Provides a failure-handling mechanism, called a fallback, for an encoded input byte sequence that cannot be converted to an output character.
See Also: DecoderFallback Members
An encoding maps a Unicode character to an encoded sequence of bytes. A particular encoding is represented by a type that is derived from the System.Text.Encoding class. Specifically, a character is encoded to a byte sequence by calling the encoding type's Encoding.GetBytes(string) method, and the byte sequence is decoded to a character array or a string by calling the Encoding.GetChars(Byte) or Encoding.GetString(Byte) method.
A decoding operation can fail if the input byte sequence cannot be mapped by the encoding. For example, an System.Text.ASCIIEncoding object cannot decode a byte sequence if that sequence represents a character that has a code point value that is outside the range U+0000 to U+007F.
When a decoding conversion cannot be performed, the .NET Framework provides a failure-handling mechanism called a fallback. Your application can use predefined .NET Framework decoder fallbacks, or it can create a custom decoder fallback derived from the System.Text.DecoderFallback and System.Text.DecoderFallbackBuffer classes.
System.Text.DecoderFallback and System.Text.DecoderFallbackBuffer are the base classes for all decoding fallback handlers in the .NET Framework. They support the following three kinds of fallback handling mechanisms:
Best-fit fallback, which maps valid Unicode characters that cannot be decoded to an approximate equivalent. For example, a best-fit fallback handler for the System.Text.ASCIIEncoding class might map Æ (U+00C6) to AE (U+0041 + U+0045). A best-fit fallback handler might also be implemented to transliterate one alphabet (such as Cyrillic) to another (such as Latin or Roman). The .NET Framework does not provide any public best-fit fallback implementations.
Replacement fallback, which replaces each character that cannot be decoded with a predefined string. The .NET Framework provides a predefined replacement fallback handler. The System.Text.DecoderReplacementFallback class replaces each byte sequence that cannot be decoded with a question mark character ("?", or U+003F) or a REPLACEMENT CHARACTER (U+FFFD). You can customize the replacement string by specifying a substitute in the call to the DecoderReplacementFallback.#ctor(string) constructor. After the substitute string is emitted, the decoding operation continues converting the remainder of the input.
Exception fallback, which throws an exception when a byte sequence cannot be decoded. The .NET Framework provides a predefined exception fallback handler. The System.Text.DecoderExceptionFallback class throws a System.Text.DecoderFallbackException when an invalid byte sequence is encountered, and the decoding operation terminates.
If you choose to implement a custom solution, you must override the following abstract members of the System.Text.DecoderFallback class:
The DecoderFallback.CreateFallbackBuffer method, which returns a class instance derived from System.Text.DecoderFallbackBuffer. Depending on the type of fallback handler that you are developing, the System.Text.DecoderFallbackBuffer implementation is responsible for performing the mapping or replacement, or for throwing the exception.
The DecoderFallback.MaxCharCount property, which returns the maximum number of characters that the fallback implementation can return. For an exception fallback handler, its value should be zero.
For more information about encoding, decoding, and fallback strategies, see Character Encoding in the .NET Framework.