C# Edit and Continue: error 4021
Deleting a [type | member | parameter | constraint | using] will prevent the debug session from continuing while Edit and Continue is enabled
This error indicates that you tried to delete a type, member, parameter, constraint, or using statement; whose deletion Edit and Continue does not support while debugging.
To correct this error
Undo the changes, and then continue debugging without the changes
-or-
On the Debug menu, click Stop Debugging, then make the changes, and start a new debugging session.