I added Using System.Configuration but still it shows error that "Configuration Manager is not found in this contex??"

using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using BEL_BookApp;

namespace DEL_BookApp
{

    class BookDetaile_DAL
    {

        SqlConnection cn = new .SqlConnection(ConfigurationManager.ConnectionStrings["DB"].COnnectionString);
    }
}

Here Configuration manager is showing error.although I had written using system.configuration