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 Wednesday, 17 January 2007
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I recently had an email discussion where I was describing why I needed to solve the problem I described in this article for WPF and this article for Windows Forms.

 

In both cases the issue is that data binding doesn’t refresh the value from the data source after it updates the data source from the UI. This means that any changes to the value that occur in the property set code aren’t reflected in the UI.

 

The question he posed to me was whether it was a good idea to have a property set block actually change the value. In most programming models, goes the thought, assigning a property to a value can’t result in that property value changing. So any changes to the value that occur in the set block of a property are counter-intuitive, and so you simply shouldn’t change the value in the setter code.

 

Here’s my response:

 

The idea of a setter (which is really just a mutator method by another name) changing a value doesn't (or shouldn't) seem counter-intuitive at all.

 

If we were talking about assigning a value to a public field I’d agree entirely. But we are not. Instead we’re talking about assigning a value to a property, and that’s very different.

 

If all we wanted were public fields, we wouldn't need the concept of "property" at all. The concept of "property" is merely a formalization of the following:

 

  1. public fields are bad
  2. private fields are exposed through an accessor method
  3. private fields are changed through a mutator method
  4. creating and using accessor/mutator methods is awkward without a standard mechanism

 

So the concept of "property" exists to standardize and formalize the idea that we need controlled access to private fields, and a standard way to change their value through a mutator method.

 

Consider the business rule that says a document id must follow a certain form - like SOP433. The first three characters must be alpha and upper case, the last three must be numeric. This is an incredibly common scenario for document, product, customer and other user-entered id values.

 

Only a poor UI would force the user to actually enter upper case values. The user should be able to type what they want, and the software will fix it.

 

But putting the upper case rule in the UI is bad, because code in the UI isn't reusable, and tends to become obsolete very rapidly as technology and/or the UI design changes. There's nothing more expensive over the life of an application than a line of code in the UI. So while it is possible to implement this rule in a validation control, in JavaScript, in a button click event handler - none of those are good solutions to the real problem.

 

Yet if that rule is placed purely in the backend system, then the user can't get any sort of interactive response. The form must be "posted" or "transmitted" to the backend before the processing can occur. Users want to immediately see the value be upper case or they get nervous.

 

So then we're stuck. Many people implement the rule twice. Once in the UI to make the user happy, and once in the backend, which is the real rule implementation. And then they try to keep those rules in sync forever - the result being an expensive, unreliable and hard to maintain system.

 

I've watched this cycle occur for 20 years now, and it is the same time after time. And it sucks.

 

This, right here, is why VB got such a bad name through the 1990’s. The VB forms designer made it way too easy to write all the logic in the UI, and without any other clear alternative that's what happened. The resulting applications are very fragile and are impossible to upgrade to the next technology (like .NET). Today, as we talk, many thousands of lines of code are being written in Windows Forms and Web Forms in exactly the same way. Those poor people will have a hell of a time upgrading to WPF, because none of their code is reusable.

 

What's needed is one location for this rule. Business objects offer a workable solution here. If the object implements the rule, and the object runs on the client workstation, then (without code in the UI) the user gets immediate response and the rule is satisfied. And the rule is reusable, because the object is reusable - in a way that UI code never can be (or at least never has been).

 

That same object, with that same interactive rule, can be used behind Windows Forms, Web Forms, WPF and even a web services interface. The rule is always applied, because it is right there in the object. And for interactive UIs it is immediate, because it is in the field's mutator method (the property setter).

 

So in my mind the idea of changing a value in a setter isn't counter-intuitive at all - it is the obvious design purpose behind the property setter (mutator). Any other alternative is really just a ridiculously complex way of implementing public fields. And worse, it leaves us where we've been for 20+ years, with duplicate code and expensive, unreliable software.

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