use cookie problem for shoppingcart

hi,

i have the following code which adds the items to the shoppingcart by session

but i need to use cookies instead!! when i use cookies i get error converting cookie to shoppingcat!!

Imports Microsoft.VisualBasic


Public Class ShoppingCart

#Region "Properties"

    Private _items As List(Of CartItem)
    Public Property Items() As List(Of CartItem)
        Get
            Return _items
        End Get
        Private Set(ByVal value As List(Of CartItem))
            _items = value
        End Set
    End Property

#End Region

#Region "Singleton Implementation"


    Public Shared ReadOnly Instance As ShoppingCart

    Shared Sub New()

        If HttpContext.Current.Session("ASPNETShoppingCart") Is Nothing Then
            Instance = New ShoppingCart()
            Instance.Items = New List(Of CartItem)
            HttpContext.Current.Session("ASPNETShoppingCart") = Instance
        Else
            Instance = CType(HttpContext.Current.Session("ASPNETShoppingCart"), ShoppingCart)
        End If

    End Sub


    Protected Sub New()

    End Sub

#End Region

#Region "Item Modification Methods"


    Public Sub AddItem(ByVal productId As Integer)

        Dim newItem = New CartItem(productId)

        If Items.Contains(newItem) Then
            For Each item As CartItem In Items
                If item.Equals(newItem) Then
                    item.Quantity += 1
                    Return
                End If
            Next
        Else
            newItem.Quantity = 1
            Items.Add(newItem)
        End If

    End Sub


    Public Sub SetItemQuantity(ByVal productId As Integer, ByVal quantity As Integer)

        If quantity = 0 Then
            RemoveItem(productId)
            Return
        End If


        Dim updatedItem = New CartItem(productId)
        For Each item As CartItem In Items
            If item.Equals(updatedItem) Then
                item.Quantity = quantity
                Return
            End If
        Next
    End Sub


    Public Sub RemoveItem(ByVal productId As Integer)
        Dim removedItem = New CartItem(productId)

        For Each item As CartItem In Items
            If item.Equals(removedItem) Then
                Items.Remove(item)
                Return
            End If
        Next
    End Sub



    Public Sub RemoveAllItems()

        For Each item As CartItem In Items
            '  Items.Remove(item.ProductId)
        Next
    End Sub

#End Region

#Region "Reporting Methods"


    Public Function GetSubTotal() As Decimal
        Dim subTotal As Decimal = 0
        For Each item As CartItem In Items
            subTotal += item.TotalPrice
        Next

        Return subTotal
    End Function

    Public Function GetTotalItemsInCart() As String
        Dim Total As Decimal = 0
        For Each item As CartItem In Items
            Total += item.Quantity
        Next
        Return Total
    End Function




#End Region

End Class