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ICustomFormatter Interface

Defines a method that supports custom formatting of the value of an object.

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

[ComVisibleAttribute(true)]
public interface ICustomFormatter

The ICustomFormatter type exposes the following members.

  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsFormatConverts the value of a specified object to an equivalent string representation using specified format and culture-specific formatting information.
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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 two .NET Framework composite string formatting methods that include an IFormatProvider parameter. Specifically, the ICustomFormatter interface can provide custom formatting of the value of an object passed to the String.Format(IFormatProvider, String, Object[]) and StringBuilder.AppendFormat(IFormatProvider, String, Object[]) methods.

Providing a custom representation of an object's value requires that you do the following:

  1. Define a class that implements the ICustomFormatter interface and its single member, the Format method.

  2. 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.

  3. Pass the IFormatProvider implementation as the provider argument of the String.Format(IFormatProvider, String, Object[]) method or a comparable 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.

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 null. 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 null, it calls the arg parameter's IFormattable.ToString implementation, if one exists, or its parameterless ToString method, if one does not. If arg is null, the method returns String.Empty.

using System;
using System.Globalization;
using System.Numerics;

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 if (arg is BigInteger) {
         bytes = ((BigInteger) arg).ToByteArray();
      }
      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 class Example
{
   public static void Main()
   {
      Console.WindowWidth = 100;

      byte byteValue = 124;
      Console.WriteLine(String.Format(new BinaryFormatter(), 
                                      "{0} (binary: {0:B}) (hex: {0:H})", byteValue));

      int intValue = 23045;
      Console.WriteLine(String.Format(new BinaryFormatter(), 
                                      "{0} (binary: {0:B}) (hex: {0:H})", intValue));

      ulong ulngValue = 31906574882;
      Console.WriteLine(String.Format(new BinaryFormatter(), 
                                      "{0}\n   (binary: {0:B})\n   (hex: {0:H})", 
                                      ulngValue));

      BigInteger bigIntValue = BigInteger.Multiply(Int64.MaxValue, 2);
      Console.WriteLine(String.Format(new BinaryFormatter(), 
                                      "{0}\n   (binary: {0:B})\n   (hex: {0:H})", 
                                      bigIntValue));
   }
}
// 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) 
//    18446744073709551614 
//       (binary: 00000000 11111111 11111111 11111111 11111111 11111111 11111111 11111111 11111110) 
//       (hex: 00 ff ff ff ff ff ff ff fe)

.NET Framework

Supported in: 4.5.2, 4.5.1, 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

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

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.

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