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ICustomFormatter.Format Method

Updated: February 2009

Converts the value of a specified object to an equivalent string representation using specified format and culture-specific formatting information.

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

Function Format ( _
	format As String, _
	arg As Object, _
	formatProvider As IFormatProvider _
) As String
Dim instance As ICustomFormatter 
Dim format As String 
Dim arg As Object 
Dim formatProvider As IFormatProvider 
Dim returnValue As String 

returnValue = instance.Format(format, _
	arg, formatProvider)


Type: System.String

A format string containing formatting specifications.

Type: System.Object

An object to format.

Type: System.IFormatProvider

An object that supplies format information about the current instance.

Return Value

Type: System.String
The string representation of the value of arg, formatted as specified by format and formatProvider.

ICustomFormatter.Format is a callback method. It is called by a method that supports custom formatting, such as String.Format(IFormatProvider, String, Object()). The implementation is called once for each format item in a composite format string. For example, in the following statement, the ICustomFormatter.Format method is called three times.

Console.WriteLine(String.Format(New BinaryFormatter(), _
                                "{0} (binary: {0:B}) (hex: {0:H})", byteValue))

The arg parameter is the object in the object list whose zero-based position corresponds to the index of a particular format item.

The format parameter contains a format string, which is the formatString component of a format item. If the format item has no formatString component, the value of format is Nothing. If format is Nothing, depending on the type of arg, you may be able to use the default format specification of your choice.

The formatProvider parameter is the IFormatProvider implementation that provides formatting for arg. Typically, it is an instance of your ICustomFormatter implementation. If formatProvider is Nothing, ignore that parameter.

Your implementation of the Format method must include the following functionality so the .NET Framework can provide formatting you do not support. If your format method does not support a format, determine whether the object being formatted implements the IFormattable interface. If it does, invoke the IFormattable.ToString method of that interface. Otherwise, invoke the default Object.ToString method of the underlying object. The following code illustrates this pattern.

If TypeOf arg Is IFormattable Then 
   Return DirectCast(arg, IFormattable).ToString(fmt, CultureInfo.CurrentCulture)
ElseIf arg IsNot Nothing Then 
   Return arg.ToString()

The following example implements ICustomFormatter to allow binary, octal, and hexadecimal formatting of integral values. Its ICustomFormatter.Format implementation determines whether the format parameter is one of the three supported format strings ("B" for binary, "O" for octal, and "H" for hexadecimal) and formats the arg parameter appropriately. Otherwise, if arg is not Nothing, it calls the arg parameter's IFormattable.ToString implementation, if one exists, or its parameterless ToString method, if one does not. If arg is Nothing, the method returns String.Empty.

Imports System.Globalization

Public Class BinaryFormatter : Implements IFormatProvider, ICustomFormatter
   ' IFormatProvider.GetFormat implementation. 
   Public Function GetFormat(formatType As Type) As Object _
                   Implements IFormatProvider.GetFormat
      ' Determine whether custom formatting object is requested. 
      If formatType Is GetType(ICustomFormatter) Then 
         Return Me 
         Return Nothing 
      End If 
   End Function    

   ' Format number in binary (B), octal (O), or hexadecimal (H). 
   Public Function Format(fmt As String, arg As Object, _
                          formatProvider As IFormatProvider) As String _
                   Implements ICustomFormatter.Format

     ' Handle format string. 
      Dim base As Integer 
      ' Handle null or empty format string, string with precision specifier. 
      Dim thisFmt As String = String.Empty
      ' Extract first character of format string (precision specifiers 
      ' are not supported by BinaryFormatter). 
      If Not String.IsNullOrEmpty(fmt) Then
         thisFmt = CStr(IIf(fmt.Length > 1, fmt.Substring(0, 1), fmt))
      End If 

      ' Get a byte array representing the numeric value. 
      Dim bytes() As Byte
      If TypeOf(arg) Is SByte Then 
         Dim byteString As String = CType(arg, SByte).ToString("X2")
         bytes = New Byte(0) { Byte.Parse(byteString, System.Globalization.NumberStyles.HexNumber ) }
      ElseIf TypeOf(arg) Is Byte Then
         bytes = New Byte(0) { CType(arg, Byte) }
      ElseIf TypeOf(arg) Is Int16 Then
         bytes = BitConverter.GetBytes(CType(arg, Int16))
      ElseIf TypeOf(arg) Is Int32 Then
         bytes = BitConverter.GetBytes(CType(arg, Int32))
      ElseIf TypeOf(arg) Is Int64 Then
         bytes = BitConverter.GetBytes(CType(arg, Int64))
      ElseIf TypeOf(arg) Is UInt16 Then
         bytes = BitConverter.GetBytes(CType(arg, UInt16))
      ElseIf TypeOf(arg) Is UInt32 Then
         bytes = BitConverter.GetBytes(CType(arg, UInt64))
      ElseIf TypeOf(arg) Is UInt64 Then
         bytes = BitConverter.GetBytes(CType(arg, UInt64))                  
            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 

      Select Case thisFmt.ToUpper()
         ' Binary formatting. 
         Case "B"
            base = 2        
         Case "O"
            base = 8
         Case "H"
            base = 16
         ' Handle unsupported format strings. 
         Case Else 
               Return HandleOtherFormats(fmt, arg) 
            Catch e As FormatException 
               Throw New FormatException(String.Format("The format of '{0}' is invalid.", fmt), e)
            End Try 
      End Select 

      ' Return a formatted string. 
      Dim numericString As String = String.Empty
      For ctr As Integer = bytes.GetUpperBound(0) To bytes.GetLowerBound(0) Step -1
         Dim byteString As String = Convert.ToString(bytes(ctr), base)
         If base = 2 Then
            byteString = New String("0"c, 8 - byteString.Length) + byteString
         ElseIf base = 8 Then
            byteString = New String("0"c, 4 - byteString.Length) + byteString
         ' Base is 16. 
            byteString = New String("0"c, 2 - byteString.Length) + byteString
         End If
         numericString +=  byteString + " " 
      Return numericString.Trim()
   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

BinaryFormatter can then be used to provide custom formatting by passing a BinaryFormatter object as the provider parameter of the Format method, as the following example shows.

Public Module Example
   Public Sub Main
      Console.WindowWidth = 100

      Dim byteValue As Byte = 124
      Console.WriteLine(String.Format(New BinaryFormatter(), _
                                      "{0} (binary: {0:B}) (hex: {0:H})", byteValue))

      Dim intValue As Integer = 23045
      Console.WriteLine(String.Format(New BinaryFormatter(), _
                                      "{0} (binary: {0:B}) (hex: {0:H})", intValue))

      Dim ulngValue As ULong = 31906574882
      Console.WriteLine(String.Format(New BinaryFormatter(), _
                                      "{0} {1}   (binary: {0:B}) {1}   (hex: {0:H})", _
                                      ulngValue, vbCrLf))
   End Sub 
End Module 
' The example displays the following output: 
'    124 (binary: 01111100) (hex: 7c) 
'    23045 (binary: 00000000 00000000 01011010 00000101) (hex: 00 00 5a 05) 
'    31906574882 
'       (binary: 00000000 00000000 00000000 00000111 01101101 11000111 10110010 00100010) 
'       (hex: 00 00 00 07 6d c7 b2 22)

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




February 2009

Expanded the Remarks section and added an example.

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