String.Format

The Format methods are a set of overloads (opens new window) in the System.String (opens new window) class used to create strings that combine objects into specific string representations. This information can be applied to String.Format (opens new window), various WriteLine methods as well as other methods in the .NET framework.

Since C# 6.0

Since C# 6.0 it is possible to use string interpolation in place of String.Format.

string name = "John";
string lastname = "Doe";
Console.WriteLine($"Hello {name} {lastname}!");

Hello John Doe!

^{More examples for this under the topic C# 6.0 features: String interpolation (opens new window).}

Places where String.Format is 'embedded' in the framework

There are several places where you can use String.Format indirectly: The secret is to look for the overload with the signature string format, params object[] args, e.g.:

Console.WriteLine(String.Format("{0} - {1}", name, value));

Can be replaced with shorter version:

Console.WriteLine("{0} - {1}", name, value);

There are other methods which also use String.Formate.g.:

Debug.WriteLine(); // and Print()
StringBuilder.AppendFormat();

Create a custom format provider

public class CustomFormat : IFormatProvider, ICustomFormatter
{
    public string Format(string format, object arg, IFormatProvider formatProvider)
    {
        if (!this.Equals(formatProvider))
        {
            return null;
        }

        if (format == "Reverse")
        {
            return String.Join("", arg.ToString().Reverse());
        }

        return arg.ToString();
    }

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

Usage:

String.Format(new CustomFormat(), "-> {0:Reverse} <-", "Hello World");

Output:

-> dlroW olleH <-

Date Formatting

DateTime date = new DateTime(2016, 07, 06, 18, 30, 14);
// Format: year, month, day hours, minutes, seconds

Console.Write(String.Format("{0:dd}",date)); 

//Format by Culture info
String.Format(new System.Globalization.CultureInfo("mn-MN"),"{0:dddd}",date);

Console.Write($"{date:ddd}");

output :

06
Лхагва
06

Specifier	Meaning	Sample	Result
d	Date	{0:d}	7/6/2016
dd	Day, zero-padded	{0:dd}	06
ddd	Short day name	{0:ddd}	Wed
dddd	Full day name	{0:dddd}	Wednesday
D	Long date	{0:D}	Wednesday, July 6, 2016
f	Full date and time, short	{0:f}	Wednesday, July 6, 2016 6:30 PM
ff	Second fractions, 2 digits	{0:ff}	20
fff	Second fractions, 3 digits	{0:fff}	201
ffff	Second fractions, 4 digits	{0:ffff}	2016
F	Full date and time, long	{0:F}	Wednesday, July 6, 2016 6:30:14 PM
g	Default date and time	{0:g}	7/6/2016 6:30 PM
gg	Era	{0:gg}	A.D
hh	Hour (2 digits, 12H)	{0:hh}	06
HH	Hour (2 digits, 24H)	{0:HH}	18
M	Month and day	{0:M}	July 6
mm	Minutes, zero-padded	{0:mm}	30
MM	Month, zero-padded	{0:MM}	07
MMM	3-letter month name	{0:MMM}	Jul
MMMM	Full month name	{0:MMMM}	July
ss	Seconds	{0:ss}	14
r	RFC1123 date	{0:r}	Wed, 06 Jul 2016 18:30:14 GMT
s	Sortable date string	{0:s}	2016-07-06T18:30:14
t	Short time	{0:t}	6:30 PM
T	Long time	{0:T}	6:30:14 PM
tt	AM/PM	{0:tt}	PM
u	Universal sortable local time	{0:u}	2016-07-06 18:30:14Z
U	Universal GMT	{0:U}	Wednesday, July 6, 2016 9:30:14 AM
Y	Month and year	{0:Y}	July 2016
yy	2 digit year	{0:yy}	16
yyyy	4 digit year	{0:yyyy}	2016
zz	2 digit timezone offset	{0:zz}	+09
zzz	full time zone offset	{0:zzz}	+09:00

Currency Formatting

The "c" (or currency) format specifier converts a number to a string that represents a currency amount.

string.Format("{0:c}", 112.236677) // $112.23 - defaults to system

Precision

Default is 2. Use c1, c2, c3 and so on to control precision.

string.Format("{0:C1}", 112.236677) //$112.2
string.Format("{0:C3}", 112.236677) //$112.237
string.Format("{0:C4}", 112.236677) //$112.2367
string.Format("{0:C9}", 112.236677) //$112.236677000

Currency Symbol

1. Pass CultureInfo instance to use custom culture symbol.

string.Format(new CultureInfo("en-US"), "{0:c}", 112.236677); //$112.24
string.Format(new CultureInfo("de-DE"), "{0:c}", 112.236677); //112,24 €
string.Format(new CultureInfo("hi-IN"), "{0:c}", 112.236677); //₹ 112.24

1. Use any string as currency symbol. Use NumberFormatInfo as to customize currency symbol.

NumberFormatInfo nfi = new CultureInfo( "en-US", false ).NumberFormat;
nfi = (NumberFormatInfo) nfi.Clone();
nfi.CurrencySymbol = "?";
string.Format(nfi, "{0:C}", 112.236677); //?112.24
nfi.CurrencySymbol = "?%^&";
string.Format(nfi, "{0:C}", 112.236677); //?%^&112.24

Position of Currency Symbol

Use CurrencyPositivePattern (opens new window) for positive values and CurrencyNegativePattern (opens new window) for negative values.

