C#详解format函数，各种格式化

一、String Format for Double

Digits after decimal point

This example formats double to string with fixed number of decimal places. For two decimal places use pattern „0.00“. If a float number has less decimal places, the rest digits on the right will be zeroes. If it has more decimal places, the number will be rounded.

[C#]

// just two decimal places
String.Format("{0:0.00}", 123.4567);      // "123.46"
String.Format("{0:0.00}", 123.4);         // "123.40"
String.Format("{0:0.00}", 123.0);         // "123.00"

Next example formats double to string with floating number of decimal places. E.g. for maximal two decimal places use pattern „0.##“.

[C#]

// max. two decimal places
String.Format("{0:0.##}", 123.4567);      // "123.46"
String.Format("{0:0.##}", 123.4);         // "123.4"
String.Format("{0:0.##}", 123.0);         // "123"

Digits before decimal point

If you want a float number to have any minimal number of digits before decimal point use N-times zero before decimal point. E.g. pattern „00.0“ formats a float number to string with at least two digits before decimal point and one digit after that.

[C#]

// at least two digits before decimal point
String.Format("{0:00.0}", 123.4567);      // "123.5"
String.Format("{0:00.0}", 23.4567);       // "23.5"
String.Format("{0:00.0}", 3.4567);        // "03.5"
String.Format("{0:00.0}", -3.4567);       // "-03.5"

Thousands separator

To format double to string with use of thousands separator use zero and comma separator before an usual float formatting pattern, e.g. pattern „0,0.0“ formats the number to use thousands separators and to have one decimal place.

[C#]

String.Format("{0:0,0.0}", 12345.67);     // "12,345.7"
String.Format("{0:0,0}", 12345.67);       // "12,346"

Zero

Float numbers between zero and one can be formatted in two ways, with or without leading zero before decimal point. To format number without a leading zero use # before point. For example „#.0“ formats number to have one decimal place and zero to N digits before decimal point (e.g. „.5“ or „123.5“).

Following code shows how can be formatted a zero (of double type).

[C#]

String.Format("{0:0.0}", 0.0);            // "0.0"
String.Format("{0:0.#}", 0.0);            // "0"
String.Format("{0:#.0}", 0.0);            // ".0"
String.Format("{0:#.#}", 0.0);            // ""

Align numbers with spaces

To align float number to the right use comma „,“ option before the colon. Type comma followed by a number of spaces, e.g. „0,10:0.0“ (this can be used only in String.Format method, not indouble.ToString method). To align numbers to the left use negative number of spaces.

[C#]

String.Format("{0,10:0.0}", 123.4567);    // "     123.5"
String.Format("{0,-10:0.0}", 123.4567);   // "123.5     "
String.Format("{0,10:0.0}", -123.4567);   // "    -123.5"
String.Format("{0,-10:0.0}", -123.4567);  // "-123.5    "

Custom formatting for negative numbers and zero

If you need to use custom format for negative float numbers or zero, use semicolon separator „;“ to split pattern to three sections. The first section formats positive numbers, the second section formats negative numbers and the third section formats zero. If you omit the last section, zero will be formatted using the first section.

[C#]

String.Format("{0:0.00;minus 0.00;zero}", 123.4567);   // "123.46"
String.Format("{0:0.00;minus 0.00;zero}", -123.4567);  // "minus 123.46"
String.Format("{0:0.00;minus 0.00;zero}", 0.0);        // "zero"

Some funny examples

As you could notice in the previous example, you can put any text into formatting pattern, e.g. before an usual pattern „my text 0.0“. You can even put any text between the zeroes, e.g. „0aaa.bbb0“.

[C#]

String.Format("{0:my number is 0.0}", 12.3);   // "my number is 12.3"
String.Format("{0:0aaa.bbb0}", 12.3);          // "12aaa.bbb3"

二、String Format for Int

Integer numbers can be formatted in .NET in many ways. You can use static method String.Formator instance method int.ToString. Following examples show how to align numbers (with spaces or zeroes), how to format negative numbers or how to do custom formatting like phone numbers.

Add zeroes before number

To add zeroes before a number, use colon separator „:“ and write as many zeroes as you want.

[C#]

String.Format("{0:00000}", 15);          // "00015"
String.Format("{0:00000}", -15);         // "-00015"

Align number to the right or left

To align number to the right, use comma „,“ followed by a number of characters. This alignment option must be before the colon separator.

[C#]

String.Format("{0,5}", 15);              // "   15"
String.Format("{0,-5}", 15);             // "15   "
String.Format("{0,5:000}", 15);          // "  015"
String.Format("{0,-5:000}", 15);         // "015  "

Different formatting for negative numbers and zero

You can have special format for negative numbers and zero. Use semicolon separator „;“ to separate formatting to two or three sections. The second section is format for negative numbers, the third section is for zero.

