// FirstName field is not written to System.assertEquals(false, c.isSet('FirstName')); c.firstName = null; //FirstName field is written to System.assertEquals(true, c.isSet('FirstName')); Contact newContact = new Contact(LastName = 'Joyce'); insert(newContact); //Insert a new contact with last name Joyce Contact c = [SELECT FirstName FROM Contact WHERE Id = :newContact.Id]; //FirstName field in query System.assertEquals(true, c.isSet(Contact.FirstName)); //LastName field not in query System.assertEquals(false, c.isSet(Contact.LastName)); sObject . isSet(fieldName) sObject . isSet(field)