How to solve “add/remove operation is impossible

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

4) reopen the project

Then we shall can add new message handler without any issue