In WPF, should my custom button with behaviour be an extended Button, a CustomControl, or just a ControlTemplate?

You have probably used attached properties without knowing it, things like Grid. Row, and DockPanel.Dock.

After reading the relevant chapters in Sells & Griffiths' Programming WPF book (an excellent book), I think I can now answer my own question. According to these guys, a UserControl is "intended to offer a similar functionality to user controls in other UI frameworks". By this, they mean a user control as a kind of invisible panel that contains a collection of controls, such as a particular form.

One further note is that they "do not usually support customization through templates".

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