public double SlopePercent {get;}

var tinLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<TinLayer>().FirstOrDefault();
await QueuedTask.Run(() =>
{
  // get elevation, slope and aspect values
  SurfaceValues values = tinLayer.GetSurfaceValues(mapPoint);
  var elev = values.Elevation;
  var slopeRadians = values.Slope;
  var slopeDegrees = values.SlopeDegrees;
  var slopePercent = values.SlopePercent;
  var aspectRadians = values.Aspect;
  var aspectDegrees = values.AspectDegrees;
});