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Composite Formatting


The .NET Framework composite formatting feature takes a list of objects and a composite format string as input. A composite format string consists of fixed text intermixed with indexed placeholders, called format items, that correspond to the objects in the list. The formatting operation yields a result string that consists of the original fixed text intermixed with the string representation of the objects in the list.

The composite formatting feature is supported by methods such as the following:

A composite format string and object list are used as arguments of methods that support the composite formatting feature. A composite format string consists of zero or more runs of fixed text intermixed with one or more format items. The fixed text is any string that you choose, and each format item corresponds to an object or boxed structure in the list. The composite formatting feature returns a new result string where each format item is replaced by the string representation of the corresponding object in the list.

Consider the following Format code fragment.

      string name = "Fred";
      String.Format("Name = {0}, hours = {1:hh}", name, DateTime.Now);

The fixed text is "Name = " and ", hours = ". The format items are "{0}", whose index is 0, which corresponds to the object name, and "{1:hh}", whose index is 1, which corresponds to the object DateTime.Now.

Each format item takes the following form and consists of the following components:

{ index[,alignment][:formatString]}

The matching braces ("{" and "}") are required.

Index Component

The mandatory index component, also called a parameter specifier, is a number starting from 0 that identifies a corresponding item in the list of objects. That is, the format item whose parameter specifier is 0 formats the first object in the list, the format item whose parameter specifier is 1 formats the second object in the list, and so on. The following example includes four parameter specifiers, numbered zero through three, to represent prime numbers less than ten:

   string primes;
   primes = String.Format("Prime numbers less than 10: {0}, {1}, {2}, {3}",
                          2, 3, 5, 7 );
   // The example displays the following output:
   //      Prime numbers less than 10: 2, 3, 5, 7

Multiple format items can refer to the same element in the list of objects by specifying the same parameter specifier. For example, you can format the same numeric value in hexadecimal, scientific, and number format by specifying a composite format string such as : "0x{0:X} {0:E} {0:N}", as the following example shows.

   string multiple = String.Format("0x{0:X} {0:E} {0:N}",
   // The example displays the following output:
   //      0x7FFFFFFFFFFFFFFF 9.223372E+018 9,223,372,036,854,775,807.00

Each format item can refer to any object in the list. For example, if there are three objects, you can format the second, first, and third object by specifying a composite format string like this: "{1} {0} {2}". An object that is not referenced by a format item is ignored. A FormatException is thrown at runtime if a parameter specifier designates an item outside the bounds of the list of objects.

Alignment Component

The optional alignment component is a signed integer indicating the preferred formatted field width. If the value of alignment is less than the length of the formatted string, alignment is ignored and the length of the formatted string is used as the field width. The formatted data in the field is right-aligned if alignment is positive and left-aligned if alignment is negative. If padding is necessary, white space is used. The comma is required if alignment is specified.

The following example defines two arrays, one containing the names of employees and the other containing the hours they worked over a two-week period. The composite format string left-aligns the names in a 20-character field, and right-aligns their hours in a 5-character field. Note that the "N1" standard format string is also used to format the hours with one fractional digit.

using System;

public class Example
   public static void Main()
      string[] names = { "Adam", "Bridgette", "Carla", "Daniel",
                         "Ebenezer", "Francine", "George" };
      decimal[] hours = { 40, 6.667m, 40.39m, 82, 40.333m, 80,
                                 16.75m };

      Console.WriteLine("{0,-20} {1,5}\n", "Name", "Hours");
      for (int ctr = 0; ctr < names.Length; ctr++)
         Console.WriteLine("{0,-20} {1,5:N1}", names[ctr], hours[ctr]);

// The example displays the following output:
//       Name                 Hours
//       Adam                  40.0
//       Bridgette              6.7
//       Carla                 40.4
//       Daniel                82.0
//       Ebenezer              40.3
//       Francine              80.0
//       George                16.8

Format String Component

The optional formatString component is a format string that is appropriate for the type of object being formatted. Specify a standard or custom numeric format string if the corresponding object is a numeric value, a standard or custom date and time format string if the corresponding object is a DateTime object, or an enumeration format string if the corresponding object is an enumeration value. If formatString is not specified, the general ("G") format specifier for a numeric, date and time, or enumeration type is used. The colon is required if formatString is specified.

The following table lists types or categories of types in the .NET Framework class library that support a predefined set of format strings, and provides links to the topics that list the supported format strings. Note that string formatting is an extensible mechanism that makes it possible to define new format strings for all existing types as well as to define a set of format strings supported by an application-defined type. For more information, see the IFormattable and ICustomFormatter interface topics.

Type or type categorySee
Date and time types (DateTime, DateTimeOffset)Standard Date and Time Format Strings

 Custom Date and Time Format Strings
Enumeration types (all types derived from System.Enum)Enumeration Format Strings
Numeric types (BigInteger, Byte, Decimal, Double, Int16, Int32, Int64, SByte, Single, UInt16, UInt32, UInt64)Standard Numeric Format Strings

 Custom Numeric Format Strings
TimeSpanStandard TimeSpan Format Strings

 Custom TimeSpan Format Strings

Escaping Braces

Opening and closing braces are interpreted as starting and ending a format item. Consequently, you must use an escape sequence to display a literal opening brace or closing brace. Specify two opening braces ("{{") in the fixed text to display one opening brace ("{"), or two closing braces ("}}") to display one closing brace ("}"). Braces in a format item are interpreted sequentially in the order they are encountered. Interpreting nested braces is not supported.

