There is no mouse double-click event on UIElements in Silverlight.
Some people think this is an odd omission. Silverlight's cousin WPF can fire a double-click event, so why not Silverlight? Even JavaScript can catch double-click events inside a web page (although in true web fashion, the exact behavior depends on the combination of browser version, operating system, and the current phase of the moon – see Jan Wolter's Javascript Madness: Mouse Events for all the gory details).
You can still detect a double click using a mouse up event handler and a timestamp - but should you? The omission of a double-click event wasn't because of a technical limitation – the omission is a deliberate design decision. Silverlight is built for the web, and many consider double-clicking on the web to be a usability no-no.
Here is what Peter Chng has to say:
Take this comment by about the new Yahoo! Photos site; the user laments about the interface requiring a double-click to open a full-size image rather than just a traditional single-click as on most other websites. (The double-click detection is done via JavaScript)
I'll admit that while I was first confused by this action, I thought little of that user's comment - I mean, how hard can it be to learn a simple action like that? But, after some more thinking, I've come to agree with the comment - breaking the pattern of how a user navigates on a website is not a good idea, even if it's done to try to make your website feel more like a desktop application.
The New York Times website includes a feature that launches a search (in a pop-up window) when you double click a word inside an article. Digital Inspiration believes this behavior is user unfriendly, while Dion thinks the feature is a bug.
You might also have noticed the only mouse button events in Silverlight are LEFT mouse buttons events. Macs don't have a right mouse button, and my guess is they never will (adding one now would be like admitting defeat). Exposing a right-click event could only invite more usability problems.
Silverlight has some wonderful potential. Just remember to innovate – don't aggravate.
