How to set the code selection programatically in AvalonEdit?

Question

I'm using AvalonEdit in a Microsoft Surface application. Since the Contacts are handled a little bit different there than in a normal WPF application, I want to set the code selection programatically.

By code selection I mean this part of the code whichs background is changed if the user select it with the mouse.

I now the DocumentLine or the VisualLine where the selection should start and where it should end. But how can I now make this selection?