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Example analysis of @ usage in C#

Apr 21, 2017 pm 01:32 PM
@c#

本文实例汇总了C#中@的用法,对C#程序设计来说有不错的借鉴价值。具体如下:

一 字符串中的用法

1.学过C#的人都知道C# 中字符串常量可以以@ 开头声名,这样的优点是转义序列“不”被处理,按“原样”输出,即我们不需要对转义字符加上 \ (反斜扛),就可以轻松coding。如,

string filePath = @"c:\Docs\Source\a.txt" // rather than "c:\\Docs\\Source\\a.txt"

2.如要在一个用 @ 引起来的字符串中包括一个双引号,就需要使用两对双引号了。这时候你不能使用 \ 来转义爽引号了,因为在这里 \ 的转义用途已经被 @  “屏蔽”掉了。如,

@"""Ahoy!"" cried the captain."  // 输出为: "Ahoy!" cried the captain.

这有点像SQL中的单引号常量处理方式:

DECLARE @msg varchar(100) 
SET @msg = ''Ahoy!'' cried the captain.' -- 输出为: 'Ahoy!' cried the captain.

3.@会识别换行符

其实这个特性,我不知道怎么描述,只是偶然发现的,先来看看下面的代码:

string script = @" 
<script type=""type/javascript""> 
function doSomething() 
{ 
} 
</script>";

这段代码在cs文件中写js,结构就很清晰了,正常情况我们是这样coding的:

string script2 = "<script type=\"type/javascript\">function doSomething(){}</script>";

或者:

string script3 = 
"<script type=\"type/javascript\">" + 
"function doSomething(){ " + 
"}</script>";

通常我们会选择后者,因为js代码一般比较长,或者方法体很大,或者需要连接其他变量,这样结构比较清晰。

注意:如果“拼接”的次数很多,应该考虑使用StringBuilder了,有助于提高性能

还有一种场景,也很常见,在程序中拼接 SQL 语句,如

private const string SQL_INS_USER = @" 
INSERT INTO t_User([UserName], [Password], Email)  
 VALUES(@UserName, @Password, @Email)";

这样就像写存储过程一般,保持相当高的代码清晰度。然而,我们需要关注一个问题:字符串长度看下面的测试代码:

private const string SQL_INS_USER1 = @" 
  INSERT INTO t_User([UserName], [Password], Email)  
 VALUES(@UserName, @Password, @Email)"; 

private const string SQL_INS_USER2 = @"INSERT INTO t_User([UserName], [Password], Email)  
 VALUES(@UserName, @Password, @Email)"; 

private const string SQL_INS_USER3 = @"INSERT INTO t_User([UserName], [Password], Email) VALUES(@UserName, @Password, @Email)";  

static void Main(string[] args) 
{ 
  Console.WriteLine(SQL_INS_USER1.Length);  // 126  
  Console.WriteLine(SQL_INS_USER2.Length);  // 112 
  Console.WriteLine(SQL_INS_USER3.Length);  // 86 
}

这里可以看到三个字符串长度分别相差了,14=126-112和26=112-86,注意观察了,在代码编辑器中,SQL_INS_USER1 中第一个换行符号之后,我缩进13个空格(INSERT之前),而
SQL_INS_USER2 中第一个换行符号之后,我缩进25个空格(VALUES之前),
那么,加上一个换行符,刚刚好 14和26

如此编写代码,虽然提高了代码的清晰度和简便性,却无行中带来了另一个问题:字符长度!
很多场景下我们希望字符串越短越好,如,通过ADO.NET 发送 SQL 语句给数据库执行。
所以还是慎用之!

二 标识符中的用法

在 C#  规范中, @  可以作为标识符(类名、变量名、方法名等)的第一个字符,以允许C# 中保留关键字作为自己定义的标识符。

如下代码:

class @class 
{ 
  public static void @static(bool @bool) { 
   if (@bool) 
     System.Console.WriteLine("true"); 
   else 
     System.Console.WriteLine("false"); 
  }   
} 
class Class1 
{ 
  static void M() { 
   cl\u0061ss.st\u0061tic(true); 
  } 
}

注意,@虽然出现在标识符中,但不作为标识符本身的一部分。
因此,以上示例,定义了一个名为 class 的类,并包含一个名为 static 的方法,以及一个参数名为了 bool 的形参。

这样,对于跨语言的移植带来了便利。因为,某个单词在 C#  中作为保留关键字,但是在其他语言中也许不是。

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