Compile error on ExecuteAsynch method

I finally figured it out. That was hard, like pulling teeth.

          IEntityQuery query = (IEntityQuery)PredicateBuilder.FilterQuery(Mgr.Members, aCompositePredicateDescription);

             Mgr.ExecuteQueryAsync<Member>((IEntityQuery<Member>)query,
                 (op) => {
                     var results = op.Results;
                     results.ForEach(Members.Add);
                 }
                 );

            return Members;
           

    I am passing a query object to my GetMembers() method that holds a variable number of search parameters. After building the query with PredicateBuilder, I want to execute the query asynchronously and in the same method use lambda statements to fill the collection instead of using a callback operation. I am getting a compile error on the ExecuteAsynch method. How do I write this method correctly? It is the line in blue at the bottom of the method.
   
       public ObservableCollection<Member> GetMembers(MemberQO q)
        {
            List<PredicateDescription> predicateDescriptions = new List<PredicateDescription>();

            PredicateDescription aPredicateDescription;

            if (q.SSN != string.Empty)
            {
                aPredicateDescription = PredicateBuilder.Make(typeof(Member), "SocSecNo", FilterOperator.IsEqualTo,
                                                              q.SSN);
                predicateDescriptions.Add(aPredicateDescription);
            }

            if (q.LastName != string.Empty)
                predicateDescriptions.Add(PredicateBuilder.Make(typeof(Member), "LastName", FilterOperator.StartsWith,            q.LastName));

            if (q.FirstName != string.Empty)
                predicateDescriptions.Add(PredicateBuilder.Make(typeof(Member), "FirstName", FilterOperator.StartsWith, q.FirstName));

            if (q.EmployeeNo != string.Empty)
                predicateDescriptions.Add(PredicateBuilder.Make(typeof(Member), "EmployeeNumber", FilterOperator.IsEqualTo, q.EmployeeNo));

            if (q.CardNo != 0)
                predicateDescriptions.Add(PredicateBuilder.Make(typeof(Member), "CardNo", FilterOperator.IsEqualTo, q.CardNo));

            if (q.Status != string.Empty)
                predicateDescriptions.Add(PredicateBuilder.Make(typeof(Member), "Status", FilterOperator.IsEqualTo, q.Status));

            if (q.Office != string.Empty)
                predicateDescriptions.Add(PredicateBuilder.Make(typeof(Member), "Office", FilterOperator.IsEqualTo, q.Office));
      
     CompositePredicateDescription aCompositePredicateDescription = PredicateBuilder.And(predicateDescriptions.ToArray());

            var query = PredicateBuilder.FilterQuery(Mgr.Members, aCompositePredicateDescription);

           query.ExecuteAsync<Member>(op => op.Results.ForEach(Members.Add));
           return Members;
     }