ASP.NET - how to fix errror: is not allowed here because it does not extend class 'System.Web.UI.Page'.

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Ensure that namespace in code behind is correct
<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication12._Default" %>

must be the same as

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

namespace WebApplication12
{