IFormattable Interface (System)

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Microsoft Visual Studio 2008/.NET Framework 3.5

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.NET Framework Class Library

IFormattable Interface

Updated: April 2009

Provides functionality to format the value of an object into a string representation.

Namespace: System
Assembly: mscorlib (in mscorlib.dll)

Syntax

Visual Basic (Declaration)

<ComVisibleAttribute(True)> _
Public Interface IFormattable

Visual Basic (Usage)

Dim instance As IFormattable

[ComVisibleAttribute(true)]
public interface IFormattable

Visual C++

[ComVisibleAttribute(true)]
public interface class IFormattable

JScript

public interface IFormattable

Remarks

The IFormattable interface converts an object to its string representation based on a format string and a format provider.

A format string typically defines the general appearance of an object. For example, the .NET Framework supports the following:

Standard format strings for formatting enumeration values (see Enumeration Format Strings).
Standard and custom format strings for formatting numeric values (see Standard Numeric Format Strings and Custom Numeric Format Strings).
Standard and custom format string for formatting date and time values (see Standard Date and Time Format Strings and Custom Date and Time Format Strings). You can also define your own format strings to support formatting of your application-defined types.

A format provider returns a formatting object that typically defines the symbols used in converting an object to its string representation. For example, when you convert a number to a currency value, a format provider defines the currency symbol that appears in the result string. The .NET Framework defines three format providers:

The System.Globalization..::.CultureInfo class, which returns either a NumberFormatInfo object for formatting numeric values, or a DateTimeFormatInfo object for formatting date and time values.
The System.Globalization..::.NumberFormatInfo class, which returns an instance of itself for formatting numeric values.
The System.Globalization..::.DateTimeFormatInfo class, which returns an instance of itself for formatting date and time values.

In addition, you can define your own custom format providers to supply culture-specific, profession-specific, or industry-specific information used in formatting. For more information about implementing custom formatting by using a custom format provider, see ICustomFormatter.

The IFormattable interface defines a single method, ToString, that supplies formatting services for the implementing type. The ToString method can be called directly. In addition, it is called automatically by the Convert..::.ToString(Object) and Convert..::.ToString(Object, IFormatProvider) methods, and by methods that use the composite formatting feature in the .NET Framework. Such methods include Console..::.WriteLine(String, Object), String..::.Format, and StringBuilder..::.AppendFormat(String, Object), among others. The ToString method is called for each format item in the method's format string.

[The T:System.IFormattable] interface is implemented by the base data types.

Notes to Implementers:

Classes that require more control over the formatting of strings than Object..::.ToString provides should implement IFormattable.

A class that implements IFormattable must support the "G" (general) format specifier. Besides the "G" specifier, the class can define the list of format specifiers that it supports. In addition, the class must be prepared to handle a format specifier that is nullNothingnullptra null reference (Nothing in Visual Basic). For more information about formatting and formatting codes, see Formatting Overview

Examples

The following example defines a Temperature class that implements the IFormattable interface. The class supports four format specifiers: "G" and "C", which indicate that the temperature is to be displayed in Celsius; "F", which indicates that the temperature is to be displayed in Fahrenheit; and "K", which indicates that the temperature is to be displayed in Kelvin. In addition, the IFormattable..::.ToString implementation also can handle a format string that is nullNothingnullptra null reference (Nothing in Visual Basic) or empty. The other two ToString methods defined by the Temperature class simply wrap a call to the IFormattable..::.ToString implementation.

Visual Basic

Imports System.Globalization

Public Class Temperature : Implements IFormattable
   Private temp As Decimal

   Public Sub New(temperature As Decimal)
      If temperature < -273.15 Then _ 
        Throw New ArgumentOutOfRangeException(String.Format("{0} is less than absolute zero.", _
                                              temperature))
      Me.temp = temperature
   End Sub

   Public ReadOnly Property Celsius As Decimal
      Get
         Return temp
      End Get
   End Property

   Public ReadOnly Property Fahrenheit As Decimal
      Get
         Return temp * 9 / 5 + 32
      End Get
   End Property

   Public ReadOnly Property Kelvin As Decimal
      Get
         Return temp + 273.15d
      End Get
   End Property

   Public Overrides Function ToString() As String
      Return Me.ToString("G", CultureInfo.CurrentCulture)
   End Function

   Public Overloads Function ToString(fmt As String) As String
      Return Me.ToString(fmt, CultureInfo.CurrentCulture)
   End Function

   Public Overloads Function ToString(fmt As String, provider As IFormatProvider) _
                   As String _
                   Implements IFormattable.ToString
      If String.IsNullOrEmpty(fmt) Then fmt = "G"
      If provider Is Nothing Then provider = CultureInfo.CurrentCulture

