#define Preprocessing directives (c++,c#,vb.net)

In C + +, we can define the so-called symbolic constants by #define This instruction, which is directly compiled into binary code. In this case, it is basically the same as the effect of the constant value variable defined with Const. Of

For example:

But #define also has the function of changing compiler behavior, for example, depending on whether or not you want to compile a code

The following code is available in many C + + source files

#if　!defined(AFX_MAINFRM_H__0EFEA8A4_112C_4528_B7A8_761F1B02C20C__INCLUDED_)　//如果没有定义
#define　AFX_MAINFRM_H__0EFEA8A4_112C_4528_B7A8_761F1B02C20C__INCLUDED_　//那么就定义
#if　_MSC_VER　>　1000　//如果某个符号常量大于1000
#pragma　once
#endif　//　_MSC_VER　>　1000

So, this behavior is. NET inside is how to express it?

1. #define is inherited as a compilation instruction in C #.

#define DEBUG

"Note" In C #, you cannot assign a value to a defined symbol, for example, the following statement is invalid

#define Debug=true

That is, #define只需要定义即可, not (and not necessarily) specifying the value of this symbol

In subsequent code, it is also a syntax such as #if to determine whether a symbol is defined.

2.VB. NET also has a similar implementation, but it does not use the #define keyword, but instead uses the #const keyword

#Const　MINI　=　1
#Const　VERSION　=　1.0
Const　MINI　As　String　=　"4"
#Region　"这是一个代码块"
Sub　Main()
#If　MINI　=　1　Then
Console.WriteLine("Hello.world")
#End　If
End　Sub
#End　Region