VS2010: “Type or namespace name could not be found”, problem & solution

I recently converted a VS2008 project to VS2010. An interesting probelm is that the project compiles and runs well in VS2008, however, when compiling in VS2010, errors occur:

“The type or namespace name  could not be found (are you missing an assembly reference?)… ”

Wow, what does this mean?

A search in Google shows that there are a lot of different explanations, but it seems there is no one that works, at least for my case.

Finally, I got the trick:

In the project properties, change the Dotnet framework from 2.0,3.0 or 3.5 to 4, compile and run!

Happy coding!