ASP.NET "'Session' threw an exception of type 'System.NullReferenceException'" master page

Question

I'm building a website with a shopping cart.
I want all Cart's Item will appear on master page.

This is my SiteMaster.Master.cs:

  protected void Page_Load(object sender, EventArgs e)
    {
        System.Web.Security.MembershipUser user = System.Web.Security.Membership.GetUser();

        IBLClient b = new FrontEnd();
        if (user != null)
            Session["UserID"] = b.GetUserIDByUsername(user.UserName);

        if (Session["Cart"] == null)
        {
            Session["Cart"] = new List<string>();
        }

        if (!(Roles.IsUserInRole("Admin") || Roles.IsUserInRole("Seller")))
        {
            ManagePage.Visible = false;

        }

    }


    public BindingList<Product> GetProductsCart()
    {
        IBLClient b = new FrontEnd();
        BindingList<Product> ProductsList = new BindingList<Product>();


        if (Session["Cart"] != null)
        {
            foreach (string ProductId in (List<string>)Session["Cart"])
            {
                ProductsList.Add(b.GetProduct(int.Parse(ProductId)));
            }
        }

        return ProductsList;
    }

And this is my Source code:

 <asp:DataList ID="ProductsList" runat="server" CellPadding="4" DataSourceID="ProductsListObjectDataSource" ForeColor="#333333">
                                    <AlternatingItemStyle BackColor="White" />
                                    <ItemTemplate>
                                    </ItemTemplate>
                                    <SelectedItemStyle BackColor="#C5BBAF" Font-Bold="True" ForeColor="#333333" />
                                </asp:DataList>
                                <asp:ObjectDataSource ID="ProductsListObjectDataSource" runat="server" SelectMethod="GetProductsCart" TypeName="WebGUI.SiteMaster"></asp:ObjectDataSource>

When the Page is load it's look like this:

But after the ObjectDataSource's SelectMethod GetProductsCart() function, I got this exeption and the this changed.

Any idea why it's happening?
What should I do to avoid it?

Answer 1

I should do context.Session this is the problem!!