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The #if and #endif directives enable conditional compilation of a sequence of code based upon whether an expression involving one or more symbols evaluates to true A symbol is true if it has been defined It is false otherwise Thus, if a symbol has been defined by a #define directive, it will evaluate as true The general form of #if is #if symbol-expression statement sequence #endif If the expression following #if is true, the code that is between it and #endif is compiled Otherwise, the intervening code is skipped The #endif directive marks the end of an #if block A symbol expression can be as simple as just the name of a symbol You can also use these operators in a symbol expression: !, = =, !=, &&, and || Parentheses are also allowed Here s an example:
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// Demonstrate #if, #endif, and #define #define EXPERIMENTAL using System; class Test { static void Main() { #if EXPERIMENTAL ConsoleWriteLine("Compiled for experimental version");
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#endif ConsoleWriteLine("This is in all versions");
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PART I PART I PART I
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Compiled for experimental version This is in all versions
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The program defines the symbol EXPERIMENTAL Thus, when the #if is encountered, the symbol expression evaluates to true, and the first WriteLine( ) statement is compiled If you remove the definition of EXPERIMENTAL and recompile the program, the first WriteLine( ) statement will not be compiled, because the #if will evaluate to false In all cases, the second WriteLine( ) statement is compiled because it is not part of the #if block As explained, you can use a symbol expression in an #if For example,
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// Use a symbol expression #define EXPERIMENTAL #define TRIAL using System; class Test { static void Main() { #if EXPERIMENTAL ConsoleWriteLine("Compiled for experimental version"); #endif #if EXPERIMENTAL && TRIAL ConsoleErrorWriteLine("Testing experimental trial version"); #endif ConsoleWriteLine("This is in all versions"); } }
The output from this program is shown here:
Compiled for experimental version Testing experimental trial version This is in all versions
In this example, two symbols are defined, EXPERIMENTAL and TRIAL The second WriteLine( ) statement is compiled only if both are defined You can use the ! to compile code when a symbol is not defined For example,
#if !EXPERIMENTAL ConsoleWriteLine("Code is not experimental!"); #endif
The call to WriteLine( ) will be compiled only if EXPERIMENTAL has not been defined
Part I:
The C# Language
#else and #elif
The #else directive works much like the else that is part of the C# language: It establishes an alternative if #if fails The previous example can be expanded as shown here:
// Demonstrate #else #define EXPERIMENTAL using System; class Test { static void Main() { #if EXPERIMENTAL ConsoleWriteLine("Compiled for experimental version"); #else ConsoleWriteLine("Compiled for release"); #endif #if EXPERIMENTAL && TRIAL ConsoleErrorWriteLine("Testing experimental trial version"); #else ConsoleErrorWriteLine("Not experimental trial version"); #endif ConsoleWriteLine("This is in all versions"); } }
The output is shown here:
Compiled for experimental version Not experimental trial version This is in all versions
Since TRIAL is not defined, the #else portion of the second conditional code sequence is used Notice that #else marks both the end of the #if block and the beginning of the #else block This is necessary because there can only be one #endif associated with any #if Furthermore, there can be only one #else associated with any #if The #elif directive means else if and establishes an if-else-if chain for multiple compilation options #elif is followed by a symbol expression If the expression is true, that block of code is compiled and no other #elif expressions are tested Otherwise, the next block in the series is checked If no #elif succeeds, then if there is a #else, the code sequence associated with the #else is compiled Otherwise, no code in the entire #if is compiled The general form for #elif is #if symbol-expression statement sequence #elif symbol-expression statement sequence #elif symbol-expression
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statement sequence // #endif Here s an example:
// Demonstrate #elif #define RELEASE using System; class Test { static void Main() { #if EXPERIMENTAL ConsoleWriteLine("Compiled for experimental version"); #elif RELEASE ConsoleWriteLine("Compiled for release"); #else ConsoleWriteLine("Compiled for internal testing"); #endif #if TRIAL && !RELEASE ConsoleWriteLine("Trial version"); #endif ConsoleWriteLine("This is in all versions"); } }
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