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 Wednesday, 20 June 2007
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I’ve been spending some quality time with WPF and Visual Studio 2008 Beta 1 (Orcas) over the past few months. A couple observations I’ve made:

Observation ichi:

Putting code in a Page/Window constructor is bad. Yes, I know it is the “C# way”, but it is bad. The “VB way” of putting code in the Loaded event handler is better.

Why?

Because any exceptions thrown in the constructor prevent the page/window from loading at all, and you have to catch those exceptions in the code that is creating the page/window. In many navigation scenarios you can’t catch them, so the user gets an ugly WPF exception dialog.

However, if you do all your init work in the Loaded event handler, the page/window instance already exists. Navigation has already happened, so the “calling code” is no longer responsible for your page/window going haywire. Instead, you can actually handle the exception and show a nice dialog, explaining to the user what happened, and you can (if desired) navigate somewhere else or whatever.

Observation ni:

Handing control events with += or AddHandler is superior to using WithEvents/Handles. Yes, I am a strong advocate of WithEvents/Handles, and it is one of the major missing features in C#, but for WPF it turns out to be a negative.

Why?

Because the Handles clause links the name of the control to the code behind. The AddHandler or += approach, while less attractive in many ways, only couples the event name to the code behind.

In other words, when using Handles you get code like:

Private Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) _
  Handles Button1.Click

While the AddHandler approach results in this code:

Private Sub SaveData(ByVal sender As Object, ByVal e As EventArgs)

The value of the second approach is that any control can route an event to SaveData(). It doesn’t have to be a button, much less Button1. It actually doesn’t even have to be a Click event. It could be almost any event.

The result is that the Presentation (XAML) is less coupled to the UI (VB/C# code) this way than when using Handles.

Unfortunately the Handles approach is more explicit and thus more readable (which is why C# should really get the Handles feature), so we’re left trading readability in order to gain loose coupling…

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