add/remove operation is impossible, because the code element 'Cxxx' is read only

How to solve “add/remove operation is impossible, because the code element 'Cxxx' is read only”

With Visual Studio 2008, we may meet “add/remove operation is impossible, because the code element 'Cxxx' is read only” error when we want to add a new message handler in “properties” window. In order to solve this issue, we shall:

1) Close current project

2) Enter the directory where the project located

3) delete the .ncb and .suo file  (When I meet this problem, I only delete the .ncb file, and it becomes ok.)

4) reopen the project

Then we shall can add new message handler without any issue