request to review the shopping cart

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C#

ASP.NET

hi all,

i am a mca student and i was trying to prepare my collage project. to make my project more realistic i want to add a cart to my project. which i am not aware how to do. in order to get the ans to my question i gone through one article and as per the guidance i have a prepared a demo sales cart.

cart was properly functional. problem occurred when i uploaded my demo project on free hosting site. i found that cart is not empty. when i updated the information in the cart from computer A was also appearing in the computer B. i think same cart is used between all users.

below is what i have created, i want some to review this code. i know i am asking to much, but your help will definitely help in getting good grade.

first i created 3 classes: cartitem, product, shoppingcart. and code is as follows:

1) product class:

public class Product
{
    public int Id { get; set; }
    public decimal Price { get; set; }
    public string Description { get; set; }

    public Product(int id)
    {
        this.Id = id;
        switch (id) {
            case 1:
                this.Price = 19.95m;
                this.Description = "Shoes";
                break;
            case 2:
                this.Price = 9.95m;
                this.Description = "Shirt";
                break;
            case 3:
                this.Price = 14.95m;
                this.Description = "Pants";
                break;
        }
    }

2) Shopping cart

public class ShoppingCart {
	#region Properties
	
	public List<cartitem> Items { get; private set; }
	
	#endregion

	#region Singleton Implementation

	// Readonly properties can only be set in initialization or in a constructor
	public static readonly ShoppingCart Instance;
	// The static constructor is called as soon as the class is loaded into memory
	static ShoppingCart() {
		// If the cart is not in the session, create one and put it there
		// Otherwise, get it from the session
		if (HttpContext.Current.Session["ASPNETShoppingCart"] == null) {
			Instance = new ShoppingCart();
			Instance.Items = new List<cartitem>();
			HttpContext.Current.Session["ASPNETShoppingCart"] = Instance;
		} else {
			Instance = (ShoppingCart)HttpContext.Current.Session["ASPNETShoppingCart"];
		}
	}

	// A protected constructor ensures that an object can't be created from outside
	protected ShoppingCart() { }

	#endregion

	#region Item Modification Methods
	/**
	 * AddItem() - Adds an item to the shopping 
	 */
	public void AddItem(int productId) {
		// Create a new item to add to the cart
		CartItem newItem = new CartItem(productId);

		// If this item already exists in our list of items, increase the quantity
		// Otherwise, add the new item to the list
		if (Items.Contains(newItem)) {
			foreach (CartItem item in Items) {
				if (item.Equals(newItem)) {
					item.Quantity++;
					return;
				}
			}
		} else {
			newItem.Quantity = 1;
			Items.Add(newItem);
		}
	}

3) cartitem class

public class CartItem : IEquatable<cartitem> {
	#region Properties

	// A place to store the quantity in the cart
	// This property has an implicit getter and setter.
	public int Quantity { get; set; }

	private int _productId;
	public int ProductId {
		get { return _productId; }
		set {
			// To ensure that the Prod object will be re-created
			_product = null;
			_productId = value;
		}
	}

	private Product _product = null;
	public Product Prod {
		get {
			// Lazy initialization - the object won't be created until it is needed
			if (_product == null) {
				_product = new Product(ProductId);
			}
			return _product;
		}
	}

	public string Description {
		get { return Prod.Description; }
	}

	public decimal UnitPrice {
		get { return Prod.Price; }
	}

	public decimal TotalPrice {
		get { return UnitPrice * Quantity; }
	}

	#endregion

	// CartItem constructor just needs a productId
	public CartItem(int productId) {
		this.ProductId = productId;
	}

	/**
	 * Equals() - Needed to implement the IEquatable interface
	 *    Tests whether or not this item is equal to the parameter
	 *    This method is called by the Contains() method in the List class
	 *    We used this Contains() method in the ShoppingCart AddItem() method
	 */
	public bool Equals(CartItem item) {
		return item.ProductId == this.ProductId;
	}

Note: I have also created the web page which have link button. code is as follows

protected void btnAddShoes_Click(object sender, EventArgs e) {
        // Add product 1 to the shopping cart
        ShoppingCart.Instance.AddItem(1);

        // Redirect the user to view their shopping cart
        Response.Redirect("ViewCart.aspx");

Can any one please review the code and advise what is wrong init