ConfigurationManager in WPF

This one use System.Configuration namespace

using System.Configuration;

Or add System.Configuration in reference

System.ConfigurationManager.ConnectionStrings["Trackboard"].ConnectionString;
System.ConfigurationManager.ConnectionStrings[0].ConnectionString;

I've figured it out! I shouldn't have created a new config file. There is a default app.config file in the project. Now everything is fine.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
    </configSections>
    <connectionStrings>
        <add name="Trackboard.Properties.Settings.TrackboardConnectionString"
            connectionString="Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\DATABASE\Trackboard.mdf;Integrated Security=True"
            providerName="System.Data.SqlClient" />
    </connectionStrings>
</configuration>

private static string ConnStr = ConfigurationManager.ConnectionStrings["Trackboard.Properties.Settings.TrackboardConnectionString"].ConnectionString;

Just add an app.config and not web.config because it is not a web application.

And after that it's too simple, just add a reference to to System.Configuration and then use this.

var ConnStr = ConfigurationManager.AppSettings["Trackboard"];