This documentation is archived and is not being maintained.

DecoderFallbackException Class

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 following code example demonstrates the DecoderExceptionFallback and DecoderFallbackException classes.

// This example demonstrates the DecoderExceptionFallback 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 DecoderExceptionFallback parameter specifies that an exception 
// is thrown if a character cannot be encoded.  
// An encoder exception fallback is also specified, but in this code 
// example the encoding operation cannot fail.  

    Encoding ae = Encoding.GetEncoding(
                  new EncoderExceptionFallback(), 
                  new DecoderExceptionFallback());
    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);

// -------------------------------------------------------------------------- 
// Attempt to decode the encoded bytes. However, an exception is thrown  
// before the byte sequence can be decoded.

    Console.WriteLine("Compare the decoded bytes to the input string...");

    try {
        decodedString = ae.GetString(encodedBytes);
    catch (DecoderFallbackException dfe)
        Console.WriteLine("\n*** THE CODE EXAMPLE TERMINATES HERE AS INTENDED. ***");

// This statement is never executed.
    Console.WriteLine("This statement is never executed.");
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...
System.Text.DecoderFallbackException: Unable to translate bytes [FF] at index 0 from speci
fied code page to Unicode.
   at System.Text.DecoderExceptionFallbackBuffer.Throw(Byte[] bytesUnknown, Int32 index)
   at System.Text.DecoderExceptionFallbackBuffer.Fallback(Byte[] bytesUnknown, Int32 index
   at System.Text.DecoderFallbackBuffer.InternalFallback(Byte[] bytes, Byte* pBytes)
   at System.Text.ASCIIEncoding.GetCharCount(Byte* bytes, Int32 count, DecoderNLS decoder)

   at System.String.CreateStringFromEncoding(Byte* bytes, Int32 byteLength, Encoding encod
   at System.Text.ASCIIEncoding.GetString(Byte[] bytes, Int32 byteIndex, Int32 byteCount)
   at System.Text.Encoding.GetString(Byte[] bytes)
   at Sample.Main()




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

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0