Connection String in wpf

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WPF

Hii... I'm developing WPF application..I want to know how to declare connectionstring in App.Config..Please help me out..

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;

namespace Demo_wpf
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
        private void btnSubmit_Click(object sender, RoutedEventArgs e)
        {

        }




    }
}

app config Code:-

<configuration>
  <connectionStrings>
    <add name="connectionString" connectionString="Data Source=ASHISH-PC;Initial Catalog=b;Integrated Security=True" providerName="System.Data.SqlClient"/>
  </connectionStrings>
</configuration>

But it gives an error that is:- The name 'ConfigurationManager' does not exist in the current context