'System.Web.HttpUnhandledException' in System.Web.dll Error When I create a new page in my project

I have a project and I took it from someone else who I couldnt contact with him anymore. I want to add a new page in the project and I got this error 'System.Web.HttpUnhandledException' in System.Web.dll

this is my aspx page:

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true"
CodeBehind="example.aspx.cs" Inherits="Web.example" %>

and this is my cs code:

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.Services;
using UserManagement.Manager;
using System.Windows.Forms;
using System.IO;
using System.Text;

namespace Web
{
   public partial class example : xPage
   {
    public string id = "";
    public static DataTable table = new DataTable();
   }
}

I did not put anything in the page but I got this error. Can anybody help me? Thank you!