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#if (C# Reference)

#if lets you begin a conditional directive, testing a symbol or symbols to see if they evaluate to true. If they do evaluate to true, the compiler evaluates all the code between the #if and the nearest #endif directive. For example,

#define DEBUG
// ...
#if DEBUG
    Console.WriteLine("Debug version");
#endif

You can use the operators == (equality), != (inequality), && (and), and || (or) to evaluate multiple symbols. 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 condition of one or more symbols. This can be most 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 VC_V7
using System;
public class MyClass 
{
    static void Main() 
    {
#if (DEBUG && !VC_V7)
        Console.WriteLine("DEBUG is defined");
#elif (!DEBUG && VC_V7)
        Console.WriteLine("VC_V7 is defined");
#elif (DEBUG && VC_V7)
        Console.WriteLine("DEBUG and VC_V7 are defined");
#else
        Console.WriteLine("DEBUG and VC_V7 are not defined");
#endif
    }
}

Output

 
DEBUG and VC_V7 are defined

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