public Point ClientPoint {get;}

internal class GetMapCoordinates : MapTool
{
  protected override void OnToolMouseDown(MapViewMouseButtonEventArgs e)
  {
    if (e.ChangedButton == System.Windows.Input.MouseButton.Left)
      e.Handled = true; //Handle the event args to get the call to the corresponding async method
  }

  protected override Task HandleMouseDownAsync(MapViewMouseButtonEventArgs e)
  {
    return QueuedTask.Run(() =>
    {
      //Convert the clicked point in client coordinates to the corresponding map coordinates.
      var mapPoint = MapView.Active.ClientToMap(e.ClientPoint);
      ArcGIS.Desktop.Framework.Dialogs.MessageBox.Show(string.Format("X: {0} Y: {1} Z: {2}",
                  mapPoint.X, mapPoint.Y, mapPoint.Z), "Map Coordinates");
    });
  }
}