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Encoding is the process of turning a text string into a sequence of bytes. Conversely, decoding is the process of turning a byte sequence into a text string. The .NET APIs for encoding and decoding represents these sequences as byte arrays. Let s look at the code in Example 10-80 that illustrates this. First, we ll encode some text using the UTF-8 and ASCII encodings, and write the byte values we see to the console.
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static void Main(string[] args) { string listenUp = "Listen up!";
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byte[] utf8Bytes = Encoding.UTF8.GetBytes(listenUp); byte[] asciiBytes = Encoding.ASCII.GetBytes(listenUp); Console.WriteLine("UTF-8"); Console.WriteLine("-----"); foreach (var encodedByte in utf8Bytes) { Console.Write(encodedByte); Console.Write(" "); } Console.WriteLine(); Console.WriteLine(); Console.WriteLine("ASCII"); Console.WriteLine("-----"); foreach (var encodedByte in asciiBytes) { Console.Write(encodedByte); Console.Write(" "); } } Console.ReadKey();
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The framework provides us with the Encoding class. This has a set of static properties that provide us with specific instances of an Encoding object for a particular scheme. In this case, we re using UTF8 and ASCII, which actually return instances of UTF8Encoding and ASCIIEncoding, respectively.
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Under normal circumstances, you do not need to know the actual type of these instances; you can just talk to the object returned through its Encoding base class.
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GetBytes returns us the byte array that corresponds to the actual in-memory represen-
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tation of a string, encoded using the relevant scheme. If we build and run this code, we see the following output:
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UTF-8 ----76 105 115 116 101 110 32 117 112 33 ASCII ----76 105 115 116 101 110 32 117 112 33
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Notice that our encodings are identical in this case, just as promised. For basic Latin characters, UTF-8 and ASCII are compatible. (Unlike Notepad, the .NET UTF8Encod ing does not choose to add a BOM by default, so unless you use characters outside the
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ASCII range this will in fact produce files that can be understood by anything that knows how to process ASCII.) Let s make a quick change to the string we re trying to change, and translate it into French. Replace the first line inside the Main method with Example 10-81. Notice that we ve got a capital E with an acute accent at the beginning.
string listenUp = " coute-moi!";
If you don t have a French keyboard and you re wondering how to insert that E-acute character, there are a number of ways to do it. If you know the decimal representation of the Unicode code point, you can hold down the Alt key and type the number on the numeric keypad (and then release the Alt key). So Alt-0163 will insert the symbol for the UK currency, , and Alt-0201 produces . This doesn t work for the normal number keys, though, so if you don t have a numeric keypad most laptops don t this isn t much help. Possibly the most fun, though, is to run the charmap.exe application. The program icon for it in the Start menu is buried pretty deeply, so it s easier to type charmap into a command prompt, the Start Run box, or the Windows 7 Start menu search box. This is very instructive, and allows you to explore the various different character sets and (if you check the Advanced view box) encodings. You can see an image of it in Figure 10-2. Alternatively, you could just escape the character the string literal "\u00C9coutez moi" will produce the same result. And this has the advantage of not requiring nonASCII values in your source file. Visual Studio is perfectly able to edit various file encodings, including UTF-8, so you can put non-ASCII characters in strings without having to escape them, and you can even use them in identifiers. But some text-oriented tools are not so flexible, so there may be advantages in keeping your source code purely ASCII. Now, when we run again, we get the following output:
UTF-8 ----195 137 99 111 117 116 101 45 109 111 105 33 ASCII ----63 99 111 117 116 101 45 109 111 105 33
We ve quite clearly not got the same output in each case. The UTF-8 case starts with 195, 137, while the ASCII starts with 63. After this preamble, they re again identical. So, let s try decoding those two byte arrays back into strings, and see what happens. Insert the code in Example 10-82 before the call to Console.ReadKey.
string decodedUtf8 = Encoding.UTF8.GetString(utf8Bytes); string decodedAscii = Encoding.ASCII.GetString(asciiBytes); Console.WriteLine(); Console.WriteLine(); Console.WriteLine("Decoded UTF-8"); Console.WriteLine("-------------"); Console.WriteLine(decodedUtf8); Console.WriteLine(); Console.WriteLine(); Console.WriteLine("Decoded ASCII"); Console.WriteLine("-------------"); Console.WriteLine(decodedAscii);
We re now using the GetString method on our Encoding objects, to decode the byte array back into a string. Here s the output:
UTF-8 ----195 137 99 111 117 116 101 45 109 111 105 33 ASCII ----63 99 111 117 116 101 45 109 111 105 33 Decoded UTF-8 ------------ coute-moi! Decoded ASCII ------------ coute-moi!
The UTF-8 bytes have decoded back to our original string. This is because the UTF-8 encoding supports the E-acute character, and it does so by inserting two bytes into the array: 195 137. On the other hand, our ASCII bytes have been decoded and we see that the first character has become a question mark. If you look at the encoded bytes, you ll see that the first byte is 63, which (if you look it up in an ASCII table somewhere) corresponds to the question mark character. So this isn t the fault of the decoder. The encoder, when faced with a character it didn t understand, inserted a question mark.
So, you need to be careful that any encoding you choose is capable of supporting the characters you are using (or be prepared for the information loss if it doesn t).
OK, we ve seen an example of the one-byte-per-character ASCII representation, and the at-least-one-byte-per-character UTF-8 representation. Let s have a look at the underlying at-least-two-bytes-per-character UTF-16 encoding that the framework uses internally Example 10-83 uses this.
static void Main(string[] args) { string listenUpFR = " coute-moi!"; byte[] utf16Bytes = Encoding.Unicode.GetBytes(listenUpFR); Console.WriteLine("UTF-16"); Console.WriteLine("-----"); foreach (var encodedByte in utf16Bytes) {
} }
Console.Write(encodedByte); Console.Write(" ");
Console.ReadKey();
Notice that we re using the Unicode encoding this time. If we compile and run, we see the following output:
UTF-16 ----201 0 99 0 111 0 117 0 116 0 101 0 45 0 109 0 111 0 105 0 33 0
It is interesting to compare this with the ASCII output we had before:
ASCII ----63 99 111 117 116 101 45 109 111 105 33
The first character is different, because UTF-16 can encode the E-acute correctly; thereafter, every other byte in the UTF-16 array is zero, and the next byte corresponds to the ASCII value. As we said earlier, the Unicode standard is highly compatible with ASCII, and each 16-bit value (i.e., pair of bytes) corresponds to the equivalent 7-bit value in the ASCII encoding. There s one more note to make about this byte array, which has to do with the order of the bytes. This is easier to see if we first update the program to show the values in hex, using the formatting function we learned about earlier, as Example 10-84 shows.
static void Main(string[] args) { string listenUpFR = " coute-moi!"; byte[] utf16Bytes = