Yesterday I tweeted a link to an article about overloading that I’d just finished. In that article, all my examples look a bit like this:
using System;
class Test
{
static void Foo(int x, int y = 5)
{
Console.WriteLine("Foo(int x, int y = 5)");
}
static void Foo(double x)
{
Console.WriteLine("Foo(double x)");
}
static void Main()
{
Foo(10);
}
}
Each example is followed by an explanation of the output.
Fairly soon afterwards, I received an email from a reader who disagreed with my choices for sample code. ere are a few extracts from the email exchange. Please read them carefully – they really form the context of the rest of this post.
This is really not proper. When a method can do more than one thing, you might offer what are called ‘convenience overloads’, which make it easier for the consuming developer. When you start swaying away so much that you have wildly different arguments, then it’s probably time to refactor and consider creating a second method. With your example with "Foo", it’s hard to tell which is the case.
My point is, the ‘convenience overloads’ should all directly or indirectly call the one REAL method. I’m not a fan of "test", "foo", and "bar", because they rarely make the point clearer, and often make it more confusing. So let me use something more realistic. So let me use something more realistic. This nonsensical example, but hopefully is clear:
...
The point here was to make you aware of the oversight. I do what I can to try to stop bad ideas from propagating, particularly now that you're writing books. When developers read your book and consider it an "authority" on the topic, they take your example as if it's a model for what they should do. I just hope your more mindful of that in your code samples in the future.
...
Specific to this overload issue, this has come up many times for me. Developers will write 3 overloads that do wildly different things or worse, will have 98% of the same code repeated. We try to catch this in a code review, but sometimes we will get pushback because they read it in a book (hence, my comments).
...
I assume your audience is regular developer, right? In other words, the .NET Framework developers at Microsoft perhaps aren't the ones reading your books, but it's thousands of App Developer I and App Developer II that do business development? I just mean that there are far, far more "regular developers" than seasoned, expert developers who will be able to discern the difference and know what is proper. You are DEFINING what is proper in your book, you become an authority on the matter!
Anyhow, all my point was it to realize how far your influence goes once you become an author. Even the simplest, throwaway example can be seen as a best-practice beacon.
Now, this gave me pause for thought. Indeed, I went back and edited the overloading article - not to change the examples, but to make the article's scope clearer. It's describing the mechanics of overloading, rather than suggesting when it is and isn't appropriate to use overloading at all.
I don't think I'm actually wrong here, but I wanted to explore it a little more in this post, and get feedback. First I'd like to suggest a few categorizations - these aren't the only possible ones, of course, but I think they divide the spectrum reasonably. Here I'll give example examples in another area: overriding and polymorphism. I'll just describe the options first, and then we can talk about the pros and cons afterwards.
Totally abstract - no code being presented at all
Sometimes we talk about code without actually giving any examples at all. In order to override a member, it has to be declared as `virtual` in a base class, and then the overriding member uses the `override `modifier. When the virtual member is called, it is dispatched to the most specific implementation which overrides it, even if the caller is unaware of the existence of the implementation class.
Working but pointless code
This is the level my overloading article worked at. Here, you write code whose sole purpose is to demonstrate the mechanics of the feature you're describing. So in this case we might have:
using System;
public class C1
{
public virtual void M()
{
Console.WriteLine("C1.M");
}
}
public class C2 : C1
{
public override void M()
{
Console.WriteLine("C2.M");
}
}
public class C3
{
static void Main()
{
C1 c = new C2();
c.M();
}
}
Now this is a reasonably extreme example; as a matter of personal preference I tend to use class names like "Test" or "Program" as the entry point, perhaps "BaseClass" and "DerivedClass" where "C1" and "C2" are used here, and "Foo" instead of "M" for the method name. Obviously "Foo" has no more real meaning than "M" as a name - I just get uncomfortable for some reason around single character identifiers other than for local variables. Arguably "M" is better as it stands for "method" and I could use "P" for a property etc. Whatever we choose, we're talking about metasyntactic variables really.
