window form in connection string could not get on Dbconnection page

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C#

sir i am using app config file for creating connection and access this connection on Dbconnection.class page but i get error in ConfigurationManager in this pls solve it
and tell me what is the problem in this code.

using System.Data.SqlClient;
using System.Configuration;


namespace hello42_win
{
       
   public class Dbconnection : Form
    {
        SqlConnection sqlConn;


        public SqlConnection OpenConnection()
        {
                       sqlConn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConString"].ConnectionString);

            if (sqlConn.State != ConnectionState.Open)
            {
                sqlConn.Open();
            }
            return sqlConn;
        }
        public SqlConnection CloseConnection()
        {
            if (sqlConn.State != ConnectionState.Closed)
            {
                sqlConn.Close();
            }
            return sqlConn;
        }

    }
}