exExecuteNonQuery: Connection property has not been initialized.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
using System.Configuration;
namespace air
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        static string constring = ConfigurationManager.ConnectionStrings["Cnstr"].ConnectionString;
        static SqlConnection con = new SqlConnection(constring);

        private void button1_Click(object sender, EventArgs e)
        {

            SqlCommand cmd = new SqlCommand("in_mem");
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@MemNo", txtmbm.Text);
            cmd.Parameters.AddWithValue("@name", txtnomname.Text);
            cmd.Parameters.AddWithValue("@deptno",txtdept.Text);
            cmd.Parameters.AddWithValue("@empno",txtemp.Text);
            cmd.Parameters.AddWithValue("@d_o_m", txtdom.Text);
            cmd.Parameters.AddWithValue("@d_o_b", txtdob.Text);
            cmd.Parameters.AddWithValue("@d_o_r", txtdor.Text);
            cmd.Parameters.AddWithValue("@nominee_name",txtnomname.Text);
            cmd.Parameters.AddWithValue("@relationship", txtrlt.Text);
            cmd.Parameters.AddWithValue("@address",txtaddr.Text);
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
        }
    }
}
and my connection string

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings >
    <add name="cnstr" providerName="System.Data.SqlClient"
              connectionString="Data Source=localhost;Initial Catalog=airindia_db; Integrated Security=true" />
  </connectionStrings>
</configuration>