Complete programs indicative of design in a non-business context
This is the level at which I would probably choose to demonstrate overriding. It's certainly the one I've used for talking about generic variance. Here, the goal is to give the audience a flavour of the purpose of the feature as well as demonstrating the mechanics, but to stay in the simplistic realm of non-business examples. To adapt one of my normal examples - where I'd actually use an interface instead of an abstract class - we might end up with an example like this:
using System;
using System.Collections.Generic;
public abstract class Shape
{
public abstract double Area { get; }
}
public class Square : Shape
{
private readonly double side;
public Square(double side)
{
this.side = side;
}
public override double Area { get { return side * side; } }
}
public class Circle : Shape
{
public readonly double radius;
public Circle(double radius)
{
this.radius = radius;
}
public override double Area { get { return Math.PI * radius * radius; } }
}
public class ShapeDemo
{
static void Main()
{
List<Shape> shapes = new List<Shape>
{
new Square(10),
new Circle(5)
};
foreach (Shape shape in shapes)
{
Console.WriteLine(shape.Area);
}
}
}
Now these are pretty tame shapes - they don't even have a location. If I were really going to demonstrate an abstract class I might try to work out something I could do in the base class to make it sensibly a non-interface... but at least we're demonstrating the property being overridden.
Business-like partial example
Here we'll use classes which sound like they could be in a real business application... but we won't fill in all the useful logic, or worry about any properties that aren't needed for the demonstation.
using System;
using System.Collections.Generic;
public abstract class Employee
{
private readonly DateTime joinDate;
private readonly decimal salary;
public virtual int BonusPercentage { get { return 0; } }
public decimal Salary { get { return salary; } }
public DateTime JoinDate { get { return joinDate; } }
public int YearsOfService
{
get { return DateTime.Now.Year - joinDate.Year; }
}
public Employee(decimal salary, DateTime joinDate)
{
this.salary = salary;
this.joinDate = joinDate;
}
}
public abstract class Manager : Employee
{
public override int BonusPercentage { get { return 15; } }
}
public abstract class PreIpoContract : Employee
{
public override int BonusPercentage
{
get { return YearsOfService * 2; }
}
}
Now this particular code sample won't even compile: we haven't provided the necessary constructors in the derived classes. Note how the employees don't have names, and there are no relationships between employees and their managers, either.
Obviously we could have filled in all the rest of the code, ending up with a complete solution to an imaginary business need. Other examples at this level may well include customers and orders. One interesting thing to note here: admittedly I've only been working in the industry for 16 years, and only 12 years full time, but I don't think I've ever written a Customer or Order class as part of my job.
Full application example
No, I'm not going to provide an example of this. Usually this is the sort of thing which a book might work up to over the course of the complete text, and you'll end up with a wiki, or an e-commerce site, or an indexed library of books with complete web site around it. If you think I'm going to spend days or even weeks coding something like that just for this blog post, you'll be disappointed :)
Anyway, the idea of this is that it does something genuinely useful, and you can easily lift whole sections of it into other projects - or at least the design of it.
Which approach is best?
I'm sure you know what's coming here: it depends. In particular, I believe it depends on:
Your readership
Are they likely to copy and paste your example into production code without further thought? Arguably in that case the first option might be the best: they may not understand it, but at least it means your code won't be injuring a project.
Simply put, didactic code is not production code. The parables in the Bible aren't meant to be gripping stories with compelling characterization: they're meant to make a point. Scales aren't meant to sound like wonderful music: they're meant to help you improve your abilities to make a nice sound when you're playing real music.
The point you're trying to put across
If I'm trying to explain the mechanics of a feature, I find the second option to be useful. The reader doesn't need to try to take in the context of what the code is trying to accomplish, because it's explicitly not trying to do anything of any use. It's just demonstrating how the language or platform behaves in a particular scenario.
