i have used app.config and the connection are mentioned in connectionclass1.cs but it show the error the string contain null value

Question

public class connectionClass1
    {
       SqlConnection Con;
       SqlConnection cn;
       SqlCommand cmd;
       SqlDataAdapter Da = new SqlDataAdapter();

    public SqlConnection GetConnection()
    {
          string connectionString = ConfigurationManager.ConnectionStrings["WindowsFormsApplication5.Properties.Settings.terumopenpolConnectionString"].ConnectionString;
        SqlConnection Con = new SqlConnection(connectionString);
        Con.Open();
        return Con;
    }

    public  connectionClass1()
    {
          string connectionString = ConfigurationManager.ConnectionStrings["WindowsFormsApplication5.Properties.Settings.terumopenpolConnectionString"].ConnectionString.ToString();
         SqlConnection cn = new SqlConnection(connectionString);
    }

these was the connection string I have given but it shows the error of I didn't set the instant of object and the string contain null value