InitializeError Message: Failed to load a platform extension in Silverlight

If you get an error like:

Unhandled Error in Silverlight Application Code: 2153 Category: InitializeError Message: Failed to load a platform extension. Possibly corrupt or invalid file: …

when you try to launch your Silverlight application (especially if you have the project set to use an autogenerated test page), you should read:

http://forums.silverlight.net/t/185747.aspx/1

In my case the suggestion to turn off “Reduce XAP size by using application library caching” setting at the Silverlight library project settings fixed the issue.

I was using the internal Visual Studio development web server (Cassini), but some people at the discussion thread above also say it can occur in IIS7, with the problem actually being in compression settings (need to turn off option "Enable dynamic content compression”) and they point to the following discussion:

http://forums.iis.net/p/1164340/2023839.aspx#2023839