DecoderExceptionFallback Class
Throws DecoderFallbackException if an encoded input byte sequence cannot be converted to a decoded output character. This class cannot be inherited.
Assembly: mscorlib (in mscorlib.dll)
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.
A decoding operation can fail if the input byte sequence cannot be mapped by the encoding. For example, an ASCIIEncoding object cannot decode a byte sequence that yields a character having a 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 decoder fallback, or it can create a custom decoder fallback derived from the DecoderFallback and DecoderFallbackBuffer classes.
The .NET Framework provides two predefined classes that implement different fallback strategies for handling decoding conversion failures. The DecoderReplacementFallback class substitutes a string provided in place of any input byte sequence that cannot be converted. After the substitute string is emitted, the decoding operation continues converting the remainder of the input. In contrast, the DecoderExceptionFallback class throws a DecoderFallbackException when an invalid byte sequence is encountered.
The following code example demonstrates the DecoderExceptionFallback and DecoderFallbackException classes.
' This example demonstrates the DecoderExceptionFallback class. Imports System Imports System.Text Class Sample Public Shared Sub 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. Dim eef As New EncoderExceptionFallback() Dim def As New DecoderExceptionFallback() Dim ae As Encoding = Encoding.GetEncoding("us-ascii", eef, def) Dim inputString As String = "XYZ" Dim decodedString As String Dim twoNewLines As String = vbCrLf & vbCrLf Dim numberOfEncodedBytes As Integer = ae.GetByteCount(inputString) ' Counteract the compiler adding an extra element. Dim encodedBytes(numberOfEncodedBytes - 1) As Byte ' -------------------------------------------------------------------------- Console.Clear() ' Display the name of the encoding. Console.WriteLine("The name of the encoding is ""{0}""." & vbCrLf, ae.WebName) ' Display the input string in text. Console.WriteLine("Input string ({0} characters): ""{1}""", inputString.Length, inputString) ' Display the input string in hexadecimal. ' Each element is converted to an integer with Convert.ToInt32. Console.Write("Input string in hexadecimal: ") Dim c As Char For Each c In inputString.ToCharArray() Console.Write("0x{0:X2} ", Convert.ToInt32(c)) Next c Console.Write(twoNewLines) ' -------------------------------------------------------------------------- ' Encode the input string. Console.WriteLine("Encode the input string...") numberOfEncodedBytes = ae.GetBytes(inputString, 0, inputString.Length, _ encodedBytes, 0) ' Display the encoded bytes. ' Each element is converted to an integer with Convert.ToInt32. Console.WriteLine("Encoded bytes in hexadecimal ({0} bytes):" & vbCrLf, _ numberOfEncodedBytes) Dim b As Byte For Each b In encodedBytes Console.Write("0x{0:X2} ", Convert.ToInt32(b)) Next b Console.Write(twoNewLines) ' -------------------------------------------------------------------------- ' 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) = &HFF encodedBytes(2) = &HFF Console.WriteLine("Display the corrupted byte sequence...") ' Each element is converted to an integer with Convert.ToInt32. Console.WriteLine("Encoded bytes in hexadecimal ({0} bytes):" & vbCrLf, _ numberOfEncodedBytes) For Each b In encodedBytes Console.Write("0x{0:X2} ", Convert.ToInt32(b)) Next b Console.Write(twoNewLines) ' -------------------------------------------------------------------------- ' 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 dfe As DecoderFallbackException Console.WriteLine(dfe) Console.WriteLine(vbCrLf & _ "*** THE CODE EXAMPLE TERMINATES HERE AS INTENDED. ***") Return End Try ' This statement is never executed. Console.WriteLine("This statement is never executed.") End Sub 'Main End Class 'Sample ' '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 'ing) ' at System.Text.ASCIIEncoding.GetString(Byte[] bytes, Int32 byteIndex, Int32 byteCount) ' at System.Text.Encoding.GetString(Byte[] bytes) ' at Sample.Main() ' '*** THE CODE EXAMPLE TERMINATES HERE AS INTENDED. *** ' '
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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.