C#中的String.Format的格式参数详解

作者：袖梨 2022-06-25

一、定义
String.Format是将指定的 String类型的数据中的每个格式项替换为相应对象的值的文本等效项。
如:

（1）

代码如下	复制代码
string p1 = "Jackie"; string p2 = "Aillo"; Response.Write(String.Format("Hello {0}, I'm {1}", p1, p2));

（2）

代码如下	复制代码
Response.Write(String.Format("Hello {0}, I'm {1}", "Jackie", "Aillo"));

这二者的效果是一样的。都是将最后面的两项的值分别替换第一项的{0}和{1}。
输出的结果是：Hello Jackie, I'm Aillo

(3)

代码如下	复制代码
12345.ToString("n"); //生成 12,345.00 12345.ToString("C"); //生成￥12,345.00 12345.ToString("e"); //生成 1.234500e+004 12345.ToString("f4"); //生成 12345.0000 12345.ToString("x"); //生成 3039 (16进制) 12345.ToString("p"); //生成 1,234,500.00%

二、String.Format的多格式定义：
这里所谓的多格式是指一个格式项中可以定义1~3个格式参数，每种格式参数用分号（；）隔开。带2个和3个格式参数的格式项所对应的值必须是数值类型的，这样才能判断是否为负数、正数、零。
带1个格式参数：

代码如下	复制代码
//以科学计数法的格式输出 double p1 = 1000000; Response.Write(String.Format("{0:E2}", p1));

带2个格式参数：

/*当格式项对应的值为非负数，则选择第一种格式；值为负数则选第二种格式*/

代码如下	复制代码
double p1 = 10000; double p2 = -2420.50; Response.Write(String.Format("{0:#,###0.00;#,###0.000;} ", p1)); Response.Write(String.Format("{0:#,###0.00;#,###0.000;}", p2));

带3个格式参数：

/*当格式项对应的值为正数则选择第一张格式；
负数则为第二中格式；
值等于零则为第三种格式*/

代码如下	复制代码
1double p1 = 10000; double p2 = -2420.50; double p3 = 0.00; Response.Write(String.Format("{0:#,###0.00;#,###0.000;#,###0.0000} ", p1)); Response.Write(String.Format("{0:#,###0.00;#,###0.000;#,###0.0000} ", p3)); Response.Write(String.Format("{0:#,###0.00;#,###0.000;#,###0.0000}", p2));

补充：

{0:N2} 中的N3,f3表示格式化之后数据的类型以及小数的位数。如：N2表示带2个小数的数字；
与此类似：

N或者n 表示数字
F或者f 表示固定点
E或者e 表示科学计数法
D或者d 表示十进制数
X或者x 表示十六进制
G或者g 表示常规
C或者c 表示货币

String.Format的格式

C#格式化数值结果表

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字符	说明	示例	输出
C	货币	string.Format("{0:C3}", 2)	＄2.000
D	十进制	string.Format("{0:D3}", 2)	002
E	科学计数法	1.20E+001	1.20E+001
G	常规	string.Format("{0:G}", 2)	2
N	用分号隔开的数字	string.Format("{0:N}", 250000)	250,000.00
X	十六进制	string.Format("{0:X000}", 12)	C
		string.Format("{0:000.000}", 12.2)	012.200

Strings

There really isn't any formatting within a strong, beyond it's alignment. Alignment works for any argument being printed in a String.Format call.

Sample	Generates
String.Format("->{1,10}<-", "Hello");	-> Hello<-
String.Format("->{1,-10}<-", "Hello");	->Hello <-

Numbers

Basic number formatting specifiers:

Specifier	Type	Format	Output (Passed Double 1.42)	Output (Passed Int -12400)
c	Currency	{0:c}	＄1.42	-＄12,400
d	Decimal (Whole number)	{0:d}	System. FormatException	-12400
e	Scientific	{0:e}	1.420000e+000	-1.240000e+004
f	Fixed point	{0:f}	1.42	-12400.00
g	General	{0:g}	1.42	-12400
n	Number with commas for thousands	{0:n}	1.42	-12,400
r	Round trippable	{0:r}	1.42	System. FormatException
x	Hexadecimal	{0:x4}	System. FormatException	cf90

Custom number formatting:

Specifier	Type	Example	Output (Passed Double 1500.42)	Note
0	Zero placeholder	{0:00.0000}	1500.4200	Pads with zeroes.
#	Digit placeholder	{0:(#).##}	(1500).42
.	Decimal point	{0:0.0}	1500.4
,	Thousand separator	{0:0,0}	1,500	Must be between two zeroes.
,.	Number scaling	{0:0,.}	2	Comma adjacent to Period scales by 1000.
%	Percent	{0:0%}	150042%	Multiplies by 100, adds % sign.
e	Exponent placeholder	{0:00e+0}	15e+2	Many exponent formats available.
;	Group separator	see below

The group separator is especially useful for formatting currency values which require that negative values be enclosed in parentheses. This currency formatting example at the bottom of this document makes it obvious:

Dates

Note that date formatting is especially dependant on the system's regional settings; the example strings here are from my local locale.

Specifier	Type	Example (Passed System.DateTime.Now)
d	Short date	10/12/2002
D	Long date	December 10, 2002
t	Short time	10:11 PM
T	Long time	10:11:29 PM
f	Full date & time	December 10, 2002 10:11 PM
F	Full date & time (long)	December 10, 2002 10:11:29 PM
g	Default date & time	10/12/2002 10:11 PM
G	Default date & time (long)	10/12/2002 10:11:29 PM
M	Month day pattern	December 10
r	RFC1123 date string	Tue, 10 Dec 2002 22:11:29 GMT
s	Sortable date string	2002-12-10T22:11:29
u	Universal sortable, local time	2002-12-10 22:13:50Z
U	Universal sortable, GMT	December 11, 2002 3:13:50 AM
Y	Year month pattern	December, 2002

The 'U' specifier seems broken; that string certainly isn't sortable.

Custom date formatting:

Specifier	Type	Example	Example Output
dd	Day	{0:dd}	10
ddd	Day name	{0:ddd}	Tue
dddd	Full day name	{0:dddd}	Tuesday
f, ff, ...	Second fractions	{0:fff}	932
gg, ...	Era	{0:gg}	A.D.
hh	2 digit hour	{0:hh}	10
HH	2 digit hour, 24hr format	{0:HH}	22
mm	Minute 00-59	{0:mm}	38
MM	Month 01-12	{0:MM}	12
MMM	Month abbreviation	{0:MMM}	Dec
MMMM	Full month name	{0:MMMM}	December
ss	Seconds 00-59	{0:ss}	46
tt	AM or PM	{0:tt}	PM
yy	Year, 2 digits	{0:yy}	02
yyyy	Year	{0:yyyy}	2002
zz	Timezone offset, 2 digits	{0:zz}	-05
zzz	Full timezone offset	{0:zzz}	-05:00
:	Separator	{0:hh:mm:ss}	10:43:20
/	Separator	{0:dd/MM/yyyy}	10/12/2002