#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.
Remarks
#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.
Example
// 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