Problem with session.

i have used class files. now since i wanted to check whether the session exists or not i used static.. Here is the class file..


using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Collections.Generic;

public class ShoppingCart
{
#region Properties

public List<cartitem> Items { get; private set; }

#endregion

#region Singleton Implementation


public static readonly ShoppingCart Instance;

ShoppingCart()
{
if (HttpContext.Current.Session["Cart"] == null)
{
Instance = new ShoppingCart();
Instance.Items = new List<cartitem>();
HttpContext.Current.Session["Cart"] = Instance;
}
else
{
Instance = (ShoppingCart)HttpContext.Current.Session["Cart"];
}
}

protected ShoppingCart() { }

#endregion

#region Item Modification Methods

public void AddItem( string pid)
{
CartItem newItem = new CartItem(pid);

if (Items.Contains(newItem))
{
foreach (CartItem item in Items)
{
if (item.Equals(newItem))
{
item.Quantity++;
return;
}
}
}
else
{
newItem.Quantity = 1;
Items.Add(newItem);
}
}

public void SetItemQuantity( string pid, int quantity)
{
if (quantity == 0)
{
RemoveItem(pid);
return;
}
CartItem updatedItem = new CartItem(pid);

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

public void RemoveItem( string pid)
{
CartItem removedItem = new CartItem(pid);
Items.Remove(removedItem);
}


#endregion

#region Reporting Methods
public decimal GetSubTotal()
{
decimal subTotal = 0;
foreach (CartItem item in Items)
subTotal += item.TotalPrice + item.Tax ;

return subTotal;
}
#endregion
}