C# If Preprocessing Directive: Elif and Endif

If, Elif, Endif. The #if and #endif directives allow conditional compilation.

Along with #elif and #else, these directives add conditions to parts of a source file. They cause the compiler to skip certain parts of the source code based on defined symbols.

Tip: These directives can make your project simpler—and configurable with compile-time options.

Example. First, this program defines three symbols in the initial #define directives. The NET symbol is then undefined. In the body of the Main method, we can see that the #if PERLS block is compiled.

Here: The #if DOT || NET block is omitted but the #elif PYTHONS block is allowed. The code inside the last directive is compiled.

Define and Undef Directives

C# program that uses if conditional compilation

#define PERLS
#define PYTHONS
#define NET
#undef NET

using System;

class Program
{
    static void Main()
    {
#if PERLS
	Console.WriteLine("PERLS"); // Compiled.
#endif

#if DOT || NET
	Console.WriteLine("DOT OR NET"); // Skipped.
#elif PYTHONS
	Console.WriteLine("PYTHONS"); // Compiled.
#endif

#if (PERLS || PYTHONS) && !NET
	Console.WriteLine("PERLS OR PYTHONS"); // Compiled.
#endif
    }
}

Output

PERLS
PYTHONS
PERLS OR PYTHONS

Syntax tricks. Mainly, the goal of this program is to demonstrate the #if, #elif, and #endif syntax. Please avoid #else if, #elseif, or #elsif—they won't work. You can use the || and && operators within the conditions.

Also: You can negate a symbol, such as with !NET above, and this inverts its Boolean value.

Performance. The #if, #elif, #else, and #endif directives are processed at compile-time. They do not affect runtime. If you inspect the compiled code, you will see no traces if the #if, #elif, #else, #endif, or even #define and #undef directives.

False. Let's look at a useful #if statement: the #if false directive. Inside the #if false and #endif directives, you can put whatever you want and the program is still a valid C# program.

Tip: This is useful when developing if you are making significant changes to aspects of your code base.

Program that uses #if false

class Program
{
    static void Main()
    {
	/*
	 *
	 * This is valid C# code.
	 *
	 * */
#if false
	void a = 100 * whatever;
#endif
    }
}

Note: As an aside, #if true is supported, but it is only useful if you are going to change the directive into something more significant.

Else. The simple directive #else is also available in the C# language. This example shows that #else is conceptually equivalent to an #elif with no condition. The #else directive is similar to an else-statement.

Else

Program that uses #else directive

using System;

class Program
{
    static void Main()
    {
#if false
	Console.WriteLine(0);
#else
	Console.WriteLine(1);
#endif
    }
}

Output

1

Visual Studio. You can use Visual Studio to add definitions to your program. This will affect how the #if conditions are processed. In Visual Studio 2010, please go to Project and then to Properties.

Then: Click on Build: this presents the "Conditional compilation symbols" textbox. Type the symbols, separated by spaces, and recompile.

Visual Studio Tips and Examples

Summary. The #if, #elif, #else, and #endif preprocessing directives in the C# language are among the most useful directives. They allow you to keep a single source file but compile it based on #define directives or definitions in Visual Studio.

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