Things You Can Do With String.Format In C#

Formatting output string with String.Format is really helpful, and it’s been mostly use when you develop application using C# or VB.Net. String.Format firstly released in .NET Framework 1.1. String.Format is for replaces each format item in a specified String with the text equivalent of a corresponding object’s value.

String.Format overload function:

public static string Format(
	string format,
	Object arg0
)

public static string Format(
	string format,
	params Object[] args
)

public static string Format(
	IFormatProvider provider,
	string format,
	params Object[] args
)

Basic use:

String.Format(“Hello {0}”, “World”); //Produce: "Hello World"

As you see the argument that inside the curly braces {0}, is the string format that will replace by the arg0. Which have this following format:
{index[,alignment][:formatString]}
where:
index: index of the arguments given
alignment: If alignment is positive, the text is right-aligned in a field the given number of spaces; if it’s negative, it’s left-aligned
formatString: string format specifiers.

Another example:

DateTime dt = new DateTime(2010, 10, 15, 12, 47, 5, 0);
String.Format("{0:dd MM yyyy}", dt);  // "15 Oct 2010"

Format specifiers:
1. Numbers

2. Date and Time

More Examples

Formatting Integer
Add zero before number:

String.Format("{0:000000}", 200);          // "000200"
String.Format("{0:000000}", -200);         // "-000200"

Align number to the right or left:

String.Format("{0,6:0000}", 22);          // "  0022"
String.Format("{0,-6:0000}", 22);         // "0022  "

Custom number formatting for phone number:

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

Formatting Double
Digits after decimal point:

String.Format("{0:0.00}", 123.4567);      // "123.46"
String.Format("{0:0.00}", 123.4);         // "123.40"
String.Format("{0:0.##}", 123.4567);      // "123.46"
String.Format("{0:0.##}", 123.4);         // "123.4"

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"

Thousands separator:

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

Custom formatting for negative numbers and zero

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"

Formatting Datetime

// create date time 2008-03-09 16:05:07.123
DateTime dt = new DateTime(2008, 3, 9, 16, 5, 7, 123);
// 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)
String.Format("{0:MM/dd/yyyy}", dt);          // "03/09/2008"
String.Format("{0:dddd, MMMM d, yyyy}", dt);  // "Sunday, March 9, 2008"
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

Formatting With IFormatProvider

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

More details about String.Format on MSDN
Code source C# Examples