Biztalk Error System.String must be Xml serializable to be a message part type

I was doing some development in Biztalk 2006 with Vs 2005 . I created an orchestration and there was one message in it that has type System.String. When I tried to compile my prjoect i was getting following error

"Biztalk Error System.String must be Xml serializable to be a message part type"

I checked the System.String and it was already serializable and I was surprised why this error is occurred.

I was unable to solve the problem then at last I closed the visual studio and started it again and tried to compile my project and it was build successfully .

So the solution is : just restart the visual studio and enjoy