ASP.NET

<asp:scriptmanager... xmlns:asp="#unknown" />

<script runat="server" type="text/C#">
    void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            if (Session["dummy"] == null)
                Session["dummy"] = 1;
        }

        lbItem.Text = Session["dummy"].ToString() + " items";
    }
</script>

<asp:contentplaceholder id="contentCart" runat="server" xmlns:asp="#unknown">
<asp:updatepanel id="upCart" runat="server" updatemode="Always" rendermode="Inline">
<contenttemplate>
    <table>
    <tr>
    <td>
    <asp:label id="lbItem" runat="server" />
    </td>
    </tr>
    </table>
    </contenttemplate>
    </asp:updatepanel>
    </asp:contentplaceholder>

<script runat="server">

    protected void Page_Load(object sender, EventArgs e)
    {
        ScriptManager sm1 = (ScriptManager)this.Master.FindControl("ScriptManager1");
        sm1.RegisterAsyncPostBackControl(Button1);          
    }
    
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (TextBox2.Text != "")
        {
            Session["dummy"] = (int)Session["dummy"] + int.Parse(TextBox2.Text);
        }                 
    }
</script>