      Select Case fmt.ToUpperInvariant()
         Case "G", "C"
            Return temp.ToString("F2", provider) + " °C" 
         Case "F"
            Return Fahrenheit.ToString("F2", provider) + " °F"
         Case "K"
            Return Kelvin.ToString("F2", provider) + " °K"
         Case Else
            Throw New FormatException(String.Format("The {0} format string is not supported.", fmt))
      End Select
   End Function
End Class

using System;
using System.Globalization;

public class Temperature : IFormattable
{
   private decimal temp;

   public Temperature(decimal temperature)
   {
      if (temperature < -273.15m) 
        throw new ArgumentOutOfRangeException(String.Format("{0} is less than absolute zero.", 
                                              temperature));
      this.temp = temperature;
   }

   public decimal Celsius
   {
      get { return temp; }
   }

   public decimal Fahrenheit
   {
      get { return temp * 9 / 5 + 32; }
   }

   public decimal Kelvin
   {
      get { return temp + 273.15m; }
   }

   public override string ToString()
   {
      return this.ToString("G", CultureInfo.CurrentCulture);
   }

   public string ToString(string format)
   {
      return this.ToString(format, CultureInfo.CurrentCulture);
   }

   public string ToString(string format, IFormatProvider provider) 
   {
      if (String.IsNullOrEmpty(format)) format = "G";
      if (provider == null) provider = CultureInfo.CurrentCulture;

      switch (format.ToUpperInvariant())
      {
         case "G":
         case "C":
            return temp.ToString("F2", provider) + " °C"; 
         case "F":
            return Fahrenheit.ToString("F2", provider) + " °F";
         case "K":
            return Kelvin.ToString("F2", provider) + " °K";
         default:
            throw new FormatException(String.Format("The {0} formst string is not supported.", format));
      }
   }
}

The following example then calls the IFormattable..::.ToString implementation either directly or by using a composite format string.

Visual Basic

Module Example
   Public Sub Main()
      ' Use composite formatting with format string in the format item.
      Dim temp1 As New Temperature(0)
      Console.WriteLine("{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1)
      Console.WriteLine()

      ' Use composite formatting with a format provider.
      temp1 = New Temperature(-40)
      Console.WriteLine(String.Format(CultureInfo.CurrentCulture, "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1))
      Console.WriteLine(String.Format(New CultureInfo("fr-FR"), "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1))
      Console.WriteLine()

      ' Call ToString method with format string.
      temp1 = New Temperature(32)
      Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)", _
                        temp1.ToString("C"), temp1.ToString("K"), temp1.ToString("F"))
      Console.WriteLine()

      ' Call ToString with format string and format provider
      temp1 = New Temperature(100)      
      Dim current As NumberFormatInfo = NumberFormatInfo.CurrentInfo
      Dim nl As New CultureInfo("nl-NL") 
      Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)", _
                        temp1.ToString("C", current), temp1.ToString("K", current), temp1.ToString("F", current))
      Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)", _
                        temp1.ToString("C", nl), temp1.ToString("K", nl), temp1.ToString("F", nl))
   End Sub
End Module
' The example displays the following output:
'    0.00 °C (Celsius) = 273.15 °K (Kelvin) = 32.00 °F (Fahrenheit)
'    
'    -40.00 °C (Celsius) = 233.15 °K (Kelvin) = -40.00 °F (Fahrenheit)
'    -40,00 °C (Celsius) = 233,15 °K (Kelvin) = -40,00 °F (Fahrenheit)
'    
'    32.00 °C (Celsius) = 305.15 °K (Kelvin) = 89.60 °F (Fahrenheit)
'    
'    100.00 °C (Celsius) = 373.15 °K (Kelvin) = 212.00 °F (Fahrenheit)
'    100,00 °C (Celsius) = 373,15 °K (Kelvin) = 212,00 °F (Fahrenheit)

public class Example
{
   public static void Main()
   {
      // Use composite formatting with format string in the format item.
      Temperature temp1 = new Temperature(0);
      Console.WriteLine("{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)\n", temp1);

      // Use composite formatting with a format provider.
      temp1 = new Temperature(-40);
      Console.WriteLine(String.Format(CultureInfo.CurrentCulture, "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1));
      Console.WriteLine(String.Format(new CultureInfo("fr-FR"), "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)\n", temp1));

      // Call ToString method with format string.
      temp1 = new Temperature(32);
      Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)\n", 
                        temp1.ToString("C"), temp1.ToString("K"), temp1.ToString("F"));

      // Call ToString with format string and format provider
      temp1 = new Temperature(100)      ;
      NumberFormatInfo current = NumberFormatInfo.CurrentInfo;
      CultureInfo nl = new CultureInfo("nl-NL"); 
      Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)", 
                        temp1.ToString("C", current), temp1.ToString("K", current), temp1.ToString("F", current));
      Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)", 
                        temp1.ToString("C", nl), temp1.ToString("K", nl), temp1.ToString("F", nl));
   }
}
// The example displays the following output:
//    0.00 °C (Celsius) = 273.15 °K (Kelvin) = 32.00 °F (Fahrenheit)
//    
//    -40.00 °C (Celsius) = 233.15 °K (Kelvin) = -40.00 °F (Fahrenheit)
//    -40,00 °C (Celsius) = 233,15 °K (Kelvin) = -40,00 °F (Fahrenheit)
//    
//    32.00 °C (Celsius) = 305.15 °K (Kelvin) = 89.60 °F (Fahrenheit)
//    
//    100.00 °C (Celsius) = 373.15 °K (Kelvin) = 212.00 °F (Fahrenheit)
//    100,00 °C (Celsius) = 373,15 °K (Kelvin) = 212,00 °F (Fahrenheit)

Platforms

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Version Information