If you want to read filtered data why don't you use first parameter filterArray
of ReadMultiple
function which reads a filtered set of records
List<Customer_Filter> filterArray = new List<Customer_Filter>();
Customer_Filter nameFilter = new Customer_Filter();
nameFilter.Field = Customer_Fields.Name;
nameFilter.Criteria = "S*";
filterArray.Add(nameFilter);
Customer[] custList = service.ReadMultiple(filterArray.ToArray(), null, 100);