ช่วยแก้ Error ให้หน่อยค่ะ The name 'WebConfigurationManager' does not exist in the current context ภาษา asp.net c#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;

public partial class Default3 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
     
        string StrConn = WebConfigurationManager.ConnectionStrings["Database"].ConnectionString;
        SqlConnection Conn = new SqlConnection(StrConn);
        Conn.Open();
        string sqlProduct;
        sqlProduct = " Select iddog,namedog,agedog FROM dog ";
        SqlDataAdapter da = new SqlDataAdapter(sqlProduct, Conn);
        DataSet ds = new DataSet();
        da.Fill(ds, "dog");
        GridView1.DataSource = ds.Tables["dog"];
        GridView1.DataBind();
    }
}