public void RemoveAddOn(AddOn addOn)
{
CheckRule(new ConditionMustBeTrueRule(_addOns.Contains(addOn), "add-on not found"));
_addOns.Remove(addOn);
}
public void NewAddOn(AddOn addOn)
{
CheckRule(new ConditionMustBeTrueRule(_addOns.All(x => x.Name != addOn.Name),
"add on with same name already exists"));
_addOns.Add(addOn);
}