Since Popup doesn't derive from Control and doesn't have a template, how can I define a template so that all popups look the same? I need to design one that has a certain look and don't want to have to copy markup each time one is used.
This seems pretty easy but I can't figure out how to do it. The Child property defines a logical tree but I don't see how you can pull that out into a template and reuse it.
Depends how you want your pop-ups to behave. If they're just for displaying information in a uniform manner, than you might want to have a class that derives from Window that has the standard formats and styling wrapped around a ContentPresenter then bind the content of the presenter to a property which can represent the custom information for each pop-up.
Then its just a matter of programatically inserting whatever custom content you want before displaying the pop-up window.
Hope it helps.