Set Entity key value

Question

I want to use program keys generated sutomatcaly by my code in the insertion os every entity in my model but i don't want to make it by hand and i still getting in the same error

the problem is 

when i try to modify the  entity data model

        partial void OnContextCreated()
        {
            this.ObjectStateManager.ObjectStateManagerChanged += new System.ComponentModel.CollectionChangeEventHandler(ObjectStateManager_ObjectStateManagerChanged);
        }

        void ObjectStateManager_ObjectStateManagerChanged(object sender, System.ComponentModel.CollectionChangeEventArgs e)
        {
            if (e.Action == System.ComponentModel.CollectionChangeAction.Add && ((EntityObject)e.Element).EntityState == System.Data.EntityState.Added)
            {
                var entidade = (ObjectStateEntry)e.Element;
                
                var temp = this.CreateEntityKey(entidade.EntityKey.EntitySetName, entidade);
                EntityKeyMember[] keymember = new EntityKeyMember[1];
                keymember[0] = new EntityKeyMember(temp.EntityKeyValues[0].Key, 1000/*here a function would put the real key*/);
                EntityKey chave = new EntityKey();
                chave.EntityContainerName = entidade.EntityKey.EntityContainerName;
                chave.EntitySetName = entidade.EntityKey.EntitySetName;
                chave.EntityKeyValues = keymember;
                
                entidade.EntityKey = chave;
            }
        }

whe this code runs it says that i cant modfie the entity data model that it can only be set when it is null

any one got a workaround