The designer cannot be modified while debugging
Note
This article applies to Visual Studio 2015. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here
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
LINQ to SQL Tools in Visual Studio Walkthrough: Creating LINQ to SQL Classes (O-R Designer) LINQ to SQL