<UserControl x:Class="Testing.MyControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Grid>
    <Rectangle Width="300" Height="100" Fill="Red"/>
    <TextBlock Text="THE UserControl" TextWrapping="Wrap" HorizontalAlignment="Center" VerticalAlignment="Center"/>
  </Grid>
</UserControl>