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DecoderReplacementFallback Class

DecoderReplacementFallback Class

Provides a failure-handling mechanism, called a fallback, for an encoded input byte sequence that cannot be converted to an output character. The fallback emits a user-specified replacement string instead of a decoded input byte sequence. This class cannot be inherited.


Namespace:  System.Text
Assemblies:   System.Text.Encoding (in System.Text.Encoding.dll)
  mscorlib (in mscorlib.dll)

public sealed class DecoderReplacementFallback : DecoderFallback

The DecoderReplacementFallback type exposes the following members.

Public methodDecoderReplacementFallback()Initializes a new instance of the DecoderReplacementFallback class.
Public methodDecoderReplacementFallback(String)Initializes a new instance of the DecoderReplacementFallback class using a specified replacement string.

Public propertyDefaultStringGets the replacement string that is the value of the DecoderReplacementFallback object.
Public propertyMaxCharCountGets the number of characters in the replacement string for the DecoderReplacementFallback object. (Overrides DecoderFallback.MaxCharCount.)

Public methodCreateFallbackBufferCreates a DecoderFallbackBuffer object that is initialized with the replacement string of this DecoderReplacementFallback object. (Overrides DecoderFallback.CreateFallbackBuffer().)
Public methodEqualsIndicates whether the value of a specified object is equal to the DecoderReplacementFallback object. (Overrides Object.Equals(Object).)
Public methodGetHashCodeRetrieves the hash code for the value of the DecoderReplacementFallback object. (Overrides Object.GetHashCode().)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)

A common reason for an encoding or decoding operation to fail is if the underlying encoding class does not provide a mapping between a character and an equivalent byte sequence. For example, an ASCIIEncoding object cannot decode a byte value greater than 0x7F. If an input byte sequence cannot be converted to an output character, a DecoderReplacementFallback object emits a replacement string into the output to represent the original input byte sequence. The conversion process then continues to decode the remainder of the original input.

The replacement string used by a DecoderReplacementFallback object is determined by the call to its class constructor. Two options are available:

This class is one of two .NET Framework classes that implement different fallback strategies for handling decoding conversion failures. The other class is the DecoderExceptionFallback class, which throws a DecoderFallbackException when an invalid byte sequence is encountered.

The following code example demonstrates the DecoderReplacementFallback class.

// This example demonstrates the DecoderReplacementFallback class. 

using System;
using System.Text;

class Sample 
    public static void Main() 

// Create an encoding, which is equivalent to calling the  
// ASCIIEncoding class constructor.  
// The DecoderReplacementFallback parameter specifies that the  
// string "(error)" is to replace characters that cannot be decoded. 
// An encoder replacement fallback is also specified, but in this code 
// example the encoding operation cannot fail.  

    Encoding ae = Encoding.GetEncoding(
                  new EncoderReplacementFallback("(unknown)"), 
                  new DecoderReplacementFallback("(error)"));
    string inputString = "XYZ";
    string decodedString;
    string twoNewLines = "\n\n";
    byte[] encodedBytes = new byte[ae.GetByteCount(inputString)];
    int numberOfEncodedBytes = 0;

// --------------------------------------------------------------------------

// Display the name of the encoding.
    Console.WriteLine("The name of the encoding is \"{0}\".\n", ae.WebName);

// Display the input string in text.
    Console.WriteLine("Input string ({0} characters): \"{1}\"", 
                       inputString.Length, inputString);

// Display the input string in hexadecimal.
    Console.Write("Input string in hexadecimal: ");
    foreach (char c in inputString.ToCharArray()) 
        Console.Write("0x{0:X2} ", (int)c);

// -------------------------------------------------------------------------- 
// Encode the input string. 

    Console.WriteLine("Encode the input string...");
    numberOfEncodedBytes = ae.GetBytes(inputString, 0, inputString.Length, 
                                       encodedBytes, 0);

// Display the encoded bytes.
    Console.WriteLine("Encoded bytes in hexadecimal ({0} bytes):\n", 
    foreach (byte b in encodedBytes)
        Console.Write("0x{0:X2} ", (int)b);

// -------------------------------------------------------------------------- 

// Replace the encoded byte sequences for the characters 'X' and 'Z' with the  
// value 0xFF, which is outside the valid range of 0x00 to 0x7F for  
// ASCIIEncoding. The resulting byte sequence is actually the beginning of  
// this code example because it is the input to the decoder operation, and  
// is equivalent to a corrupted or improperly encoded byte sequence. 

    encodedBytes[0] = 0xFF;
    encodedBytes[2] = 0xFF;

    Console.WriteLine("Display the corrupted byte sequence...");
    Console.WriteLine("Encoded bytes in hexadecimal ({0} bytes):\n", 
    foreach (byte b in encodedBytes)
        Console.Write("0x{0:X2} ", (int)b);

// -------------------------------------------------------------------------- 
// Decode the encoded bytes.

    Console.WriteLine("Compare the decoded bytes to the input string...");
    decodedString = ae.GetString(encodedBytes);

// Display the input string and the decoded string for comparison.
    Console.WriteLine("Input string:  \"{0}\"", inputString);
    Console.WriteLine("Decoded string:\"{0}\"", decodedString);
This code example produces the following results:

The name of the encoding is "us-ascii".

Input string (3 characters): "XYZ"
Input string in hexadecimal: 0x58 0x59 0x5A

Encode the input string...
Encoded bytes in hexadecimal (3 bytes):

0x58 0x59 0x5A

Display the corrupted byte sequence...
Encoded bytes in hexadecimal (3 bytes):

0xFF 0x59 0xFF

Compare the decoded bytes to the input string...
Input string:  "XYZ"
Decoded string:"(error)Y(error)"


.NET Framework

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

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
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