Connection String Error

Question

Hi

 public partial class Form1 : Form
    {
        string ConnectionString = ConfigurationSettings.AppSettings["cnn"];
        string str;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
           
        }

        private void Btn_Click_1(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection(ConnectionString);
            con.Open();
            str = "select count(*) from temp where usrname='" + txtusername.Text.Trim() + "' and usrpassword='" + txtpassword.Text.Trim() + "'";
            SqlCommand cmd = new SqlCommand(str, con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds, "login");

            long x;
            x = Convert.ToInt64(cmd.ExecuteScalar());
            if (x == 1)
            {
                MessageBox.Show("Hello");
            }
            else
            {
                MessageBox.Show("Invalid UserName & Password");
            }
            con.Close();
        }
    }

Error : The connectionstring property has not been initialised.

App.Config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings>
    <add name="cnn"
         connectionString="Persist Security Info=False;
         Data Source=localhost;
         Initial Catalog=ha;
         Integrated Security=SSPI;
         Trusted_Connection=TRUE;" />
  </connectionStrings>
</configuration>

Thanks