Facebook sdk to share custom object

I'm trying to implement the share on facebook through the sdk facebook in a Xamarin.forms PCL project. I managed to implement smoothly the share of Android , but I have problems for iOS . In particular, the share of a link works , but nothing happens if I try to do the share of custom objects . I can not find any complete example to help me solve this problem .
Below is my code to share:

ShareOpenGraphObject obj = new MyShareOpenGraphObject ();
            obj.SetString("myapp:myobject", "og:type");
            obj.SetString (title, "og:title");
            obj.SetString ("xxxxxxxxx", "fb:app_id");
            obj.SetString (des, "og:description");

            ShareOpenGraphAction action = new MyShareOpenGraphAction{
                ActionType = "myapp:myaction"
            };
            action.SetObject(obj, "myapp:myobject");

            // Create the content
            ShareOpenGraphContent content = new ShareOpenGraphContent{
                PreviewPropertyName = "myapp:myobject",
                Action = action
            };

            var shareDialog = new MyShareDialog
            {
                FromViewController = UIApplication.SharedApplication.KeyWindow.RootViewController,
                Mode = ShareDialogMode.Automatic
            };
            shareDialog.SetShareContent(content);
            shareDialog.Show();

Below my classes:

 public class MyShareDialog : ShareDialog
    {
        public MyShareDialog () : base(NSObjectFlag.Empty)
        {

        }
    }

public class MyShareOpenGraphAction : ShareOpenGraphAction
    {
        public MyShareOpenGraphAction (): base(NSObjectFlag.Empty)
        {
        }
    }

public class MyShareOpenGraphObject: ShareOpenGraphObject
    {
        public MyShareOpenGraphObject () : base(NSObjectFlag.Empty)
        {
        }
    }