By default, this replacement string is a question mark, but you can call a class constructor overload to choose a different string. Typically, the replacement string is a single character, although this is not a requirement. You can also implement a replacement class for an encoding. However, you are free to choose any replacement string, and it can contain multiple characters. Instead of providing a best-fit fallback or a replacement string, an encoder can throw an EncoderFallbackException if it is unable to encode a set of characters, and a decoder can throw a DecoderFallbackException if it is unable to decode a byte array.
To throw an exception in encoding and decoding operations, you supply an EncoderExceptionFallback object and a DecoderExceptionFallback object, respectively, to the Encoding.
You can also implement a custom exception handler for an encoding operation. The EncoderFallbackException and DecoderFallbackException objects provide the following information about the condition that caused the exception:.
The EncoderFallbackException object includes an IsUnknownSurrogate method, which indicates whether the character or characters that cannot be encoded represent an unknown surrogate pair in which case, the method returns true or an unknown single character in which case, the method returns false.
The characters in the surrogate pair are available from the EncoderFallbackException. CharUnknownLow properties. The unknown single character is available from the EncoderFallbackException. CharUnknown property. The EncoderFallbackException. Index property indicates the position in the string at which the first character that could not be encoded was found.
The DecoderFallbackException object includes a BytesUnknown property that returns an array of bytes that cannot be decoded. The DecoderFallbackException. Index property indicates the starting position of the unknown bytes. Although the EncoderFallbackException and DecoderFallbackException objects provide adequate diagnostic information about the exception, they do not provide access to the encoding or decoding buffer. Therefore, they do not allow invalid data to be replaced or corrected within the encoding or decoding method.
In addition to the best-fit mapping that is implemented internally by code pages,. NET includes the following classes for implementing a fallback strategy:. In addition, you can implement a custom solution that uses best-fit fallback, replacement fallback, or exception fallback, by following these steps:. Derive a class from EncoderFallback for encoding operations, and from DecoderFallback for decoding operations.
Derive a class from EncoderFallbackBuffer for encoding operations, and from DecoderFallbackBuffer for decoding operations. For exception fallback, if the predefined EncoderFallbackException and DecoderFallbackException classes do not meet your needs, derive a class from an exception object such as Exception or ArgumentException. To implement a custom fallback solution, you must create a class that inherits from EncoderFallback for encoding operations, and from DecoderFallback for decoding operations.
Instances of these classes are passed to the Encoding. GetEncoding String, EncoderFallback, DecoderFallback method and serve as the intermediary between the encoding class and the fallback implementation.
When you create a custom fallback solution for an encoder or decoder, you must implement the following members:. The EncoderFallback. MaxCharCount or DecoderFallback. MaxCharCount property, which returns the maximum possible number of characters that the best-fit, replacement, or exception fallback can return to replace a single character. For a custom exception fallback, its value is zero.
CreateFallbackBuffer or DecoderFallback. The method is called by the encoder when it encounters the first character that it is unable to successfully encode, or by the decoder when it encounters the first byte that it is unable to successfully decode.
To implement a custom fallback solution, you must also create a class that inherits from EncoderFallbackBuffer for encoding operations, and from DecoderFallbackBuffer for decoding operations. CreateFallbackBuffer method is called by the encoder when it encounters the first character that it is not able to encode, and the DecoderFallback.
CreateFallbackBuffer method is called by the decoder when it encounters one or more bytes that it is not able to decode. Each instance represents a buffer that contains the fallback characters that will replace the character that cannot be encoded or the byte sequence that cannot be decoded.
The EncoderFallbackBuffer. Fallback or DecoderFallbackBuffer. Fallback method. Fallback is called by the encoder to provide the fallback buffer with information about the character that it cannot encode. Because the character to be encoded may be a surrogate pair, this method is overloaded. One overload is passed the character to be encoded and its index in the string.
The second overload is passed the high and low surrogate along with its index in the string. The DecoderFallbackBuffer. Fallback method is called by the decoder to provide the fallback buffer with information about the bytes that it cannot decode. This method is passed an array of bytes that it cannot decode, along with the index of the first byte.
