Issue Add Media URL With Item In .Net Core

I’m using this code to add media url :

var thumbnails = new ThumbnailDisplayUrls();
thumbnails.Large = (Uri)jo.SelectToken(“result.records.record[” + i + “].rdf:RDF.Entity.thumbnail”);
thumbnails.Medium = (Uri)jo.SelectToken(“result.records.record[” + i + “].rdf:RDF.Entity.thumbnail”);
thumbnails.Square = (Uri)jo.SelectToken(“result.records.record[” + i + “].rdf:RDF.Entity.thumbnail”);
ksamsokModel.ThumbnailDisplayUrls = thumbnails;

                            var owners = new OOwner[1];
                            owners[0] = new OOwner();
                            owners[0].Id = new Uri("http://demo/api/media/3653");
                            owners[0].OId = 3653;
                            ksamsokModel.OMedia = owners;