Conditional compilation directives for C #

The conditional compilation directive has the following four types:

#if

#elfi

#else

#endif

These conditional compilation directives are used to conditionally include or exclude part of a program segment. They have a similar effect to the IF statement in C #. You can use the logical operator with (&&) in the instruction, or (| |) and take the inverse operator (!) Wait The order in which they appear in the program must be this:

A #if statement (must have)

0 or more #elif statements

0 or a #else statement

A #endif statement (must have)

Here are some examples to illustrate their use.

#define Debug
Class Class1
{
#if Debug
void Trace (String s) {}
}
Another example:

#define A
#define B
#undef C
class D
{
  #if c
    void F () {}
  #elif A && b
  void I () {}< c9/> #else
     void G () {}
  #endif
}

It is not difficult to see that the actual compilation of the effect is equivalent to:

Class C
{
void I () {}
}
In this example, please note the use of the #elif directive, which means: "Else if", using #elif can add one or more branches to the #if instruction.

#if指令可以嵌套使用, for example:

#define DEBUG  //debugging on

#undef Trace  //tracing off

class Purchase Transaction
{
  void Commit () {
    #if Debug
      checkconsistency ();
      #if Trace
          WriteToLog (this. ToString ());
      #endif
    #enfif
    commithelper ();
  }

Preprocessing directives are not executed if they appear in the middle of other output-output elements. For example, the following program tries to show Hello World when debug is defined, otherwise the hello everyone is displayed, but the result is a laugh and cry:

Program Listing 8-6:

Using System;
Class Hello
{
 static void Main () {
    System.Console.WriteLine (@ "Hello,
 #if Debug
 World #else
     everyone
 #endif
     ");
 }
The output of the program is as follows:

Hello,
#if Debug World
#else
  everyone
#endif