Today I opened a few of my Silverlight 4 projects only to be greeted by the following error when attempting to start debuggin:

"Unable to start debugging. The Silverlight Developer Runtime is not installed. Please install a matching version"

What?!

The tools were installed.

I remembered that I’d recently installed a new version of the Silverlight 4 runtime that had been required to view this new theme:

http://www.silverlight.net/content/samples/sl4/themes/jetpack.html

I’d have included a screenshot of the theme, but that would mean using a VM and going through the ordeal again.

I searched around and found a few notes on this forum…

http://forums.silverlight.net/forums/p/188683/433695.aspx

It is true that the tools include a special debug version of the silverlight runtime, however, it’s missing an important step:

You need to uninstall the currently installed Silverlight 4 end-user runtime before re-installing the Silverlight 4 Tools for Visual Studio – http://go.microsoft.com/fwlink/?LinkID=188039 – otherwise you haven’t reverted to the older developer runtime required and just waste a good 10 minutes for nothing.

I’m not sure how Microsoft is going to handle this going forward.  The builds are just slightly different, but the tools will not tolerate the difference.