connection string is not read?

When I execute the following code, for cnString, I get:

Data Source=.\SQLExpress12;Att;AttachDbFilename=|DataDirectory|\NORTHWND.MDF;Integrated Security=True;User Instance=True

Whereas I am expecting sting like: TestToGetStringFrom XML File as it is embedded in the cnNorthwind.xml file.

Question: Why it is not reading it correctly? Besides, where does the program gets the above string anyway?
 

Here is cnNorthwind:
<?xml version="1.0" encoding="utf-8" ?>
<connection>
  TestToGetStringFrom XML File
  <!-- Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\NORTHWND.mdf;Integrated Security=True;Connect Timeout=30 -->
</connection>

This is the code reading cnNorthwind:
namespace CodeSampleCS
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            XmlTextReader xmlReader = new XmlTextReader("csNorthwind.xml");
            cnString = xmlReader.ReadElementString("connection");
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            MessageBox.Show("msg 1" + cnString);
            Application.Run(new frmCS01(cnString));
            MessageBox.Show("msg 2" + cnString);
        }

        public static string cnString { get; set; }
    }
}

FYI: This App.config
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
    </configSections>
    <connectionStrings>
        <add name="CodeSampleCS.Properties.Settings.NORTHWNDConnectionString"
            connectionString="TestToGetStringFrom App.config File"
            providerName="System.Data.SqlClient" />
    </connectionStrings>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

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