Prefer
public class Order
{
    public OrderLine AddOrderLine(OrderLine orderLine)
    {
        if (orderLine == null) throw new ArgumentNullException(nameof(orderLine));
        ...
    }
}
Over
public class Order
{
    public OrderLine AddOrderLine(OrderLine orderLine)
    {
        if (orderLine == null) throw new ArgumentNullException("orderLine");
        ...
    }
}    
Using the nameof feature makes it easier to refactor method parameters.
 
                