ASP.Net MVC Error: System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null

Connection problem with the database

System.NullReferenceException: 'The object reference is not defined to an instance of an object.'

System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null.

My connection string :

<add name="Constring" connectionString="data source=.; database = namedatabase; integrated security = SSPI" providerName="System.Data.EntityClient" />

Code :

public ActionResult Affhicherinfo(string code)
{
    List<Resultat> infogenes = new List<Resultat>(); 
    string constr = ConfigurationManager.ConnectionStrings["sConnectionString"].ConnectionString; 
    using (SqlConnection con = new SqlConnection(constr))
    {
        string query = " SELECT  Nom,Id FROM Information_General where nom_de_site= @code "; 
        using (SqlCommand cmd = new SqlCommand(query))
        {
            cmd.Connection = con; con.Open(); 
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    infogenes.Add(new Resultat { Nom = sdr["Nom"].ToString(), Id = Convert.ToInt32(sdr["Id"]) });
                }
            }
            con.Close();
        }
    }
    return View(infogenes);
}