The way escaped braces are interpreted can lead to unexpected results. For example, consider the format item "{{{0:D}}}", which is intended to display an opening brace, a numeric value formatted as a decimal number, and a closing brace. However, the format item is actually interpreted in the following manner:

  1. The first two opening braces ("{{") are escaped and yield one opening brace.

  2. The next three characters ("{0:") are interpreted as the start of a format item.

  3. The next character ("D") would be interpreted as the Decimal standard numeric format specifier, but the next two escaped braces ("}}") yield a single brace. Because the resulting string ("D}") is not a standard numeric format specifier, the resulting string is interpreted as a custom format string that means display the literal string "D}".

  4. The last brace ("}") is interpreted as the end of the format item.

  5. The final result that is displayed is the literal string, "{D}". The numeric value that was to be formatted is not displayed.

One way to write your code to avoid misinterpreting escaped braces and format items is to format the braces and format item separately. That is, in the first format operation display a literal opening brace, in the next operation display the result of the format item, then in the final operation display a literal closing brace. The following example illustrates this approach.

      int value = 6324;
      string output = string.Format("{0}{1:D}{2}", 
                                   "{", value, "}");
      // The example displays the following output:
      //       {6324}                            

Processing Order

If the call to the composite formatting method includes an IFormatProvider argument whose value is not null, the runtime calls its IFormatProvider.GetFormat method to request an ICustomFormatter implementation. If the method is able to return an ICustomFormatter implementation, it is cached for later use.

Each value in the parameter list that corresponds to a format item is converted to a string by performing the following steps. If any condition in the first three steps is true, the string representation of the value is returned in that step, and subsequent steps are not executed.

  1. If the value to be formatted is null, an empty string ("") is returned.

  2. If an ICustomFormatter implementation is available, the runtime calls its Format method. It passes the method the format item's formatString value, if one is present, or null if it is not, along with the IFormatProvider implementation.

  3. If the value implements the IFormattable interface, the interface's ToString(String, IFormatProvider) method is called. The method is passed the formatString value, if one is present in the format item, or null if it is not. The IFormatProvider argument is determined as follows:

  4. The type's parameterless ToString method, which either overrides Object.ToString() or inherits the behavior of its base class, is called. In this case, the format string specified by the formatString component in the format item, if it is present, is ignored.

Alignment is applied after the preceding steps have been performed.

The following example shows one string created using composite formatting and another created using an object's ToString method. Both types of formatting produce equivalent results.

      string FormatString1 = String.Format("{0:dddd MMMM}", DateTime.Now);
      string FormatString2 = DateTime.Now.ToString("dddd MMMM");

Assuming that the current day is a Thursday in May, the value of both strings in the preceding example is Thursday May in the U.S. English culture.

Console.WriteLine exposes the same functionality as String.Format. The only difference between the two methods is that String.Format returns its result as a string, while Console.WriteLine writes the result to the output stream associated with the Console object. The following example uses the Console.WriteLine method to format the value of MyInt to a currency value.

      int MyInt = 100;
      Console.WriteLine("{0:C}", MyInt);
      // The example displays the following output 
      // if en-US is the current culture:
      //        $100.00

The following example demonstrates formatting multiple objects, including formatting one object two different ways.

      string myName = "Fred";
      Console.WriteLine(String.Format("Name = {0}, hours = {1:hh}, minutes = {1:mm}",
            myName, DateTime.Now));
      // Depending on the current time, the example displays output like the following:
      //    Name = Fred, hours = 11, minutes = 30                 

The following example demonstrates the use of alignment in formatting. The arguments that are formatted are placed between vertical bar characters (|) to highlight the resulting alignment.

      string myFName = "Fred";
      string myLName = "Opals";
      int myInt = 100;
      string FormatFName = String.Format("First Name = |{0,10}|", myFName);
      string FormatLName = String.Format("Last Name = |{0,10}|", myLName);
      string FormatPrice = String.Format("Price = |{0,10:C}|", myInt); 
      FormatFName = String.Format("First Name = |{0,-10}|", myFName);
      FormatLName = String.Format("Last Name = |{0,-10}|", myLName);
      FormatPrice = String.Format("Price = |{0,-10:C}|", myInt);
      // The example displays the following output on a system whose current
      // culture is en-US:
      //          First Name = |      Fred|
      //          Last Name = |     Opals|
      //          Price = |   $100.00|
      //          First Name = |Fred      |
      //          Last Name = |Opals     |
      //          Price = |$100.00   |

Formatting Types
Standard Numeric Format Strings
Custom Numeric Format Strings
Standard Date and Time Format Strings
Custom Date and Time Format Strings
Standard TimeSpan Format Strings
Custom TimeSpan Format Strings
Enumeration Format Strings