The designer cannot be modified while debugging
Visual Studio 2015
For the latest documentation on Visual Studio 2017, see The designer cannot be modified while debugging on docs.microsoft.com. 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.
LINQ to SQL Tools in Visual Studio
Walkthrough: Creating LINQ to SQL Classes (O-R Designer)
LINQ to SQL
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