[V8i C#] ECFramework Can't get DataSet from QueryResultsCass.

Hello all,

I try to query same data from my spatial database I use ECFramework to do this.

Everything work fine but when I try to get DataSet from QueryResults Method thrown this exeption:  Instance must refer to class in the query

Exception thrown: 'Bentley.Exceptions.ProgrammerException' in Bentley.EC.Persistence.2.0.dll.

ECQuery query = QueryHelper.CreateQuery(AddInMain.XfmStorage.ECSchema, "My_Class_Name");

QueryHelper.SelectProperties(query, "Fullname", "R_Strid");
query.SelectClause.SelectDistinctValues = true;
PersistenceService persistenceservice = PersistenceServiceFactory.GetService();

QueryResults SQueryResults = persistenceservice.ExecuteQuery(Repository, query, 0, Bentley.EC.Persistence.LoadModifiers.SuppressOptionalECProperties, null);

DataSet ds = SQueryResults.GetAllAsDataSet();

Any idea what I do wrong?