C# Edit and Continue: error 4021 

Error Message

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.

See Also

Reference

using (C# Reference)
Supported Code Changes (C#)
Edit and Continue (Visual C#)

Other Resources

Types (C# Reference)
Edit and Continue Errors and Warnings (C#)