When the C# compiler encounters an #if directive, followed eventually by an #endif directive, it will compile the code between the directives only if the specified symbol is defined. Unlike C and C++, you cannot assign a numeric value to a symbol; the #if statement in C# is Boolean and only tests whether the symbol has been defined or not. For example,

#define DEBUG //
… #if DEBUG Console.WriteLine(“Debug version”);

You can use the operators == (equality), != (inequality) only to test for true or false . True means the symbol is defined. The statement #if DEBUG has the same meaning as #if (DEBUG == true). The operators && (and), and || (or) can be used to evaluate whether multiple symbols have been defined. You can also group symbols and operators with parentheses.

#if, along with the #else#elif#endif#define, and #undef directives, lets you include or exclude code based on the existence of one or more symbols. This can be useful when compiling code for a debug build or when compiling for a specific configuration.

A conditional directive beginning with a #if directive must explicitly be terminated with a #endif directive.

#define lets you define a symbol, such that, by using the symbol as the expression passed to the #if directive, the expression will evaluate to true.

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 with #define is the file in which it was defined.

// preprocessor_if.cs
#define DEBUG
#define MYTEST
using System;
public class MyClass { static void Main()
{ #if (DEBUG && !MYTEST) Console.WriteLine(“DEBUG is defined”);
#elif (!DEBUG && MYTEST) Console.WriteLine(“MYTEST is defined”);
#elif (DEBUG && MYTEST) Console.WriteLine(“DEBUG and MYTEST are defined”);
#else Console.WriteLine(“DEBUG and MYTEST are not defined”);
#endif }
DEBUG and MYTEST are defined

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