C# ASP.net Shopping cart

Using examples I crated shopping cart,
But  issue is if i add any products , its will appear all the other client web browser.
for that i need to add unique Cart identifier.
Could you please show me where i have to use that code i already attached my code

using System.Collections.Generic;
using System.Web;

/**
 * The ShoppingCart class
 * 
 * Holds the items that are in the cart and provides methods for their manipulation
 */
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;
    public const string CartSessionKey = "CartId";
    // 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[CartSessionKey] == null)
        {
            Instance = new ShoppingCart();
            Instance.Items = new List<CartItem>();
            HttpContext.Current.Session[CartSessionKey] = 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);
        }
    }

    /**
     * SetItemQuantity() - Changes the quantity of an item in the cart
     */
    public void SetItemQuantity(int productId, int quantity)
    {
        // If we are setting the quantity to 0, remove the item entirely
        if (quantity == 0)
        {
            RemoveItem(productId);
            return;
        }

        // Find the item and update the quantity
        CartItem updatedItem = new CartItem(productId);

        foreach (CartItem item in Items)
        {
            if (item.Equals(updatedItem))
            {
                item.Quantity = quantity;
                return;
            }
        }
    }

    public string GetCartId()
    {
        if (HttpContext.Current.Session[CartSessionKey] == null)
        {
            if (!string.IsNullOrWhiteSpace(HttpContext.Current.User.Identity.Name))
            {
                HttpContext.Current.Session[CartSessionKey] = HttpContext.Current.User.Identity.Name;
            }
            else
            {
                // Generate a new random GUID using System.Guid class.     
                Guid tempCartId = Guid.NewGuid();
                HttpContext.Current.Session[CartSessionKey] = tempCartId.ToString();
            }
        }
        return HttpContext.Current.Session[CartSessionKey].ToString();
    }
    
    /**
     * RemoveItem() - Removes an item from the shopping cart
     */
    public void RemoveItem(int productId)
    {
        CartItem removedItem = new CartItem(productId);
        Items.Remove(removedItem);
    }
    #endregion

    #region Reporting Methods
    /**
	 * GetSubTotal() - returns the total price of all of the items
	 *                 before tax, shipping, etc.
	 */
    public decimal GetSubTotal()
    {
        decimal subTotal = 0;
        foreach (CartItem item in Items)
            subTotal += item.TotalPrice;

        return subTotal;
    }
    #endregion
}

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