Hi,
Mostly people jump to python for this kind of thing? Or use custom nodes… Rhythm has one and it is open source (thanks JP! https://github.com/johnpierson/RhythmForDynamo/blob/master/src/Rhythm/Revit/Application/Application.cs)
If using the python node is acceptable to you, this thread maybe gives some pointers…
I am not an expert on design script, but I think part of the reason people jump to these other languages is the control they offer for working with the api (and other dlls).
There is this post? But it doesn’t make much sense to me. DesignScript Libraries - #4 by aclarke
Best of luck, let us know how you get on 
Mark