How to get opened workitem result document and selected workitems in Visual studio 2015 ?

Hi All,
Earlier in visual studio 2012,I was using DocumentServices class from Microsoft.VisualStudio.TeamFoundation.WorkItemTracking namespace to get opened document and work items. Now I have moved to Visual studio 2015 and cannot find this namespace in VS2015.

1)How to get Microsoft.VisualStudio.TeamFoundation.WorkItemTracking namespace?
2)Is there any other way to get opened workitem document and selected workitems in Visual studio 2015?

Thanks in advance.