[C#]

String.Format("{0:#;minus #}", 15);      // "15"
String.Format("{0:#;minus #}", -15);     // "minus 15"
String.Format("{0:#;minus #;zero}", 0);  // "zero"

Custom number formatting (e.g. phone number)

Numbers can be formatted also to any custom format, e.g. like phone numbers or serial numbers.

[C#]

String.Format("{0:+### ### ### ###}", 447900123456); // "+447 900 123 456"
String.Format("{0:##-####-####}", 8958712551);       // "89-5871-2551"

三、String Format for DateTime

This example shows how to format DateTime using String.Format method. All formatting can be done also using DateTime.ToString method.

Custom DateTime Formatting

There are following custom format specifiers y (year), M (month), d (day), h (hour 12), H (hour 24),m (minute), s (second), f (second fraction), F (second fraction, trailing zeroes are trimmed), t (P.M or A.M) and z (time zone).

Following examples demonstrate how are the format specifiers rewritten to the output.

[C#]

// create date time 2008-03-09 16:05:07.123
DateTime dt = new DateTime(2008, 3, 9, 16, 5, 7, 123);

String.Format("{0:y yy yyy yyyy}", dt);  // "8 08 008 2008"   year
String.Format("{0:M MM MMM MMMM}", dt);  // "3 03 Mar March"  month
String.Format("{0:d dd ddd dddd}", dt);  // "9 09 Sun Sunday" day
String.Format("{0:h hh H HH}",     dt);  // "4 04 16 16"      hour 12/24
String.Format("{0:m mm}",          dt);  // "5 05"            minute
String.Format("{0:s ss}",          dt);  // "7 07"            second
String.Format("{0:f ff fff ffff}", dt);  // "1 12 123 1230"   sec.fraction
String.Format("{0:F FF FFF FFFF}", dt);  // "1 12 123 123"    without zeroes
String.Format("{0:t tt}",          dt);  // "P PM"            A.M. or P.M.
String.Format("{0:z zz zzz}",      dt);  // "-6 -06 -06:00"   time zone

You can use also date separator / (slash) and time sepatator : (colon). These characters will be rewritten to characters defined in the current DateTimeForma­tInfo.DateSepa­rator andDateTimeForma­tInfo.TimeSepa­rator.

[C#]

// date separator in german culture is "." (so "/" changes to ".")
String.Format("{0:d/M/yyyy HH:mm:ss}", dt); // "9/3/2008 16:05:07" - english (en-US)
String.Format("{0:d/M/yyyy HH:mm:ss}", dt); // "9.3.2008 16:05:07" - german (de-DE)

Here are some examples of custom date and time formatting:

[C#]

// month/day numbers without/with leading zeroes
String.Format("{0:M/d/yyyy}", dt);            // "3/9/2008"
String.Format("{0:MM/dd/yyyy}", dt);          // "03/09/2008"

// day/month names
String.Format("{0:ddd, MMM d, yyyy}", dt);    // "Sun, Mar 9, 2008"
String.Format("{0:dddd, MMMM d, yyyy}", dt);  // "Sunday, March 9, 2008"

// two/four digit year
String.Format("{0:MM/dd/yy}", dt);            // "03/09/08"
String.Format("{0:MM/dd/yyyy}", dt);          // "03/09/2008"

Standard DateTime Formatting

In DateTimeForma­tInfo there are defined standard patterns for the current culture. For example property ShortTimePattern is string that contains value h:mm tt for en-US culture and value HH:mmfor de-DE culture.

Following table shows patterns defined in DateTimeForma­tInfo and their values for en-US culture. First column contains format specifiers for the String.Format method.

Specifier	DateTimeFormatInfo property	Pattern value (for en-US culture)
t	ShortTimePattern	h:mm tt
d	ShortDatePattern	M/d/yyyy
T	LongTimePattern	h:mm:ss tt
D	LongDatePattern	dddd, MMMM dd, yyyy
f	(combination of D and t)	dddd, MMMM dd, yyyy h:mm tt
F	FullDateTimePattern	dddd, MMMM dd, yyyy h:mm:ss tt
g	(combination of d and t)	M/d/yyyy h:mm tt
G	(combination of d and T)	M/d/yyyy h:mm:ss tt
m, M	MonthDayPattern	MMMM dd
y, Y	YearMonthPattern	MMMM, yyyy
r, R	RFC1123Pattern	ddd, dd MMM yyyy HH':'mm':'ss 'GMT' (*)
s	SortableDateTi­mePattern	yyyy'-'MM'-'dd'T'HH':'mm':'ss (*)
u	UniversalSorta­bleDateTimePat­tern	yyyy'-'MM'-'dd HH':'mm':'ss'Z' (*)
		(*) = culture independent

Following examples show usage of standard format specifiers in String.Format method and the resulting output.

