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Server Side
General Rules
Get what you want. Don't retrieve an Entity with all the fields.
Update what you need. When updating an entity create a new instance of the target entity with the required attributes and use this instance to perform the update.
csharpvar contactToUpdate = new Entity('contact',contactId) { ["firstname"] = "Refael", ["lastname"] = "Eitan", } _service.Update(contactToUpdate)