string connectionString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        try
        {
            using (OdbcConnection connect = new OdbcConnection(connectionString))
            {
                connect.Open();
                OdbcCommand cmd = new OdbcCommand("SELECT username, password FROM receptionist", connect);
                OdbcDataReader reader = cmd.ExecuteReader();



                if (username_login.Text == username && password_login.Text == password)
                {
                    this.Hide();
                    MessageBox.Show("Invalid User", "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    this.Close();
                }
                else MessageBox.Show("Invalid User", "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                connect.Close();



            }



        }

        catch (OdbcException ex)
        {
            MessageBox.Show(ex.Message, "error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }

<?xml version="1.0"?>
<configuration>
   <configSections>
      <sectionGroup name="applicationSettings" type="System.Configuration.ApplicationSettingsGroup, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" >
      <section name="yourProjectName.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
</sectionGroup>