You can't set startup form sample.cs .cs is code behind.
You set sample.xaml
Open your App.xaml and set Startup Uri Sample.xaml
<pre lang="HTML"><application x:class="MyApp.App" xmlns:x="#unknown">
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
StartupUri="Sample.xaml">
<application.resources>
</application.resources>
</application>