新手求助：缺少using指令或程序集引用

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Data.OleDb;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using Infragistics.WebUI.UltraWebNavigator;
我已经引用了的。
private void Page_Load(object sender, System.EventArgs e)
{
// 在此处放置用户代码以初始化页面
//rootNode = UltraWebTree2.Nodes.Add("First Root Node"); 
if(!this.IsPostBack) 
           {string northwind = @"C:\Program Files\Infragistics\NetAdvantage 2004 Volume 2\ASP.NET\Samples\Data\Nwind.mdb";
DataSet customers = new DataSet();
OleDbConnection con = new OleDbConnection( String.Format( "Data Source=\"{0}\";Mode=Share Deny None;Provider=\"Microsoft.Jet.OLEDB.4.0\";", northwind));
OleDbDataAdapter cmdCustomers = new OleDbDataAdapter( "Select * from Customers", con);
OleDbDataAdapter cmdOrders = new OleDbDataAdapter( "Select * from Orders, Employees where Orders.EmployeeID = Employees.EmployeeID", con);
OleDbDataAdapter cmdOrderDetails = new OleDbDataAdapter( "Select * From [Order Details], Products where [Order Details].ProductID = Products.ProductID", con);
cmdCustomers.Fill( customers, "Customers");
cmdOrders.Fill( customers, "Orders");
cmdOrderDetails.Fill( customers, "Order_Details");
customers.Relations.Add( "CustOrders", customers.Tables[ "Customers"].Columns[ "CustomerID"], customers.Tables[ "Orders"].Columns[ "CustomerID"]);
customers.Relations.Add( "OrderDetails", customers.Tables[ "Orders"].Columns[ "OrderID"], customers.Tables[ "Order_Details"].Columns[ "OrderID"]);
          } this.UltraWebTree2.DataSource = customers.Tables[ "Customers"].DefaultView;
            
this.UltraWebTree2.Levels[0].RelationName = "CustOrders";
this.UltraWebTree2.Levels[0].ColumnName = "CompanyName";
this.UltraWebTree2.Levels[1].RelationName = "OrderDetails";
this.UltraWebTree2.Levels[1].ColumnName = "LastName";
this.UltraWebTree2.Levels[2].ColumnName = "ProductName";         this.UltraWebTree2.DataBind();
}运行提示:C:\Inetpub\wwwroot\frame\Navigation.aspx.cs(44): 找不到类型或命名空间名称“customers”(是否缺少 using 指令或程序集引用？)这行代码：this.UltraWebTree2.DataSource = customers.Tables[ "Customers"].DefaultView;