找不到类型或命名空间名称“Admin”（是否缺少uing指令或程序集引用？）

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using StudentApartment.BusinessLogicLayer;

public partial class enter : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        //构建一个管理员帐号信息类,然后将用户名和密码传入该类
        Admin admin = new Admin();
        admin.AdminUserName = UserName.Text;
        admin.AdminPassword = Password.Text;

        if (admin.checkAdmin()) //如果验证用户名和密码都成功
        {
            //设置登陆成功的session变量
            Session["adminUserName"] = UserName.Text;
            Response.Redirect("default.aspx");         //转入到主操作界面
        }
        else
        {
            this.ErrMessage.Text = "<font color=red>" + admin.ErrMessage + "</font>";
        }
    }
    protected void Button2_Click(object sender, EventArgs e)
    {

    }
}

在线等 请高手们看看如何解决