use XAML map control lib to show locations located inside the current

I am using XAML map control lib to show some locations on OpenStreetMap from C# VS2013 WPF.
I would like to show the locations by pushpins when the zoom level > 10.

But, I need to know the corner coordinates (latitude/longitude) of the current viewport so that I only need to show the locations that are located inside the viewport rectangle. Another reason is that marking too many pushpins are not efficient in WPF because each pushpin is a UIElement. So, I only need to show the locations located in the current viewport.

  I hope that I can have this like: 

 viewport.coordinates or viewport.borders or  viewport.north/south/east/west.

Any help would be appreciated.