Article
11/18/2011

DecoderFallbackException Class

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The exception that is thrown when a decoder fallback operation fails. This class cannot be inherited.

Inheritance Hierarchy

System.Object
  System.Exception
    System.SystemException
      System.ArgumentException
        System.Text.DecoderFallbackException

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

Syntax

'Declaration
Public NotInheritable Class DecoderFallbackException _
    Inherits ArgumentException

public sealed class DecoderFallbackException : ArgumentException

The DecoderFallbackException type exposes the following members.

Constructors

	Name	Description
	DecoderFallbackException()	Initializes a new instance of the DecoderFallbackException class.
	DecoderFallbackException(String)	Initializes a new instance of the DecoderFallbackException class. A parameter specifies the error message.
	DecoderFallbackException(String, Exception)	Initializes a new instance of the DecoderFallbackException class. Parameters specify the error message and the inner exception that is the cause of this exception.
	DecoderFallbackException(String, array<Byte[], Int32)	Initializes a new instance of the DecoderFallbackException class. Parameters specify the error message, the array of bytes being decoded, and the index of the byte that cannot be decoded.

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Properties

	Name	Description
	BytesUnknown	Gets the input byte sequence that caused the exception.
	Data	Gets a collection of key/value pairs that provide additional user-defined information about the exception. (Inherited from Exception.)
	HResult	Gets or sets HRESULT, a coded numerical value that is assigned to a specific exception. (Inherited from Exception.)
	Index	Gets the index position in the input byte sequence of the byte that caused the exception.
	InnerException	Gets the Exception instance that caused the current exception. (Inherited from Exception.)
	Message	Gets the error message and the parameter name, or only the error message if no parameter name is set. (Inherited from ArgumentException.)
	StackTrace	Gets a string representation of the frames on the call stack at the time the current exception was thrown. (Inherited from Exception.)

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Methods

	Name	Description
	Equals(Object)	Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
	Finalize	Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
	GetBaseException	When overridden in a derived class, returns the Exception that is the root cause of one or more subsequent exceptions. (Inherited from Exception.)
	GetHashCode	Serves as a hash function for a particular type. (Inherited from Object.)
	GetType	Gets the runtime type of the current instance. (Inherited from Exception.)
	MemberwiseClone	Creates a shallow copy of the current Object. (Inherited from Object.)
	ToString	Creates and returns a string representation of the current exception. (Inherited from Exception.)

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Remarks

When using the standard decoders, a DecoderFallbackException is thrown only if an attempt to decode a sequence of bytes fails and the value of the throwOnInvalidBytes argument was set to true in the Encoding implementation's class constructor.

Examples

The following example provides a possible handler for a DecoderFallbackException. It decodes a UTF8-encoded string in which two bytes have been modified so that they cannot be successfully decoded. When the exception handler is invoked, it decodes the byte array on a byte-by-byte basis and substitutes the asterisk (*) character for any bytes that cannot be successfully decoded.

Imports System.Text

Public Module Example
   Public Sub Demo(outputBlock As System.Windows.Controls.TextBlock)
      ' Retrieve a UTF8 encoding object that throws an exception if the
      ' encoding/decoding operation fails. 
      Dim enc8 As Encoding = New UTF8Encoding(False, True)
      Dim inputString As String = "XYZ"
      Dim decodedString As String = String.Empty
      Dim numberOfEncodedBytes As Integer = enc8.GetByteCount(inputString)

      ' Create array to hold encoded bytes.
      Dim encodedBytes(numberOfEncodedBytes - 1) As Byte

      ' Display the input string in text.
      outputBlock.Text += String.Format("String to encode with {2} encoding: '{1}' ({0} characters)", _
                        inputString.Length, inputString, enc8.WebName) + vbCrLf 

      ' Display the hexadecimal equivalent of the Char values.
      outputBlock.Text += "Input string in hexadecimal: "
      For ctr As Integer = 0 to inputString.Length - 1
         outputBlock.Text += String.Format("0x{0:X2} ", Convert.ToInt32(inputString.Chars(ctr)))
      Next 
      outputBlock.Text += vbCrLf

      ' Encode the input string and display the encoded bytes. 
      numberOfEncodedBytes = enc8.GetBytes(inputString, 0, inputString.Length, _
                                           encodedBytes, 0)
      outputBlock.Text += String.Format("Encoded bytes in hexadecimal ({0} bytes): ", _
                         numberOfEncodedBytes) 
      For Each b As Byte In encodedBytes
         outputBlock.Text += String.Format("0x{0:X2} ", Convert.ToInt32(b))
      Next
      outputBlock.Text += vbCrLf

      ' Replace the encoded byte sequences for the characters 'X' and 'Z' with the 
      ' invalid value 0xFF.  
      encodedBytes(0) = &HFF
      encodedBytes(2) = &HFF

      outputBlock.Text += String.Format("The corrupted byte sequence in hexadecimal ({0} bytes): ", _
                         numberOfEncodedBytes)
      For Each b As Byte In encodedBytes
         outputBlock.Text += String.Format("0x{0:X2} ", Convert.ToInt32(b))
      Next b
      outputBlock.Text += vbCrLf

      ' Try to decode the corrupted bytes. (This throws an exception.)
      Try
         decodedString += enc8.GetString(encodedBytes, 0, encodedBytes.Length)
      Catch e As DecoderFallbackException
         Dim byteString As String = String.Empty
         For Each unknownByte As Byte In e.BytesUnknown
            byteString = String.Format("0x{0:X2} ", unknownByte)
         Next

         ' Retrieve decoder to translate byte array.
         Dim enc8Decoder As Decoder = enc8.GetDecoder()
         Dim index As Integer = 0         
         Dim sb As New StringBuilder()
         ' A character can be encoded in up to four bytes (though 4 is rare).

