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IFormatProvider Interface

Updated: September 2009

Provides a mechanism for retrieving an object to control formatting.

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

<ComVisibleAttribute(True)> _
Public Interface IFormatProvider
Dim instance As IFormatProvider

The IFormatProvider interface supplies an object that provides formatting information for formatting and parsing operations. Formatting operations convert the value of a type to the string representation of that value. Typical formatting methods are the ToString methods of a type, as well as Format. Parsing operations convert the string representation of a value to a type with that value. Typical parsing methods are Parse and TryParse.

The IFormatProvider interface consists of a single method, IFormatProvider.GetFormat. GetFormat is a callback method: The parsing or formatting method calls it and passes it a Type object that represents the type of object that the formatting or parsing method expects will provide formatting information. The GetFormat method is responsible for returning an object of that type.

IFormatProvider implementations are often used implicitly by formatting and parsing methods. For example, the DateTime.ToString(String) method implicitly uses an IFormatProvider implementation that represents the system's current culture. IFormatProvider implementations can also be specified explicitly by methods that have a parameter of type IFormatProvider, such as Int32.Parse(String, IFormatProvider) and String.Format(IFormatProvider, String, Object()).

The .NET Framework includes the following three predefined IFormatProvider implementations to provide culture-specific information that is used in formatting or parsing numeric and date and time values:

  • The NumberFormatInfo class, which provides information that is used to format numbers, such as the currency, thousands separator, and decimal separator symbols for a particular culture.

  • The DateTimeFormatInfo class, which provides information that is used to format dates and times, such as the date and time separator symbols for a particular culture or the order and form of a date's year, month, and day components.

  • The CultureInfo class, which represents a particular culture. Its GetFormat method returns a culture-specific NumberFormatInfo or DateTimeFormatInfo object, depending on whether the CultureInfo object is used in a formatting or parsing operation that involves numbers or dates and times.

The .NET Framework also supports custom formatting. This typically involves the creation of a formatting class that implements both IFormatProvider and ICustomFormatter. An instance of this class is then passed as a parameter to a method that performs a custom formatting operation, such as String.Format(IFormatProvider, String, Object()) The example provides an illustration of such a custom implementation that formats a number as a 12-digit account number.

The following example illustrates how an IFormatProvider implementation can change the representation of a date and time value. In this case, a single date is displayed by using CultureInfo objects that represent four different cultures.

Imports System.Globalization

Module Example
   Public Sub Main()
      Dim dateValue As Date = #06/01/2009 4:37PM#
      Dim cultures() As CultureInfo = {New CultureInfo("en-US"), _
                                       New CultureInfo("fr-FR"), _
                                       New CultureInfo("it-IT"), _
                                       New CultureInfo("de-DE") }
      For Each culture As CultureInfo In cultures
         Console.WriteLine("{0}: {1}", culture.Name, dateValue.ToString(culture))
   End Sub 
End Module 
' The example displays the following output: 
'       en-US: 6/1/2009 4:37:00 PM 
'       fr-FR: 01/06/2009 16:37:00 
'       it-IT: 01/06/2009 16.37.00 
'       de-DE: 01.06.2009 16:37:00

The following example illustrates the use of a class that implements the IFormatProvider interface and the GetFormat method. The AcctNumberFormat class converts an Int64 value that represents an account number to a formatted 12-digit account number. Its GetFormat method returns a reference to the current AcctNumberFormat instance if the formatType parameter refers to a class that implements ICustomFormatter; otherwise, GetFormat returns Nothing.

Public Class AcctNumberFormat : Implements IFormatProvider, ICustomFormatter

   Private Const ACCT_LENGTH As Integer = 12

   Public Function GetFormat(formatType As Type) As Object _
                   Implements IFormatProvider.GetFormat
      If formatType Is GetType(ICustomFormatter) Then 
         Return Me 
         Return Nothing 
      End If 
   End Function 

   Public Function Format(fmt As String, arg As Object, formatProvider As IFormatProvider) As String _
                          Implements ICustomFormatter.Format

      ' Provide default formatting if arg is not an Int64. 
       If Not TypeOf arg Is Int64 Then 
            Return HandleOtherFormats(fmt, arg) 
         Catch e As FormatException 
            Throw New FormatException(String.Format("The format of '{0}' is invalid.", fmt), e)
         End Try 
       End If    

      ' Provider default formatting for unsupported format strings. 
      Dim ufmt As String = fmt.ToUpper(CultureInfo.InvariantCulture)
      If Not (ufmt = "H" Or ufmt = "I") Then 
            Return HandleOtherFormats(fmt, arg)
         Catch e As FormatException 
            Throw New FormatException(String.Format("The format of '{0}' is invalid.", fmt), e)
         End Try 
      End If    

      ' Convert argument to a string. 
      Dim result As String = arg.ToString()

      ' If account number is less than 12 characters, pad with leading zeroes. 
      If result.Length < ACCT_LENGTH Then result = result.PadLeft(ACCT_LENGTH, "0"c)
      ' If account number is more than 12 characters, truncate to 12 characters. 
      If result.Length > ACCT_LENGTH Then result = Left(result, ACCT_LENGTH)   

      If ufmt = "I"                              ' Integer-only format.
         Return result
      ' Add hyphens for H format specifier. 
      Else                                       ' Hypenated format.
         Return Left(result, 5) & "-" & Mid(result, 6, 3) & "-" & Right(result, 4)
      End If    
   End Function    

   Private Function HandleOtherFormats(fmt As String, arg As Object) As String 
      If TypeOf arg Is IFormattable Then 
         Return DirectCast(arg, IFormattable).ToString(fmt, CultureInfo.CurrentCulture)
      ElseIf arg IsNot Nothing Then 
         Return arg.ToString()
         Return String.Empty
      End If 
   End Function 
End Class

The class that implements IFormatProvider can then be used in a call to a formatting and parsing operation. For example, the following code calls the String.Format(IFormatProvider, String, Object()) method to generate a string that contains a formatted 12-digit account number.

Imports System.Globalization

Public Enum DaysOfWeek As Long
   Monday = 1
   Tuesday = 2
End Enum 

Module TestFormatting
   Public Sub Main()
      Dim acctNumber As Long, balance As Double  
      Dim wday As DaysOfWeek 
      Dim output As String

      acctNumber = 104254567890
      balance = 16.34
      wday = DaysOfWeek.Monday

      output = String.Format(New AcctNumberFormat(), "On {2}, the balance of account {0:H} was {1:C2}.", acctNumber, balance, wday)

      wday = DaysOfWeek.Tuesday
      output = String.Format(New AcctNumberFormat(), "On {2}, the balance of account {0:I} was {1:C2}.", acctNumber, balance, wday)
   End Sub 
End Module 
' The example displays the following output: 
'    On Monday, the balance of account 10425-456-7890 was $16.34. 
'    On Tuesday, the balance of account 104254567890 was $16.34.

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.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0




September 2009

Modified the example of an IFormatProvider implementation.

Customer feedback.

February 2009

Expanded the Remarks section and added another example.

Customer feedback.