

        //Remove All Products in Cart
        public virtual void RemoveLine(Product product)
            => lineCollection.RemoveAll(l => l.Product.ProductID == product.ProductID);
        //Compute Total in Cart
        public virtual decimal ComputeTotalValue()
            => lineCollection.Sum(e => e.Product.Price * e.Quantity);

        public virtual void Clear() => lineCollection.Clear();

        public virtual IEnumerable Lines => lineCollection;
    }

and the complete Controller Class Cart:

public class CartController : Controller
    {

        private IProductRepository repository;
        private Cart Cart;
        public CartController(IProductRepository repo,Cart cart)
        {
            repository = repo;
            Cart = cart;
        }

        public ViewResult Index(string returnUrl)
        {
            return View(new CartIndexViewModel
            {
                //Cart = GetCart(),
                Cart = Cart,
                ReturnUrl = returnUrl
            });
        }
        public RedirectToActionResult AddToCart(int productId,string returnUrl)
        {
            Product product = repository.Products.FirstOrDefault(p=>p.ProductID==productId);
            if (product!=null)
            {
                //Cart cart = GetCart();
               
                //cart.AddItem(product, 1);
               // SaveCart(cart);
               Cart.AddItem(product, 1);
            }
            return RedirectToAction("Index", new { returnUrl });
        }

        public RedirectToActionResult RemoveFromCart(int productId,string returnUrl)
        {
            Product product = repository.Products
            .FirstOrDefault(p => p.ProductID == productId);
            if (product != null)
            {
                //Cart cart = GetCart();
                //cart.RemoveLine(product);
                //SaveCart(cart);
                Cart.RemoveLine(product);
            }
            return RedirectToAction("Index", new { returnUrl });
        }
     

    }

and Session Extension class:

public static class SessionExtensions
    {
        public static void SetJson(this ISession session,string key,object value)
        {
            session.SetString(key, JsonConvert.SerializeObject(value));

        }

        public static T GetJson(this ISession session,string key)
        {
            var sessiondata = session.GetString(key);
            return sessiondata == null ? default(T) : JsonConvert.DeserializeObject(sessiondata);
        }
    }

...................... ??? Why ??? i dont know what is it happen between private and Public in this piece of code

What I have tried:

i just changed Public to private and all of code work correct i don't know why?