It appears that the Silverlight Unit Test Framework automatically calls http://localhost:8000/… when you load the Silverlight control in the web browser.

If Port 8000 is already in use, you will receive an error message from the Silverlight control stating that it received a 404 error page when it issued a web request (or something similar – I don’t remember the exact error message).

Bottom line, make sure that no web service or other application is using Port 8000 on your machine. Furthermore, note that Fiddler will not display any outgoing requests to “localhost”, so you will not see this particular request in Fiddler.

Unfortunately, I cannot explain why the Silverlight Unit Test Framework uses Port 8000 but just keep this mind! If you want to know why ask Jeff Wilcox.

Happy Unit Testing!