I've found this answer which explains how to arrange a collection of rectangles inside a grid in XAML:
<Grid... >
    <Rectangle Grid.Column="0"
               Grid.Row="0"  
               Stroke="Blue"
               Fill="Transparent" />
        <TextBlock Grid.Column="0" 
                   Grid.Row="0"
                   Text="(0,0)" />
    </Rectangle>
    ...
</Grid>
Now I'm trying to do the same in C#, and I've found out that Rectangle doesn't have a Content property. So how do I add a TextBlock to a Rectangle?