Solution 1:
1. Right click on the Silverlight project
2. Locate and click the Web tab
3. Make sure that the Silverlight checkbox is checked.
Solution 2: What if the Silverlight checkbox is already checked, you have already cleaned the solution, deleted the BIN and OBJ folders and still, it doesn't work?
1. Uncheck the Silverlight checkbox
2. Run the solution (the breakpoint will not work, of course!)
3. Stop running. Go the properties again and re-check the Silverlight option.
It works for me!
Solution 3: Clear the browser's cache / temporary files
Solution 4: Usually deleting the <solutionfilename>.suo file in the solution folder fixes it.
