DecoderFallbackException Class

DecoderFallbackException Class

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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


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

public sealed class DecoderFallbackException : ArgumentException

The DecoderFallbackException type exposes the following members.

Public methodDecoderFallbackException()Initializes a new instance of the DecoderFallbackException class.
Public methodDecoderFallbackException(String)Initializes a new instance of the DecoderFallbackException class. A parameter specifies the error message.
Public methodDecoderFallbackException(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.
Public methodDecoderFallbackException(String, 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.

Public propertyBytesUnknownGets the input byte sequence that caused the exception.
Public propertyDataGets a collection of key/value pairs that provide additional user-defined information about the exception. (Inherited from Exception.)
Public propertyHelpLinkGets or sets a link to the help file associated with this exception. (Inherited from Exception.)
Public propertyHResultGets or sets HRESULT, a coded numerical value that is assigned to a specific exception. (Inherited from Exception.)
Public propertyIndexGets the index position in the input byte sequence of the byte that caused the exception.
Public propertyInnerExceptionGets the Exception instance that caused the current exception. (Inherited from Exception.)
Public propertyMessageGets the error message and the parameter name, or only the error message if no parameter name is set. (Inherited from ArgumentException.)
Public propertyParamNameGets the name of the parameter that causes this exception. (Inherited from ArgumentException.)
Public propertySourceGets or sets the name of the application or the object that causes the error. (Inherited from Exception.)
Public propertyStackTraceGets a string representation of the frames on the call stack at the time the current exception was thrown. (Inherited from Exception.)

Public methodEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetBaseExceptionWhen overridden in a derived class, returns the Exception that is the root cause of one or more subsequent exceptions. (Inherited from Exception.)
Public methodGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodGetTypeGets the runtime type of the current instance. (Inherited from Exception.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodToStringCreates and returns a string representation of the current exception. (Inherited from Exception.)

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.

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.

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): ", 
      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): ", 
      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);
                     index += nBytes;
                     success = true;
               catch (DecoderFallbackException) {
                  success = false;
                  // Do nothing.         
            // Conversion attempt failed.

            if (! success)
               // Append "*" as fallback character.
               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*'

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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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