找不到类型和名称空间------

在我的程序中，我得到的错误消息的类型和名称空间无法找到ISaleManagerUI

我不明白为什么它不工作，我不知道如何解决它。谁能给我指个正确的方向?

我有一个类叫做isalemanagerui。cs

这是下面的代码谢谢

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Web;
 using System.Web.UI;
 using System.Web.UI.WebControls;
 using System.IO;
 using Antiques;
 using AntiqueSales;
namespace Antiques
{
    public partial class About : System.Web.UI.Page, ISaleMangerUI
{
    SaleList saleList;
    protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            if (!ReferenceEquals(null, Session["ID"]))
            {
                Sale sale = new Sale((string)Session["ID"], (DateTime)Session["Date"], (string)Session["Location"], (double)Session["Picth Cost"], (int)Session["Capacity"],
                    (bool)Session["Yes"], (bool)Session["No"]), (string)Session["Charity"], (string)Session["Catering"];
                saleList = (SaleList)Application["SaleList"];
                saleList.addSale(sale);
                Application["SaleList"] = saleList;
                UpdateListbox();
            }
        }
        catch (DuplicateIdException)
        {
    //        UpdateListbox();
        }
        //lblError.Text = null;
    }