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C# String formatting

February 27, 2020

C# String formatting

Formatting is bound to a type, so depending on which type of object you pass to String.Format, you have different format specifiers.

The index specifies the position of the parameter that shall be used for that format specifier. It can be repeated!

When implementing custom formatting, try to stick to this behavior for being consistent with the default implementation.

  • Generic, alignment is applied after type formatting, so can be used for all types

    • {index[,alignment][:format]}
  • For numerical values (; separator):

    • {index[,alignment]:format >= 0;format < 0}
    • {index[,alignment]:format > 0;format < 0;format = 0}

Pitfalls and gotchas

Pitfalls for specific types are illustrated at the bottom of the type specific parts. Generally speaking, the biggest gotcha is that you should use the invariant culture if you want to have culture independent output. This matters a lot for serialization, because ToString() might yield different results depending on the culture of the current thread.


Alignment is the only formatting you can apply to a string but it works for all types. [Object := Text (String)]

DescriptionFormatUS cultureDE culture
left align{0,-10}Text......Text......
right align{0,10}......Text......Text

Gotcha 1: { must be escaped if you want to output it! Outputting if (...) { yields a System.FormatException. The correct way is to use if (...) {{.

Generic numeric values (conventions)

  • 0 zero placeholder
  • # digit placeholder
  • . decimal point
  • , thousand separator
  • % multiplies by 100, adds % sign
  • multiplies by 100, adds sign ('\u2030')
  • e exponent placeholder

Integer types

Integer patterns [Object := **1234** (Int32)]

DescriptionFormatUS cultureDE culture
zero placeholder{0:0,000.0}1,234.01.234,0
digit placeholder{0:(#) #-##}(1) 2-34(1) 2-34
decimal 6 digits (leading zeroes){0:D6}001234001234
hexadecimal (lowercase){0:x}4d24d2
hexadecimal (lowercase) 4 digits (leading zeroes){0:x4}04d204d2
hexadecimal (uppercase){0:X}4D24D2
hexadecimal (uppercase) 5 digits (leading zeroes){0:X5}004D2004D2
Gotcha 1: # is a placeholder, not detecting fractional part!{0:#.##}12341234
-> correct: use digit placeholders (if you want 2 fractional digits){0:0.00}1234.001234,00
Gotcha 2: with # digits are filled in from right to left!{0:#-##-##}-12-34-12-34
Gotcha 3: , only specifies thousand separator shall be used but not where!{0:0,0}1,2341.234

Floating point types

Double, Float used for scientific notation or currency output.

TO BE DONE (visit us again later, ...)

Enumerated types

  • g, G display the enum as text if Flags attribute or single value
  • f, F display the enum as text (finds a matching sum), if fails falls back to integer
  • d, D decimal value display (no leading zeroes)
  • x, X hexadecimal value display with at least 8 digits

TO BE DONE: examples of output with Flags, ... (give us a visit later again, ...)

Date/Time - Generic Patterns

DateTime patterns [Object := **2009-10-24T13:47:32.0534529** (DateTime)]

DescriptionFormatUS cultureDE culture
default{0}10/24/2009 1:47:32 PM24.10.2009 13:47:32
short date{0:d}10/24/200924.10.2009
long date{0:D}Saturday, October 24, 2009Samstag, 24. Oktober 2009
short time{0:t}1:47 PM13:47
long time{0:T}1:47:32 PM13:47:32
full date & time{0:f}Saturday, October 24, 2009 1:47 PMSamstag, 24. Oktober 2009 13:47
full date & time (long){0:F}Saturday, October 24, 2009 1:47:32 PMSamstag, 24. Oktober 2009 13:47:32
default date & time{0:g}10/24/2009 1:47 PM24.10.2009 13:47
default date & time (long){0:G}10/24/2009 1:47:32 PM24.10.2009 13:47:32
month day pattern{0:M}October 2424 Oktober
year month pattern{0:y}October, 2009Oktober 2009
RFC1123 date string{0:r}Sat, 24 Oct 2009 13:47:32 GMTSat, 24 Oct 2009 13:47:32 GMT
Round-trip date/time pattern{0:o}2009-10-24T13:47:32.05345292009-10-24T13:47:32.0534529
sortable date string{0:s}2009-10-24T13:47:322009-10-24T13:47:32
universal sortable{0:u}2009-10-24 13:47:32Z2009-10-24 13:47:32Z
Gotcha 1: 'U' is broken!{0:U}Saturday, October 24, 2009 11:47:32 AMSamstag, 24. Oktober 2009 11:47:32

Date/Time - Custom Patterns

DateTime custom patterns [Object := **2009-10-24T13:47:32.0534529** (DateTime)]

DescriptionFormatUS cultureDE culture
day (01-31){0:dd}2424
day name (short){0:ddd}SatSa
full day name{0:dddd}SaturdaySamstag
month (01-12){0:MM}1010
month name (short){0:MMM}OctOkt
full month name{0:MMMM}OctoberOktober
2 digits year{0:yy}0909
4 digits year{0:yyyy}20092009
Era AD/BC{0:gg}A.D.n. Chr.
2 digit hour (00-11){0:hh}0101
2 digit hour (00-23){0:HH}1313
Timezone offset, 2 digits{0:zz}+02+02
Full timezone offset{0:zzz}+02:00+02:00
minutes (00-59){0:mm}4747
seconds (00-59){0:ss}3232
1/100 seconds (00-99){0:ff}0505
milliseconds (000-999){0:fff}053053
Gotcha 1: mm are minutes!{0:dd/mm/yyyy}24/47/200924.47.2009
-> correct: MM{0:dd/MM/yyyy}24/10/200924.10.2009
Gotcha 2: / is locale dependent{0:dd/MM/yyyy}24/10/200924.10.2009
-> correct: use apostrophe '{0:dd'/'MM'/'yyyy}24/10/200924/10/2009