The designer cannot be modified while debugging
This message appears when an attempt is made to modify items on the O/R Designer when the application is running in debug mode. When the application is running in debug mode, the O/R Designer is read-only.
To correct this error
Click Stop Debugging on the Debug menu.
The application stops debugging, and items in the O/R Designer can be modified.
See Also
Tasks
Walkthrough: Creating LINQ to SQL Classes (O/R Designer)