I think you can use GeometryEngine methods then to see if your selection rectangle intersects with the graphic geometries.
private async void OnSelect(object sender, RoutedEventArgs e)
{
try
{
var rectangle = await MyMapView.SketchEditor.StartAsync(SketchCreationMode.Rectangle, false);
foreach (var overlay in MyMapView.GraphicsOverlays)
{
foreach (var graphic in overlay.Graphics)
graphic.IsSelected = GeometryEngine.Intersects(graphic.Geometry, rectangle);
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message, ex.GetType().Name);
}
}