[C#]

String.Format("{0:t}", dt);  // "4:05 PM"                         ShortTime
String.Format("{0:d}", dt);  // "3/9/2008"                        ShortDate
String.Format("{0:T}", dt);  // "4:05:07 PM"                      LongTime
String.Format("{0:D}", dt);  // "Sunday, March 09, 2008"          LongDate
String.Format("{0:f}", dt);  // "Sunday, March 09, 2008 4:05 PM"  LongDate+ShortTime
String.Format("{0:F}", dt);  // "Sunday, March 09, 2008 4:05:07 PM" FullDateTime
String.Format("{0:g}", dt);  // "3/9/2008 4:05 PM"                ShortDate+ShortTime
String.Format("{0:G}", dt);  // "3/9/2008 4:05:07 PM"             ShortDate+LongTime
String.Format("{0:m}", dt);  // "March 09"                        MonthDay
String.Format("{0:y}", dt);  // "March, 2008"                     YearMonth
String.Format("{0:r}", dt);  // "Sun, 09 Mar 2008 16:05:07 GMT"   RFC1123
String.Format("{0:s}", dt);  // "2008-03-09T16:05:07"             SortableDateTime
String.Format("{0:u}", dt);  // "2008-03-09 16:05:07Z"            UniversalSortableDateTime

四、IFormatProvider for Numbers

This example shows how to convert float to string and string to float using IFormatProvider. To get IFormatProvider you need to get CultureInfo instance first.

Get invariant or specific CultureInfo

Invariant culture is a special type of culture which is culture-insensitive. You should use this culture when you need culture-independent results, e.g. when you format or parse values in XML file. The invariant culture is internally associated with the English language. To get invariantCultureInfo instance use static property CultureInfo.In­variantCulture.

To get specific CultureInfo instance use static method CultureInfo.Get­CultureInfo with the specificculture name, e.g. for the German language CultureInfo.GetCultureInfo("de-DE").

Format and parse numbers using the IFormatProvider

Once you have the CultureInfo instance use property CultureInfo.Num­berFormat to get anIFormatProvider for numbers (the NumberFormatInfo object)

[C#]

// format float to string
float num = 1.5f;
string str = num.ToString(CultureInfo.InvariantCulture.NumberFormat);        // "1.5"
string str = num.ToString(CultureInfo.GetCultureInfo("de-DE").NumberFormat); // "1,5"

[C#]

// parse float from string
float num = float.Parse("1.5", CultureInfo.InvariantCulture.NumberFormat);
float num = float.Parse("1,5", CultureInfo.GetCultureInfo("de-DE").NumberFormat);

五、Custom IFormatProvider

The following example shows how to write a custom IFormatProvider which you can use in methodString.Format(I­FormatProvider, …).

This formatter formats doubles to 3 decimal places with a dot separator.

[C#]

public class DoubleFormatter : IFormatProvider, ICustomFormatter
{
  // always use dot separator for doubles
  private CultureInfo enUsCulture = CultureInfo.GetCultureInfo("en-US");

  public string Format(string format, object arg, IFormatProvider formatProvider)
  {
    // format doubles to 3 decimal places
    return string.Format(enUsCulture, "{0:0.000}", arg);
  }

  public object GetFormat(Type formatType)
  {
    return (formatType == typeof(ICustomFormatter)) ? this : null;
  }
}

Having this formatter, we can use it like this:

[C#]

double width = 15.77555;
double height = 12.8497979;
Console.WriteLine(
  string.Format(new DoubleFormatter(), "w={0} h={1}", width, height));

Output:

w=15.776 h=12.850

So now we have a reusable format for doubles – 3 decimal places with dot separator. That is nice, but this formatter is very simple – it formats everything (eg. DateTime) as „0:000“. This is a fast version if you know that you will only use it for formatting lots of doubles.

The real version should look like this:

[C#]

public class DoubleFormatter : IFormatProvider, ICustomFormatter
{
  // always use dot separator for doubles
  private CultureInfo enUsCulture = CultureInfo.GetCultureInfo("en-US");

  public string Format(string format, object arg, IFormatProvider formatProvider)
  {
    if (arg is double)
    {
      if (string.IsNullOrEmpty(format))
      {
        // by default, format doubles to 3 decimal places
        return string.Format(enUsCulture, "{0:0.000}", arg);
      }
      else
      {
        // if user supplied own format use it
        return ((double)arg).ToString(format, enUsCulture);
      }
    }
    // format everything else normally
    if (arg is IFormattable)
      return ((IFormattable)arg).ToString(format, formatProvider);
    else
      return arg.ToString();
  }

  public object GetFormat(Type formatType)
  {
    return (formatType == typeof(ICustomFormatter)) ? this : null;
  }
}

Example:

[C#]

Console.WriteLine(string.Format(new DoubleFormatter(),
 "Numbers {0} and {1:0.0}. Now a string {2}, a number {3}, date {4} and object: {5}",
 1.234567, -0.57123456, "Hi!", 5, DateTime.Now, new object()));

Output:

Numbers 1.235 and -0.6. Now a string Hi!, a number 5, date 12.6.2009 17:11:35
and object: System.Object