.net – CS0012：类型’System.Data.Linq.DataContext’在未引用

CS0246: The type or namespace name ‘DataClasses1DataContext’ could not be found (are you missing a using directive or an assembly reference?)

对于此.aspx文件：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class WebApplication1_admin_Places : System.Web.UI.Page
{
    protected void Page_Load(object sender,EventArgs e)
    {
        DataClasses1DataContext db = new DataClasses1DataContext();
        var query = (from m in db.Places orderby m.Name select m);
        PlacesList.DataSource = query;
        PlacesList.DataBind();
    }
}

事情是,在/文件夹我可以访问数据库,但在/ admin文件夹我得到这个错误.

我究竟做错了什么？

编辑

CS0012：类型’System.Data.Linq.DataContext’在未引用的程序集中定义.您必须添加对程序集’System.Data.Linq,Version = 3.5.0.0,Culture = neutral,PublicKeyToken = b77a5c561934e089’的引用.

那是什么意思？