Hi Wei Li,
That is a strange error, I've never encountered it myself. When I google this error I find only 2 things.
http://forums.asp.net/p/1555911/3825438.aspx
and
http://geekswithblogs.net/Ahmed_Hussein_blog/archive/2009/04/25/al.exe.aspx
the second link seems to be about VS 2008 but a very similar error.
Possibly there is something about a resource file in your project that it doesn't like.
Things I would try:
-
reboot if you have not already
-
if there are any other projects in your solution that depend on the project you changed to 4.0 make sure you also change the dependent project to 4.0
-
review the naming of your resource files
-
review all references under the references node in your project
-
try removing and re-adding each reference
Hope it helps,
Joe