HOW TO detect if Silverlight is installed

You can utilize the Silverlight.isInstalled helper method in the Silverlight.js file as beautifully explained in this article in Silverlight Developer Center on Yahoo Developer Network (!!) or adapt Nikhil K's plain Javascript code snippet.

The YDN article has a couple of other interesting points about developing with Silverlight:

An easy way to test the experience users without Silverlight will get is to use Internet Explorer 7's add-on manager to temporarily disable the Silverlight "AgControl Class" add-on.
It is important to provide users who don't have Silverlight installed the best possible user experience. This includes providing the user with a preview of the content they will get in return for taking the time to install, keeping them within your page and automatically showing the Silverlight enabled content once the installation has completed. Unfortunately, currently all other browsers except Internet Explorer on Windows will require the user to restart their browser after installation.

The inplaceInstallPrompt trick can improve the installation experience on IE. This feature has been implemented on the revamped Visual Studio Express Edition website.

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