To a human, “I once met a man with a wooden leg named Smith” is the start of a old joke. To a computer, it’s a compile error.
Class 'WoodenLeg' has no 'Name' property or the property is not accessible.
If only computers had a sense of humor…
There’s a great discussion on Jeff Palermo’s blog about entity validation patterns. Jeff takes the position that your domain objects (or entities) should not have validation logic “baked-in” to the class itself. Instead, you should separate the validation routines into separate classes that you can use to validate the object on demand.
There are two advantages to this approach. The first is that you can use different validators for the same object in different circumstances. For example, the validation you might perform prior to storing the domain object in a persistence layer could be different than the validation routine used to validate input from the GUI layer.
The second advantage is that separating the validation logic from the data itself makes it easier to work with ORM or serialization frameworks. Most of these frameworks encourage the use of Plain Old Objects, that is, objects without special attributes or interfaces that help with these mapping and serialization tasks. (See Wikipedia’s article on Plain Old Java Objects, for example.)
Those are powerful arguments, but I’m still not convinced.
As a practical matter, Jeff’s advice is sound. It’s much easier to move business logic into the helper classes that surround your entity model. You get better tool support and more flexibility. But there’s two things about his approach that bother me. Judging by some of the excellent comments on his article, other programmers are bothered by them as well.
First, stripping away behavior from your domain objects is a recognized anti-pattern in object oriented code. Martin Fowler calls it the Anemic Domain Model. It harkens back to the days of procedural programming, where data and business logic were strictly separated. If you’re an OO purist, this is a red flag.
From an OO perspective, the need to validate the same object in different ways suggests that what you actually need to do is create more objects. Rather than pass a stripped-down data-transfer object (DTO) all the way from your data storage layer up to your GUI, you should have a bunch of intermediate objects to help transition the data and enforce proper behavior.
But I’m not an OO snob. Writing a whole bunch of extra classes to move information between tiers in my application is a hassle. I’ve done it before, and we’re doing it now with Infovark, but for most projects it just isn’t justified. Especially if you have to wrestle with various application frameworks to deal with correctly modeled but more complicated domain objects.
The second objection I have is that if we follow Jeff’s advice, we have to accept that bad data will creep into our domain. Jeff knows that this is a hard sell. It’s why he titled his article “The fallacy of the always-valid entity.”
Whew. That’s rough. That requires a whole different programming mindset. What about the problem of Garbage In, Garbage Out? Can we really create programs robust enough to handle business objects that might, at any moment, contain meaningless gibberish?
I don’t know. For now, as appealing as Jeff’s idea is, I’ll stick to always-valid approach. What do you think?
Framework Design Guidelines
I’m almost embarrassed to admit that I really enjoyed Framework Design Guidelines by Krzysztof Cwalina and Brad Abrams. I mean, it’s a book about coding standards.
Maybe I ought to get out more… but before I leave the glare of my monitor behind, I’ll type up my review.
Computing languages, just like human languages, have grammar and syntax. There are correct ways to form sentences and paragraphs, with well-defined rules (and exceptions). Just like word processors can check spelling and verify basic sentence structure, most IDEs today can ensure your code will compile and run.
That doesn’t mean that your story or program is an easy or enjoyable read, though. Most newspapers have accumulated extensive guidelines for matters of style and substance, and most software companies have their own guides as well. If you’re writing as part of a team project, or writing programs intended to be used by other programmers, it’s important to make your code consistent, clear, and direct.
Just like many journalists keep a copy of The Associated Press Stylebook or the New York Times Manual of Style and Usage handy — even if they don’t actually work for the New York Times or the AP — lots of programmers keep a copy of Microsoft’s Framework Design Guidelines as reference.
Or they should. That’s probably my roots as a maintenance programmer showing.
Ideally, you’d want any code you write for other Windows programmers’ use to look as if it came from Microsoft itself. That is, you want it to feel like a natural extension of the .NET framework and not some third-party bolt-on with odd stylistic touches. You’d also like your code to use the full power and expressiveness of .NET, and not appear like some watered-down Java-esque port. (Far too many open source projects retain awkward Java-isms after being converted to .NET, in my opinion. NUnit is a notable exception.)
