C# Edit and Continue: error 4018 

Error Message

Adding 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 add a new type. Edit and Continue does not support adding new types while debugging.

Consider the following example:

class Example

{

   static void Main() { }

}

If you step into Main, then try to add int f; above the Main method, this error occurs.

To correct this error

• Choose Undo from the Debug menu to undo the changes.

  –or–

• On the Debug menu, click Stop Debugging, 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#)