DecoderExceptionFallback クラスとは何？わかりやすく解説 Weblio辞書

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.NET Framework クラスライブラリリファレンス

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DecoderExceptionFallback クラス

メモ : このクラスは、.NET Framework version 2.0 で新しく追加されたものです。

エンコード済み入力バイトシーケンスをデコード済み出力文字に変換できない場合に、DecoderFallbackException をスローします。このクラスは継承できません。

名前空間: System.Text
アセンブリ: mscorlib (mscorlib.dll 内)
構文

Visual Basic (宣言)

<SerializableAttribute> _
Public NotInheritable Class
 DecoderExceptionFallback
    Inherits DecoderFallback

Visual Basic (使用法)

Dim instance As DecoderExceptionFallback

[SerializableAttribute] 
public sealed class DecoderExceptionFallback
 : DecoderFallback

C++

[SerializableAttribute] 
public ref class DecoderExceptionFallback sealed
 : public DecoderFallback

/** @attribute SerializableAttribute() */ 
public final class DecoderExceptionFallback
 extends DecoderFallback

JScript

SerializableAttribute 
public final class DecoderExceptionFallback
 extends DecoderFallback

解説

エンコーディングでは、Unicode 文字をエンコード済みバイトシーケンスにマップします。エンコード済みバイトシーケンスは、ディスクなどの物理メディアに転送したり、通信リンクを通じて転送したりできます。文字はさまざまな方法でマップできますが、特定のエンコーディングは Encoding クラスから派生した型で表されます。具体的には、エンコーディングの型の GetBytes メソッドは文字をバイトシーケンスにエンコードし、GetChars メソッドはバイトシーケンスを文字にデコードします。

しかし、入力バイトシーケンスがエンコーディングによってマップできない場合、デコード処理は失敗する可能性があります。たとえば、ASCIIEncoding オブジェクトは、コードポイント値が U+0000 から U+007F の範囲外にある文字のバイトシーケンスをデコードできません。

エンコーディングまたはデコードによる変換が実行できない場合のために、.NET Framework では、フォールバックと呼ばれるエラー処理機構が用意されています。コードでは、定義済みの .NET Framework のデコーダフォールバックを使用するか、DecoderFallback クラスと DecoderFallbackBuffer クラスから継承したカスタムのデコーダフォールバックを作成できます。

.NET Framework では、デコード変換エラーを処理するため、異なるフォールバック方法を実装する 2 つの定義済みクラスが用意されています。DecoderReplacementFallback クラスは、変換できない入力バイトシーケンスの代わりに、指定した文字列を挿入します。代替文字列の作成後、デコード処理は、残りの入力の変換を引き続き実行します。一方、DecoderExceptionFallback クラスは、無効なバイトシーケンスを検出すると、DecoderFallbackException をスローします。

使用例

DecoderExceptionFallback クラスと DecoderFallbackException クラスのコード例を次に示します。

Visual Basic

' 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. ***
'
'

// 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(
                  "us-ascii",
                  new EncoderExceptionFallback(), 
                  new DecoderExceptionFallback());
    string inputString = "XYZ";
    string decodedString;
    string twoNewLines = "\n\n";
    byte[] encodedBytes = new byte[ae.GetByteCount(inputString)];
    int numberOfEncodedBytes = 0;

// --------------------------------------------------------------------------
    Console.Clear();

// 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);
        }
    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.
    Console.WriteLine("Encoded bytes in hexadecimal ({0}
 bytes):\n", 
                       numberOfEncodedBytes);
    foreach (byte b in encodedBytes)
        {
        Console.Write("0x{0:X2} ", (int)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] = 0xFF;
    encodedBytes[2] = 0xFF;

    Console.WriteLine("Display the corrupted byte sequence...");
    Console.WriteLine("Encoded bytes in hexadecimal ({0}
 bytes):\n", 
                       numberOfEncodedBytes);
    foreach (byte b in encodedBytes)
        {
        Console.Write("0x{0:X2} ", (int)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 (DecoderFallbackException dfe)
        {
        Console.WriteLine(dfe);
        Console.WriteLine("\n*** THE CODE EXAMPLE TERMINATES HERE AS INTENDED.
 ***");
        return;
        }

// 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
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. ***

*/

C++

// This example demonstrates the DecoderExceptionFallback class.

using namespace System;
using namespace System::Text;

int 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^ asciiEncoding = Encoding::GetEncoding("us-ascii",
        gcnew EncoderExceptionFallback(), gcnew DecoderExceptionFallback());
    String^ inputString = "XYZ";
    String^ decodedString;
    String^ twoNewLines = Environment::NewLine + Environment::NewLine ;

    array<Byte>^ encodedBytes = 
        gcnew array<Byte>(asciiEncoding->GetByteCount(inputString));
    int numberOfEncodedBytes = 0;

    // ---------------------------------------------------------------------
    Console::Clear();

    // Display the name of the encoding.
    Console::WriteLine("The name of the encoding is \"{0}\".{1}"
,
        asciiEncoding->WebName, Environment::NewLine);

    // 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:
 ");
    for each (char c in
 inputString)
    {
        Console::Write("0x{0:X2} ", c);
    }
    Console::Write(twoNewLines);

    // ---------------------------------------------------------------------
    // Encode the input string.

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

    numberOfEncodedBytes = asciiEncoding->GetBytes(inputString, 0,
        inputString->Length, encodedBytes, 0);

    // Display the encoded bytes.
    Console::WriteLine("Encoded bytes in hexadecimal ({0}
 bytes):{1}",
        numberOfEncodedBytes, Environment::NewLine);
    for each (Byte b in encodedBytes)
    {
        Console::Write("0x{0:X2} ", 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] = 0xFF;
    encodedBytes[2] = 0xFF;

    Console::WriteLine("Display the corrupted byte sequence...");
    Console::WriteLine("Encoded bytes in hexadecimal ({0}
 bytes):{1}",
        numberOfEncodedBytes, Environment::NewLine);
    for each (Byte b in encodedBytes)
    {
        Console::Write("0x{0:X2} ", 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 = asciiEncoding->GetString(encodedBytes);
        // This statement is never executed.
        Console::WriteLine("This statement is never executed.");
    }
    catch (DecoderFallbackException^ ex)
    {
        Console::WriteLine(ex);
        Console::WriteLine(
            "{0}*** THE CODE EXAMPLE TERMINATES HERE AS INTENDED. ***",
 
            Environment::NewLine);
    }
}


/*
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. ***

*/

継承階層

System.Object
System.Text.DecoderFallback
System.Text.DecoderExceptionFallback

スレッドセーフ

この型の public static (Visual Basic では Shared) メンバはすべて、スレッドセーフです。インスタンスメンバの場合は、スレッドセーフであるとは限りません。

プラットフォーム

バージョン情報

.NET Framework
サポート対象 : 2.0

参照

関連項目
DecoderExceptionFallback メンバ
System.Text 名前空間

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DecoderExceptionFallback クラスとは？わかりやすく解説