This helps your fellow programmers gain a better understanding of your code in less time. And it can also make your programming tasks easier, too. Just like design patterns can help you lay out your application architecture, programming guidelines can help you structure your code at the class or method level.
The Framework Design Guidelines covers a lot of ground in its 400 pages. It describes what conventions Microsoft uses when designing types, methods, and exceptions. It also describes the naming and design patterns that Microsoft uses in their public APIs. The topics are grouped by category and heavily cross referenced, making it easy to find your way around. The rationale of each guideline is explained, and the authors indicate the strength of each recommendation by marking it with the terms Do, Consider, Avoid or Do Not.
But the best part of the book is the stories and comments given by members of the Microsoft team. These are sprinkled throughout the book and give insight into why the guideline exists. Some of these discuss lessons Microsoft learned the hard way, places in the .NET Framework where the rules are violated, and how real-world programmers feel about certain guidelines. You can get a flavor of these by checking out the Framework Design Guidelines section of Brad Abrams‘ blog.
If you find his posts interesting or helpful, you’ll feel the same way about the book. Highly recommended.
Recently, I made a plea for the adoption of Universal Coordinated Time (UTC) in computer applications. It’s a sensible recommendation, and I stand behind it.
The folks working on HTML 5 are proposing a <time> element for the new standard. This makes sense to me. It will help eliminate some of the objections people have raised to the datetime design pattern proposed by the microformats team.
So, problem solved, right? We use UTC for time, and the usual calendar notation for dates. Neat.
Wait a minute. By “the usual” calendar notation, do we mean the modern Gregorian calendar, or…
Peter-Paul Koch, a.k.a. PPK, author of the quirksmode blog, reminds us that calculating historical calendar dates is hard. Really, really hard.
He provides an overview of the major calendar reforms in the Western world and points out that the reforms were adopted by different countries at different times. So forming a consistent timeline requires a knowledge of both time and place.
And many important historical dates float. The rules that determine when Easter occurs in the church calendar are complicated, and Orthodox and Catholic calendars disagreed for many years. In the medieval period, years were often numbered according to the local monarch’s reign. In Roman times, extra days were added to the official calendar by decree to prevent the seasons from drifting too far out of line.
If we want to make the <time> element safe for historical use, programmers would have to deal with this mess.
As useful as having universal, consistent <time> element metadata would be, that’s just too hard. Frankly, I skimmed the last bits of PPK’s article myself, and I’m actually interested in the issue. Most working programmers won’t bother.
While it’d be nice to have trustworthy time data, we’re not likely to get it. The standard should reflect that. I vote for assigning a cutoff date for the <time> element. January 1, 1970 works for me.
I didn’t set out to buy a SQL IDE three months ago, but I bought Firebird Maestro from the SQL Maestro group anyway.
What I was actually looking for was an equivalent to Red Gate‘s SQL Packager for the Firebird database. SQL Packager is a useful application that makes it easy to package and deploy database schema upgrades. Sadly, all of Red Gate’s tools are for Microsoft SQL Server only.
My next step was to look through the list of admin tools on the Firebird website. And while there’s a huge selection of tools available for Firebird, I couldn’t find anything that would help me deploy schema and data upgrades. It was a bit surprising, actually, since one of Firebird’s strengths is how easy it is to embed inside other software applications. (If I’ve missed an application that can do this, please let me know!)
During the course of my investigations, I downloaded several different tools for managing Firebird. I’m a fan of the fast and free FlameRobin, but it’s still in development and doesn’t do everything I need. Sometimes a full-featured IDE is required, and I found two that fit the profile.
The first was SQL Studio from EMS Database Management Solutions. I appreciated their customer support and the breadth of features. But ultimately, I decided upon Firebird Maestro. The differences between the two IDEs were small, but Firebird Maestro had a better tool for producing database diagrams. Since Infovark has developers working halfway around the world from each other, having a good database diagram to reference is a must. Firebird Maestro also had a simple wizard for scripting out the schema and data into a single file. We put this text file into Subversion so that we have our database structure under version control.
Firebird Maestro
It won’t help me deploy an update, but I’ve had to do that sort of thing by hand before, and I suppose I can do it again. But sometimes when you look for something you find something else just as good.