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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

#define (C# Reference)

#define lets you define a symbol. When you use the symbol as the expression that is passed to the #if directive, the expression will evaluate to true. For example:

#define DEBUG

NoteNote:

The #define directive cannot be used to declare constant values as is typically done in C and C++. Constants in C# are best defined as static members of a class or struct. If you have several such constants, consider creating a separate "Constants" class to hold them.

Symbols can be used to specify conditions for compilation. You can test for the symbol with either #if or #elif. You can also use the conditional attribute to perform conditional compilation.

You can define a symbol, but you cannot assign a value to a symbol. The #define directive must appear in the file before you use any instructions that are not also directives.

You can also define a symbol with the /define compiler option. You can undefine a symbol with #undef.

A symbol that you define with /define or with #define does not conflict with a variable of the same name. That is, a variable name should not be passed to a preprocessor directive and a symbol can only be evaluated by a preprocessor directive.

The scope of a symbol created by using #define is the file in which it was defined.

See #if for an example of how to use #define.

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