Think of it this way.. In .NET you can have partial classes. That is, classes that are defined in more than one file. The XAML is part of the class, the code is the other, so it is a partial class. Extension Methods are another way to define functionality in a different file.

>You illustrated my point. This whole concept of putting some portions in XAML and some in C# adds a level of ambiguity that seem rather stupid and makes the code more prone to maintenance problems
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>Now if the WPF was simply design tags realted to UI presentation, then I see a clear separation between UI and code. But from what I can see, WPF blends the two together. If someone could go in and change the XAML and that resulted in the app's look changing, and this didn't require a recompile, then I'd say cool.