ICustomFormatter Interface
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Defines a method that supports custom, user-defined formatting of the value of an object.
Namespace: System
Assembly: mscorlib (in mscorlib.dll)
Syntax
'Declaration
<ComVisibleAttribute(True)> _
Public Interface ICustomFormatter
[ComVisibleAttribute(true)]
public interface ICustomFormatter
The ICustomFormatter type exposes the following members.
Methods
| Name | Description | |
|---|---|---|
.gif "Public method") .gif "Supported by Silverlight for Windows Phone") .gif "Supported by Xbox 360") |
Format | Converts the value of a specified object to an equivalent string representation using specified format and culture-specific formatting information. |
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Remarks
The ICustomFormatter interface includes a single method, ICustomFormatter.Format. When this interface is implemented by a reference or value type, the Format method returns a custom-formatted string representation of an object's value.
Typically, the ICustomFormatter interface is implemented with the IFormatProvider interface to customize the behavior of .NET Framework string formatting methods that include an IFormatProvider parameter. For example, the ICustomFormatter interface can provide custom formatting of the value of an object passed to the String.Format(IFormatProvider, String, array<Object[]) method.
Providing a custom representation of an object's value requires that you do the following:
Define a class that implements the ICustomFormatter interface and its single member, the Format method.
Define a class that implements the IFormatProvider interface and its single member, the GetFormat method. The GetFormat method returns an instance of your ICustomFormatter implementation. Often, a single class implements both ICustomFormatter and IFormatProvider. In that case, the class's GetFormat implementation just returns an instance of itself.
Pass the IFormatProvider implementation as the provider argument of the String.Format(IFormatProvider, String, array<Object[]) method.
The .NET Framework method will then use your custom formatting instead of its own.
Notes to Implementers
The common language runtime attempts to use your ICustomFormatter implementation for every format item in a composite format string. As a result, you should expect that your ICustomFormatter implementation will be called to provide formatting services to objects or values that it is not designed to handle. In these cases, your Format method must call the appropriate formatting method for that object or value.
There are two kinds of ICustomFormatter implementations: intrinsic and extension.
Intrinsic implementations are implementations that provide custom formatting for an application-defined object. In this case, your implementation should include the following:
A definition of format strings that define the formatting of the object. Format strings are optional. Typically, a "G" or "g" format string defines the general (or most commonly used) format. However, you are free to define any format strings that you choose. You are also free to decide whether they are case-sensitive or case-insensitive.
A test to ensure that the type of the object passed to your Format method is your application-defined type. If it is not, you should call the object's IFormattable implementation, if one exists, or its ToString method, if it does not. You should be prepared to handle any exceptions these method calls might throw.
Code to handle a null format string, if your implementation supports format strings. The most common approach is to replace a null format string with the general format specifier.
Code to handle any format strings that your implementation supports.
Code to handle format strings that you do not support. The most common approach is to throw a FormatException, although you can provide default formatting.
Extension implementations are implementations that provide custom formatting for a type that already has formatting support. For example, you could define a CustomerNumberFormatter that formats an integral type with hyphens between specific digits. In this case, your implementation should include the following:
A definition of format strings that extend the formatting of the object. These format strings are required, but they must not conflict with the type's existing format strings. For example, if you are extending formatting for the Int32 type, you should not implement the "C", "D", "E", "F", and "G" format specifiers, among others.
A test that the type of the object passed to your Format method is a type whose formatting your extension supports. If it is not, call the object's IFormattable implementation, if one exists, or the object's parameterless ToString method, if it does not. You should be prepared to handle any exceptions these method calls might throw.
Code to handle any format strings that your extension supports.
Code to handle any format strings that your extension does not support. These should be passed on to the type's IFormattable implementation. You should be prepared to handle any exceptions these method calls might throw.
Examples
The following example implements ICustomFormatter to allow binary, octal, and hexadecimal formatting of integral values. In this example, a single class, IBinaryFormatter, implements both ICustomFormatter and IFormatProvider. Its IFormatProvider.GetFormat method determines whether the formatType parameter represents an ICustomFormatter type. If it does, BinaryFormatter returns an instance of itself; otherwise, it returns nulla null reference (Nothing in Visual Basic). 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 nulla null reference (Nothing in Visual Basic), it calls the arg parameter's IFormattable.ToString implementation, if one exists, or its parameterless ToString method, if one does not. If arg is nulla null reference (Nothing in Visual Basic), 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
Else
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))
Else
Try
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
Try
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.