If, on the other hand, you're trying to explain a design principle, then the third or fourth options are useful. The third option can also be useful for the mechanics of a feature which is particularly abstract - like generic variance, as I mentioned earlier. That goes somewhere between "complete guide to where this feature should be used" and "no guidance whatsoever" - a sort of "here's a hint at the kind of situation where it could be useful."
If you're trying to explore a technology for fun, I find the third option works very well for that situation too. For example, while looking at Reactive Extensions, I've written programs to:
- Group lines in a file by length
- Give the results of a UK general election
- Simulate the 1998 Brazil vs Norway world cup football match
- Implement drag and drop using event filtering
None of these is likely to be directly useful in a real business app - but they were more appealing than solely demonstrating a sequence of numbers being generated (although with an appropriate marble diagram generator, that can be quite fun too).
The technology you're demonstrating
This is clearly related to the previous point, but I think it bears a certain amount of separation. I believe that language topics are fairly easily demonstrated with the second and third options. Library topics often deserve a slightly higher level of abstraction - and if you're going to try to demonstrate that a whole platform is worth investing time and energy in, it's useful to have something pretty real-world to show off.
Your time and skills
You know what? I suck at the fourth and fifth options here. I can't remember writing any complete, independent systems as a software engineer, and none of them have been in line-of-business applications anyway. The closest I've come is writing standalone tools which certainly have been useful, but often take shortcuts in terms of design which I wouldn't countenance in other applications. (And yes, I'm sure there's some discussion to be had around that as well, but it's not the point of this article.)
You may think my employee example above was lousy - and I'd agree with you. It's not really a great fit for inheritance, in my view - and the bonus calculation is certainly a dubious way of forcing in some polymorphism. But it was the best I could come up with in the time available to me. This wasn't some attempt to make it appear less worthy than the other options; I really am that bad at coming up with business-like examples. Other authors (by which I mean anyone writing at all, not just book authors) may well have found much better examples, either by spending more time on them, being more experienced with line-of-business apps, or having a better imagination. Or all three.
I'm not too proud to admit the things I suck at :) If I spent many extra hours coming up with examples for everything I write about, I would get a lot less written. I'm doing this in notional "spare time" after all. So even if you would prefer the fourth option over the third, would you rather have that but see less of my (ahem) "wisdom"? Personally I think everyone's better off with me braindumping using examples in forms which I'm better at.
How to read examples
Most of this post has been from the point of view of an author. Briefly, I'd like to suggest what this might mean for readers. The onus is on the author to make this clear, of course, but I think it's worth trying to be actively better readers ourselves.
- Understand what the author is trying to achieve. Don't assume that every example will fit nicely in your application. Example code often doesn't come with any argument validation or error handling - and very rarely does it have an appropriate set of unit tests. If you're reading about how something works, don't assume that the examples are in any way realistic. They may well be simplified to demonstrate the behaviour as clearly as possible without the extra "fluff" of useful functionality.
- Think about what may be missing, particularly if the context is an evangelical one. If someone is trying to sell you on a particular technology, then of course they'll try to show it in its best possible light. Where are the pitfalls? Where does it not stack up?
- Don't assume authority means anything. I was quite happy to take Jeffrey Richter to task on boxing for example. Jeffrey Richter is a fabulous author and clearly a smart cookie, but that doesn't mean he's right about everything... and I really, really don't like the idea of anyone appealing to my supposed abilities to justify some bad decision. Judge any argument on its merits... find out what people think and why they think it, but then see how well their reasoning actually hangs together.
Conclusion
This was always going to be a somewhat biased look at this topic, because I hold a certain viewpoint which is clearly contrary to the one held by the chap who emailed me. That's why I included a reasonable chunk of his emails - to give at least some representation to the alternatives. This post has effectively been a longwinded justification of the form my examples have taken... but does it ring true?
I can't guarantee to change my writing style drastically on this front - at least not quickly - but I would very much appreciate your thoughts on this. I'm reluctant to exaggerate, but I think it may be even more important than working out whether "Jedi" was meant to be plural or singular - and I certainly received a lot of feedback on that topic.
Jon Skeet's coding blog 