NumberFormatInfo nfi = new CultureInfo( "en-US", false ).NumberFormat;        
nfi.CurrencyPositivePattern = 0;
string.Format(nfi, "{0:C}", 112.236677); //$112.24 - default
nfi.CurrencyPositivePattern = 1;
string.Format(nfi, "{0:C}", 112.236677); //112.24$
nfi.CurrencyPositivePattern = 2;
string.Format(nfi, "{0:C}", 112.236677); //$ 112.24
nfi.CurrencyPositivePattern = 3; 
string.Format(nfi, "{0:C}", 112.236677); //112.24 $

Negative pattern usage is the same as positive pattern. A lot more use cases please refer to original link.

Custom Decimal Separator

NumberFormatInfo nfi = new CultureInfo( "en-US", false ).NumberFormat;        
nfi.CurrencyPositivePattern = 0;
nfi.CurrencyDecimalSeparator = "..";
string.Format(nfi, "{0:C}", 112.236677); //$112..24

Using custom number format

NumberFormatInfo can be used for formatting both integer and float numbers.

// invariantResult is "1,234,567.89"
var invarianResult = string.Format(CultureInfo.InvariantCulture, "{0:#,###,##}", 1234567.89);

// NumberFormatInfo is one of classes that implement IFormatProvider
var customProvider = new NumberFormatInfo
{
    NumberDecimalSeparator = "_NS_", // will be used instead of ','
    NumberGroupSeparator = "_GS_", // will be used instead of '.'
};

// customResult is "1_GS_234_GS_567_NS_89"
var customResult = string.Format(customProvider, "{0:#,###.##}", 1234567.89);

Align left/ right, pad with spaces

The second value in the curly braces dictates the length of the replacement string. By adjusting the second value to be positive or negative, the alignment of the string can be changed.

string.Format("LEFT:  string: ->{0,-5}<- int: ->{1,-5}<-", "abc", 123);
string.Format("RIGHT: string: ->{0,5}<- int: ->{1,5}<-", "abc", 123);

Output:

LEFT:  string: ->abc  <- int: ->123  <-
RIGHT: string: ->  abc<- int: ->  123<-

Numeric formats

// Integral types as hex
string.Format("Hexadecimal: byte2: {0:x2}; byte4: {0:X4}; char: {1:x2}", 123, (int)'A');

// Integers with thousand separators
string.Format("Integer, thousand sep.: {0:#,#}; fixed length: >{0,10:#,#}<", 1234567);

// Integer with leading zeroes
string.Format("Integer, leading zeroes: {0:00}; ", 1);

// Decimals
string.Format("Decimal, fixed precision: {0:0.000}; as percents: {0:0.00%}", 0.12);

Output:

Hexadecimal: byte2: 7b; byte4: 007B; char: 41
Integer, thousand sep.: 1,234,567; fixed length: > 1,234,567<
Integer, leading zeroes: 01; 
Decimal, fixed precision: 0.120; as percents: 12.00%

Escaping curly brackets inside a String.Format() expression

string outsidetext = "I am outside of bracket";
string.Format("{{I am in brackets!}} {0}", outsidetext);

//Outputs "{I am in brackets!} I am outside of bracket"

ToString()

The ToString() method is present on all reference object types. This is due to all reference types being derived from Object which has the ToString() method on it. The ToString() method on the object base class returns the type name. The fragment below will print out "User" to the console.

public class User
{
    public string Name { get; set; }
    public int Id { get; set; }
}

...

var user = new User {Name = "User1", Id = 5};
Console.WriteLine(user.ToString());

However, the class User can also override ToString() in order to alter the string it returns. The code fragment below prints out "Id: 5, Name: User1" to the console.

public class User
{
    public string Name { get; set; }
    public int Id { get; set; }
    public override ToString()
    {
        return string.Format("Id: {0}, Name: {1}", Id, Name);
    }
}

...

var user = new User {Name = "User1", Id = 5};
Console.WriteLine(user.ToString());

Relationship with ToString()

While the String.Format() method is certainly useful in formatting data as strings, it may often be a bit overkill, especially when dealing with a single object as seen below :

String.Format("{0:C}", money);  // yields "$42.00"

An easier approach might be to simply use the ToString() method available on all objects within C#. It supports all of the same standard and custom formatting strings (opens new window), but doesn't require the necessary parameter mapping as there will only be a single argument :

money.ToString("C");  // yields "$42.00"

Caveats & Formatting Restrictions

While this approach may be simpler in some scenarios, the ToString() approach is limited with regards to adding left or right padding like you might do within the String.Format() method :

String.Format("{0,10:C}", money);  // yields "    $42.00"

In order to accomplish this same behavior with the ToString() method, you would need to use another method like PadLeft() or PadRight() respectively :

money.ToString("C").PadLeft(10);  // yields "    $42.00"

Syntax

• string.Format(string format, params object[] args)
• string.Format(IFormatProvider provider, string format, params object[] args)
• $"string {text} blablabla" // Since C#6

Parameters

Parameter	Details
format	A composite format string (opens new window), which defines the way args should be combined into a string.
args	A sequence of objects to be combined into a string. Since this uses a params (opens new window) argument, you can either use a comma-separated list of arguments or an actual object array.
provider	A collection of ways of formatting objects to strings. Typical values include CultureInfo.InvariantCulture (opens new window) and CultureInfo.CurrentCulture (opens new window).

Remarks

Notes:

• String.Format() handles null arguments without throwing an exception.
• There are overloads that replace the args parameter with one, two, or three object parameters.