The documentation in ArcGIS Pro states that it is possible to select an option for the MPKX map package that will make it accessible to the ArcGIS Runtime (.NET). However, I have not been able to find any reference in the ArcGIS Runtime for .NET help that identifies the class that supports opening up this package type.
Is this packaged file type truly supported by ArcGIS Runtime for .NET? And if so, how do I open it?
In the upcoming v100.3 release we are planning to add support for .mpkx files when used in conjunction with the ArcGIS Runtime Local Server - for an explanation of how this works in the current v100.2 release with .mpk files from ArcMap see Local Server—ArcGIS Runtime SDK for .NET (WPF) | ArcGIS for Developers.
To work with packages from ArcGIS Pro directly in ArcGIS Runtime you should look at the Mobile Map Package (.mmpk) see: