Thank you for your questions @HalletBruestle. An Envelope can be constructed with diagonal MapPoint corners and a SpatialReference to essentially create a rectangle. The Contains GeometryEngine extension method can be used to determine if a point falls inside the envelope. The Set max extent code sample shows how to get the maximum boundary of a geographic region.
Getting the extent of the entire map - so the entire projected surface of the Earth - that's an interesting problem. It's not going to be easy to accomplish programmatically. Interactively, you could subscribe to a GeoViewTapped event and get approximate the maximum envelop when clicking the southwestern-most and northeastern-most points of the map. However, that is a far from elegant solution. You'd be better off searching the web for those projected points on the web and hardcoding those values.
