The #if, #elif, #else, and #endif directives can be used to specify the sections of the source file that can be executed.

expression and expression-n can only test the value of (or for the existence of) identifiers defined in a #define directive (see #define and #undef). To test for equality, use == (two equal signs). To test for inequality, use != (an exclamation point and an equal sign). You can also use || (or) and && (and) to evaluate more than one identifier in a single expression, for example:

You can also test whether an identifier has been defined. Use ! to test if an identifier has not been defined. For example:

You can also use the defined preprocessor operator to test whether an identifier has been defined. For an example, see #error.

If expression evaluates to True, the statements following the #if directive are executed. If expression evaluates to False, each expression-n (if any) is evaluated in turn. When expression-n evaluates to True, the statements following the associated #elif directive are executed. If no expression-n evaluates to True, or there are no #elif directives, the statements following the #else directive, are executed. Execution then continues with the statement following the #endif directive.

The #elif and #else directives are optional. There can be as many #elif directives as required after the #if directive, but none can appear after the #else directive.

#if directives can be nested.

The following example uses the #if, #efif, and #endif directives to control the type of debugging that will be included in an mrScriptBasic script. Because the identifier MyDebugLevel has been defined with a value of 2, the value of MyValue will be written to the log: