Forget about extending WPF data-binding support
Databinding in WPF is implemented in the Binding class, which in principle looks like just a MarkupExtension. A MarkupExtension is a core piece of XAML markup "magic". It's rather simple, though:
public abstract class MarkupExtension
{
// Methods
protected MarkupExtension();
public abstract object ProvideValue(IServiceProvider serviceProvider);
}
You're supposed to return a value to assign to the property that is using the markup extension, such as:
<TextBox Text="{l:MyMarkup}" />
A binding markup extension does a LOT more...
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