Defines conditional compiler constants for Visual Basic.
Conditional compiler constants are always private to the file in which they appear. You cannot create public compiler constants using the #Const directive; you can create them only in the user interface or with the /define compiler option.
You can use only conditional compiler constants and literals in expression. Using a standard constant defined with Const causes an error. Conversely, you can use constants defined with the #Const keyword only for conditional compilation. Constants can also be undefined, in which case they have a value of Nothing.