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PropertyCriteria criteria = new PropertyCriteria();
criteria.IsNull = true;
criteria.Name = "WriterName";
PropertyCriteriaCollection pcc = new PropertyCriteriaCollection();
col.Add(criteria);
PageDataCollection pages;
pages = Global.EPDataFactory.FindPagesWithCriteria(Global.EPConfig.StartPage, pcc, EPiServer.Security.AccessControlList.NoAccess);
That code will return all pages where WriterName is null.