Getting UI dispatcher in class library

The Application class is defined in PresentationFramework.dll. You need to reference that in order to be able to access the dispatcher through Application.Current.Dispatcher.

I had same issue ie not being able to resolve Application.Current.Dispatcher and ended up passing the client gui dispatcher down to the library which just holds a Dispatcher ref (reference WindowsBase + using System.Windows.Threading).
I prefer this option that having my non GUI lib have to carry a ref to PresentationFramework.dll (which doesn't seem natural).
I guess its 6 of one, half a dozen of the other...