EncoderExceptionFallback Class


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Provides a failure-handling mechanism, called a fallback, for an input character that cannot be converted to an output byte sequence. The fallback throws an exception if an input character cannot be converted to an output byte sequence. This class cannot be inherited.

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


public sealed class EncoderExceptionFallback : EncoderFallback


Initializes a new instance of the EncoderExceptionFallback class.


Gets the maximum number of characters this instance can return.(Overrides EncoderFallback.MaxCharCount.)


Returns an encoder fallback buffer that throws an exception if it cannot convert a character sequence to a byte sequence.(Overrides EncoderFallback.CreateFallbackBuffer().)


Indicates whether the current EncoderExceptionFallback object and a specified object are equal.(Overrides Object.Equals(Object).)


Retrieves the hash code for this instance.(Overrides Object.GetHashCode().)


Gets the Type of the current instance.(Inherited from Object.)


Returns a string that represents the current object.(Inherited from Object.)

An encoding maps a Unicode character to an encoded sequence of bytes, which can subsequently be transferred to a physical medium, such as a disk, or over a communications link. Characters can be mapped in various ways, and a particular encoding is represented by a type derived from the Encoding class. Specifically, the encoding type's GetBytes method encodes a character to a byte sequence, and the GetChars method decodes a byte sequence to a character.

An encoding operation can fail if the input character cannot be represented by the encoding. For example, a ASCIIEncoding object cannot encode a character that yields a Unicode code point value that is outside the range U+0000 to U+007F.

In cases where an encoding or decoding conversion cannot be performed, the .NET Framework provides a failure-handling mechanism called a fallback. Your application can use the predefined .NET Framework encoder fallback, or it can create a custom encoder fallback derived from the EncoderFallback and EncoderFallbackBuffer classes.

The .NET Framework provides two predefined classes that implement different fallback strategies for handling encoding conversion failures. The EncoderReplacementFallback class substitutes a string provided for any input character that cannot be converted. The substitute string is encoded in place of the invalid character, and then the encoding operation continues converting the remainder of the input. In contrast, the EncoderExceptionFallback class throws a EncoderFallbackException when an invalid character is encountered.

The following code example demonstrates the EncoderExceptionFallback and EncoderFallbackException classes.

// This example demonstrates the EncoderExceptionFallback 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 EncoderExceptionFallback parameter causes an exception to
// be thrown when a character cannot be encoded. 
// A decoder exception fallback is also specified, but it is not 
// used because this example terminates during the encoding operation.  

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

// The input string consists of the Unicode characters LEFT POINTING 
// The encoding can only encode characters in the US-ASCII range of U+0000 
// through U+007F. Consequently, the characters bracketing the 'X' character
// cause an exception.
    string inputString = "\u00abX\u00bb";

    string twoNewLines = "\n\n";
    byte[] encodedBytes = new byte[ae.GetMaxByteCount(inputString.Length)];
    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);}

// --------------------------------------------------------------------------
// Attempt to encode the input string. However, an exception is thrown before
// the input string can be encoded.

    Console.WriteLine("Encode the input string...");

// The code example terminates during the call to the GetBytes() method.
    try {
        numberOfEncodedBytes = ae.GetBytes(inputString, 0, inputString.Length, 
                                           encodedBytes, 0);
    catch (EncoderFallbackException e)
        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): "�X�"
Input string in hexadecimal: 0xAB 0x58 0xBB

Encode the input string...
System.Text.EncoderFallbackException: Unable to translate Unicode character \u00AB at inde
x 0 to specified code page.
   at System.Text.EncoderExceptionFallbackBuffer.Fallback(Char charUnknown, Int32 index)
   at System.Text.EncoderFallbackBuffer.InternalFallback(Char ch, Char*& chars)
   at System.Text.ASCIIEncoding.GetBytes(Char* chars, Int32 charCount, Byte* bytes, Int32
byteCount, EncoderNLS encoder)
   at System.Text.ASCIIEncoding.GetBytes(String chars, Int32 charIndex, Int32 charCount, B
yte[] bytes, Int32 byteIndex)
   at Sample.Main()



Universal Windows Platform
Available since 10
.NET Framework
Available since 2.0

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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