IFormattable Interface

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

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

'Declaration
<ComVisibleAttribute(True)> _
Public Interface IFormattable

The IFormattable type exposes the following members.

  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsToStringFormats the value of the current instance using the specified format.
Top

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:

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:

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 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 Nothing. For more information about formatting and formatting codes, see Formatting Types in the .NET Framework

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 Nothing or empty. The other two ToString methods defined by the Temperature class simply wrap a call to the IFormattable.ToString implementation.

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

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

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)

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

Was this page helpful?
(1500 characters remaining)
Thank you for your feedback
Show:
© 2014 Microsoft