#IFNDEF

#IfNDef

Purpose

The #IfNDef command allows you to specify conditionally compiled code based on whether a symbol has not been typed (or defined). This compiler directive does the opposite of the #IfDef compiler directive.

Syntax

#IfNDef {symbol}
{code to compile if symbol is not typed}
#Else
{code to compile if symbol is typed}
#EndIf

What It Does

Any symbolic name that has a valid ICODE equivalent in the compiler's substitution table is said to be typed. Any symbolic name that has no ICODE equivalent is said to be untyped.

Any file element, label, or variable that you have declared is considered to be typed and would test false when given as an argument to #IfNDef.
On the other hand, a symbolic name such as RAZZO is meaningless to the compiler if it has not been declared as a variable, label, or some other type of data, and would cause the #IfNDef to test true.

Most often, the #IfNDef compiler directive is used to conditionally exclude code from compilation in scenarios where code is shared between Windows and Web-based applications. Web-based applications cannot use Windows-based messages and classes, such as MessageBoxes.

In this case, the programmer would check for the existence of the IS$WebApp symbol, which will test false if compiled with Web-based packages, such as AllWebAppClasses.pkg. If compiling a Windows-based application using a precompiled package such as DfAllEnt.pkg or Windows.pkg, this symbol would not be defined.

#IfNDef IS$WebApp
    send Stop_Box "The value you have typed is too large"
#EndIf

This example compiles the line of code calling the Stop_Box Windows MessageBox message only if IS$WebApp is defined.