The fallback method should return true if the fallback buffer can supply a best-fit or replacement character or characters; otherwise, it should return false. For an exception fallback, the fallback method should throw an exception. GetNextChar method, which is called repeatedly by the encoder or decoder to get the next character from the fallback buffer. Remaining or DecoderFallbackBuffer. Remaining property, which returns the number of characters remaining in the fallback buffer.
MovePrevious or DecoderFallbackBuffer. MovePrevious method, which moves the current position in the fallback buffer to the previous character. Reset or DecoderFallbackBuffer. Typically, these new character sets support a group of related languages based on the same script.
For example, the ISO character set series was created to support different European languages. Table shows the languages that are supported by the ISO character sets. Historically, character sets have provided restricted multilingual support, which has been limited to groups of languages based on similar scripts.
More recently, universal character sets have emerged to enable greatly improved solutions for multilingual support. Unicode is one such universal character set that encompasses most major scripts of the modern world. As of version 6. Different types of encoding schemes have been created by the computer industry. The character set you choose affects what kind of encoding scheme is used. This is important because different encoding schemes have different performance characteristics.
These characteristics can influence your database schema and application development. The character set you choose uses one of the following types of encoding schemes:. Single-Byte Encoding Schemes. Multibyte Encoding Schemes. Single-byte encoding schemes are efficient.
They take up the least amount of space to represent characters and are easy to process and program with because one character can be represented in one byte. Single-byte encoding schemes are classified as one of the following types:. Single-byte 7-bit encoding schemes can define up to characters and normally support just one language.
Single-byte 8-bit encoding schemes can define up to characters and often support a group of related languages. Figure shows the ISO 8-bit encoding scheme. Multibyte encoding schemes are needed to support ideographic scripts used in Asian languages like Chinese or Japanese because these languages use thousands of characters.
These encoding schemes use either a fixed number or a variable number of bytes to represent each character. Fixed-width multibyte encoding schemes. In a fixed-width multibyte encoding scheme, each character is represented by a fixed number of bytes. The number of bytes is at least two in a multibyte encoding scheme. Variable-width multibyte encoding schemes.
A variable-width encoding scheme uses one or more bytes to represent a single character. Some multibyte encoding schemes use certain bits to indicate the number of bytes that represents a character. For example, if two bytes is the maximum number of bytes used to represent a character, then the most significant bit can be used to indicate whether that byte is a single-byte character or the first byte of a double-byte character.
Shift-sensitive variable-width multibyte encoding schemes. Some variable-width encoding schemes use control codes to differentiate between single-byte and multibyte characters with the same code values. A shift-out code indicates that the following character is multibyte. A shift-in code indicates that the following character is single-byte.
Shift-sensitive encoding schemes are used primarily on IBM platforms. Note that ISO character sets cannot be used as database character sets, but they can be used for applications such as a mail server. Oracle Database uses the fol lowing naming convention for its character set names:. The parts of the names that appear between angle brackets are concatenated. The optional S or C is used to differentiate character sets that can be used only on the server S or only on the client C.
You should use the server character set S on the Macintosh platform. The Macintosh client character sets are obsolete. When discussing character set conversion or character set compatibility between databases, Oracle documentation sometimes uses the terms superset , subset , binary superset , or binary subset to describe relationship between two character sets.
The terms subset and superset , without the adjective "binary", pertain to character repertoires of two Oracle character sets, that is, to the sets of characters supported encoded by each of the character sets. By definition, character set A is a superset of character set B if A supports all characters that B supports. Character set B is a subset of character set A if A is a superset of B.
The terms binary subset and binary superset restrict the above subset-superset relationship by adding a condition on binary representation binary codes of characters of the two character sets. By definition, character set A is a binary superset of character set B if A supports all characters that B supports and all these characters have the same binary representation in A and B.
Character set B is a binary subset of character set A if A is a binary superset of B. When character set A is a binary superset of character set B, any text value encoded in B is at the same time valid in A without need for character set conversion. When A is a non-binary superset of B, a text value encoded in B can be represented in A without loss of data but may require character set conversion to transform the binary representation.