         ' Since an exception has occurred, decode on a character-by character basis.
         Do While index < encodedBytes.Length   
            Dim success As Boolean = False
            ' Each character can be from 1 to 4 bytes.   
            Const bytesPerChar As Integer = 4
            For nBytes As Integer = 1 To bytesPerChar 
               Try
                  If index + nBytes > encodedBytes.Length Then Continue For

                  Dim chars(enc8Decoder.GetCharCount(encodedBytes, index, nBytes) - 1) As Char 
                  enc8Decoder.GetChars(encodedBytes, index, nBytes, chars, 0)
                  sb.Append(chars)
                  index += nBytes
                  success = True
                  Exit For
               Catch de As DecoderFallbackException
                  success = False
                  ' Do nothing.         
               End Try   
            Next
            ' Conversion attempt failed.

            If Not success Then 
               ' Append "*" as fallback character.
               sb.Append("*")                            
               index += 1 
            End If   
         Loop
         decodedString = sb.ToString()
      End Try
      outputBlock.Text += String.Format("Decoded string: '{0}'", decodedString) + vbCrLf 
   End Sub
End Module
' The example displays the following output:
'       String to encode with utf-8 encoding: 'XYZ' (3 characters)
'       Input string in hexadecimal: 0x58 0x59 0x5A
'       Encoded bytes in hexadecimal (3 bytes): 0x58 0x59 0x5A
'       The corrupted byte sequence in hexadecimal (3 bytes): 0xFF 0x59 0xFF
'       Decoded string: '*Y*'

using System;
using System.Text;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      // Retrieve a UTF8 encoding object that throws an exception if the
      // encoding/decoding operation fails. 
      Encoding enc8 = new UTF8Encoding(false, true);
      string inputString = "XYZ";
      string decodedString = String.Empty;
      int numberOfEncodedBytes = enc8.GetByteCount(inputString);

      // Create array to hold encoded bytes.
      byte[] encodedBytes = new byte[numberOfEncodedBytes]; 

      // Display the input string in text.
      outputBlock.Text += String.Format("String to encode with {2} encoding: '{1}' ({0} characters)\n", 
                        inputString.Length, inputString, enc8.WebName);

      // Display the hexadecimal equivalent of the Char values.
      outputBlock.Text += "Input string in hexadecimal: ";
      for (int ctr = 0; ctr <= inputString.Length - 1; ctr++)
         outputBlock.Text += String.Format("0x{0:X2} ", Convert.ToInt32(inputString[ctr]));

      outputBlock.Text += "\n";

      // Encode the input string and display the encoded bytes. 
      numberOfEncodedBytes = enc8.GetBytes(inputString, 0, inputString.Length, 
                                           encodedBytes, 0);
      outputBlock.Text += String.Format("Encoded bytes in hexadecimal ({0} bytes): ", 
                         numberOfEncodedBytes);
      foreach (byte b in encodedBytes)
         outputBlock.Text += String.Format("0x{0:X2} ", Convert.ToInt32(b));

      outputBlock.Text += "\n";

      // Replace the encoded byte sequences for the characters 'X// and 'Z// with the 
      // invalid value 0xFF.  
      encodedBytes[0] = 0xFF;
      encodedBytes[2] = 0xFF;

      outputBlock.Text += String.Format("The corrupted byte sequence in hexadecimal ({0} bytes): ", 
                         numberOfEncodedBytes);
      foreach (byte b in encodedBytes)
         outputBlock.Text += String.Format("0x{0:X2} ", Convert.ToInt32(b));

      outputBlock.Text += "\n";

      // Try to decode the corrupted bytes. (This throws an exception.)
      try {
         decodedString += enc8.GetString(encodedBytes, 0, encodedBytes.Length);
      }
      catch (DecoderFallbackException e)
      {
         string byteString = String.Empty;
         foreach (byte unknownByte in e.BytesUnknown)
            byteString = String.Format("0x{0:X2} ", unknownByte);

         // Retrieve decoder to translate byte array.
         Decoder enc8Decoder = enc8.GetDecoder();
         int index = 0;         
         StringBuilder sb = new StringBuilder();
         // A character can be encoded in up to four bytes (though 4 is rare).

         // Since an exception has occurred, decode on a character-by character basis.
         while (index < encodedBytes.Length)
         {
            bool success = false;
            // Each character can be from 1 to 4 bytes.   
            const int bytesPerChar = 4;
            for (int nBytes = 1; nBytes <= bytesPerChar; nBytes++)
            { 
               try {
                  if (index + nBytes <= encodedBytes.Length)
                  {
                     char[] chars = new char[enc8Decoder.GetCharCount(encodedBytes, index, nBytes)]; 
                     enc8Decoder.GetChars(encodedBytes, index, nBytes, chars, 0);
                     sb.Append(chars);
                     index += nBytes;
                     success = true;
                     break;
                  }
               }
               catch (DecoderFallbackException) {
                  success = false;
                  // Do nothing.         
               } 
            }
            // Conversion attempt failed.

            if (! success)
            { 
               // Append "*" as fallback character.
               sb.Append("*");                            
               index += 1; 
            }   
         }
         decodedString = sb.ToString();
      }
      outputBlock.Text += String.Format("Decoded string: '{0}'\n", decodedString);   
   }
}
// The example displays the following output:
//       String to encode with utf-8 encoding: 'XYZ' (3 characters)
//       Input string in hexadecimal: 0x58 0x59 0x5A
//       Encoded bytes in hexadecimal (3 bytes): 0x58 0x59 0x5A
//       The corrupted byte sequence in hexadecimal (3 bytes): 0xFF 0x59 0xFF
//       Decoded string: '*Y*'

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.

Thread Safety

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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