Writing a design time experience for a Silverlight control can be a bit intimidating (the experience for WPF controls remains unchanged from Blend 2 – just that you now have a ton more extensibility ponts – unless you wanted to re-use the same design-time code for the WPF and Silverlight versions of your controls). Let’s try and understand how this can works (because once you know the basics, its a breeze!). Here is a project template that will help you get started immediately.
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Writing a design time experience for a Silverlight control