Oracle Database does not maintain a list of all subset-superset pairs but it does maintain a list of binary subset-superset pairs that it recognizes in various situations such as checking compatibility of a transportable tablespace or a pluggable database.
In single-byte character sets, the number of bytes and the number of characters in a string are the same. In multibyte character sets, a character or code point consists of one or more bytes. Calculating the number of characters based on byte lengths can be difficult in a variable-width character set. Calculating column lengths in bytes is called byte semantics , while measuring column lengths in characters is called character semantics.
Character semantics is useful for defining the storage requirements for multibyte strings of varying widths. Using byte semantics, this column requires 15 bytes for the Chinese characters, which are three bytes long, and 5 bytes for the English characters, which are one byte long, for a total of 20 bytes.
Using character semantics, the column requires 10 characters. This method of specifying the length semantics is recommended as it properly documents the expected semantics in creation DDL statements and makes the statements independent of any execution environment. Its default value is BYTE. For the sake of compatibility with existing application installation procedures, which may have been written before character length semantics was introduced into Oracle SQL, Oracle recommends that you leave this initialization parameter undefined or you set it to BYTE.
Otherwise, created columns may be larger than expected, causing applications to malfunction or, in some cases, cause buffer overflows. Byte semantics is the default for the database character set. Character length semantics is the default and the only allowable kind of length semantics for NCHAR data types.
Figure shows the number of bytes needed to store different kinds of characters in the UTF-8 character set. Oracle Database uses the database character set for:. In addition, the choice of database character set determines which characters can name objects in the database.
After the database is created, you cannot change the character sets, with some exceptions, without re-creating the database. Consider the following questions when you choose an Oracle Database character set for the database:. The Oracle Database character sets are listed in " Character Sets ". In the General section, click Web Options. You can select the options on the Fonts tab in the Web Options dialog box to customize the font for each character set. If you don't choose an encoding standard when you save a file, Word encodes the file as Unicode.
Usually, you can use the default Unicode encoding, because it supports most characters in most languages. If your document will be opened in a program that does not support Unicode, you can choose an encoding standard that matches that of the target program.
For example, Unicode enables you to create a Traditional Chinese language document on your English-language system. However, if the document will be opened in a Traditional Chinese language program that does not support Unicode, you can save the document with Chinese Traditional Big5 encoding. When the document is opened in the Traditional Chinese language program, all the text is displayed properly.
Note: Because Unicode is the most comprehensive standard, saving text in any other encoding may result in some characters that can no longer be displayed. For example, a document encoded in Unicode can contain Hebrew and Cyrillic text.
If this document is saved with Cyrillic Windows encoding, the Hebrew text can no longer be displayed, and if the document is saved with Hebrew Windows encoding, the Cyrillic text can no longer be displayed. If you choose an encoding standard that doesn't support the characters you used in the file, Word marks in red the characters that it cannot save.
You can preview the text in the encoding standard that you choose before you save the file. Text formatted in the Symbol font or in field codes is removed from the file when you save a file as encoded text. In the File Conversion dialog box, select the option for the encoding standard that you want to use:. To use the default encoding standard for your system, click Windows Default.
To choose a specific encoding standard, click Other encoding , and then select the encoding standard that you want from the list. Note: You can resize the File Conversion dialog box so that you can preview more of your document. If you receive a message that states, "Text marked in red will not save correctly in the chosen encoding," you can try to choose a different encoding, or you can select the Allow character substitution check box.
When you allow character substitution, Word replaces a character that cannot be displayed with the closest equivalent character in the encoding that you chose. UTF-8 encodes a character into a binary string of one, two, three, or four bytes. UTF encodes a Unicode character into a string of either two or four bytes. This distinction is evident from their names. In UTF-8, the smallest binary representation of a character is one byte, or eight bits.
In UTF, the smallest binary representation of a character is two bytes, or sixteen bits. However, they are not compatible with each other.
These systems use different algorithms to map code points to binary strings, so the binary output for any given character will look different from both methods:. UTF must encode these same characters in either two or four bytes. If a website uses a language with characters farther back in the Unicode library, UTF-8 will encode all characters as four bytes, whereas UTF might encode many of the same characters as only two bytes.
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