Else
byteString = New String("0"c, 2 - byteString.Length) + byteString
End If
numericString += byteString + " "
Next
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()
Else
Return String.Empty
End If
End Function
End Class
using System;
using System.Globalization;
public class BinaryFormatter : IFormatProvider, ICustomFormatter
{
// IFormatProvider.GetFormat implementation.
public object GetFormat(Type formatType)
{
// Determine whether custom formatting object is requested.
if (formatType == typeof(ICustomFormatter))
return this;
else
return null;
}
// Format number in binary (B), octal (O), or hexadecimal (H).
public string Format(string format, object arg, IFormatProvider formatProvider)
{
// Handle format string.
int baseNumber;
// Handle null or empty format string, string with precision specifier.
string thisFmt = String.Empty;
// Extract first character of format string (precision specifiers
// are not supported).
if (! String.IsNullOrEmpty(format))
thisFmt = format.Length > 1 ? format.Substring(0, 1) : format;
// Get a byte array representing the numeric value.
byte[] bytes;
if (arg is sbyte)
{
string byteString = ((sbyte) arg).ToString("X2");
bytes = new byte[1] { Byte.Parse(byteString, System.Globalization.NumberStyles.HexNumber ) };
}
else if (arg is byte) {
bytes = new byte[1] { (byte) arg };
}
else if (arg is short) {
bytes = BitConverter.GetBytes((short) arg);
}
else if (arg is int) {
bytes = BitConverter.GetBytes((int) arg);
}
else if (arg is long) {
bytes = BitConverter.GetBytes((long) arg);
}
else if (arg is ushort) {
bytes = BitConverter.GetBytes((ushort) arg);
}
else if (arg is uint) {
bytes = BitConverter.GetBytes((uint) arg);
}
else if (arg is ulong) {
bytes = BitConverter.GetBytes((ulong) arg);
}
else {
try {
return HandleOtherFormats(format, arg);
}
catch (FormatException e) {
throw new FormatException(String.Format("The format of '{0}' is invalid.", format), e);
}
}
switch (thisFmt.ToUpper())
{
// Binary formatting.
case "B":
baseNumber = 2;
break;
case "O":
baseNumber = 8;
break;
case "H":
baseNumber = 16;
break;
// Handle unsupported format strings.
default:
try {
return HandleOtherFormats(format, arg);
}
catch (FormatException e) {
throw new FormatException(String.Format("The format of '{0}' is invalid.", format), e);
}
}
// Return a formatted string.
string numericString = String.Empty;
for (int ctr = bytes.GetUpperBound(0); ctr >= bytes.GetLowerBound(0); ctr--)
{
string byteString = Convert.ToString(bytes[ctr], baseNumber);
if (baseNumber == 2)
byteString = new String('0', 8 - byteString.Length) + byteString;
else if (baseNumber == 8)
byteString = new String('0', 4 - byteString.Length) + byteString;
// Base is 16.
else
byteString = new String('0', 2 - byteString.Length) + byteString;
numericString += byteString + " ";
}
return numericString.Trim();
}
private string HandleOtherFormats(string format, object arg)
{
if (arg is IFormattable)
return ((IFormattable)arg).ToString(format, CultureInfo.CurrentCulture);
else if (arg != null)
return arg.ToString();
else
return String.Empty;
}
}
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 Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
Dim byteValue As Byte = 124
outputBlock.Text &= String.Format(New BinaryFormatter(), _
"{0} (binary: {0:B}) (hex: {0:H})", byteValue) & vbCrLf
Dim intValue As Integer = 23045
outputBlock.Text &= String.Format(New BinaryFormatter(), _
"{0} (binary: {0:B}) (hex: {0:H})", intValue) + vbCrLf
Dim ulngValue As ULong = 31906574882
outputBlock.Text &= String.Format(New BinaryFormatter(), _
"{0} {1} (binary: {0:B}) {1} (hex: {0:H}){1}", _
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)
public class Example
{
public static void Demo(System.Windows.Controls.TextBlock outputBlock)
{
byte byteValue = 124;
outputBlock.Text += String.Format(String.Format(new BinaryFormatter(),
"{0} (binary: {0:B}) (hex: {0:H})", byteValue)) + "\n";
int intValue = 23045;
outputBlock.Text += String.Format(String.Format(new BinaryFormatter(),
"{0} (binary: {0:B}) (hex: {0:H})", intValue)) + "\n";
ulong ulngValue = 31906574882;
outputBlock.Text += String.Format(String.Format(new BinaryFormatter(),
"{0}\n (binary: {0:B})\n (hex: {0:H})",
ulngValue)) + "\n";
}
}
// 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)
Version Information
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.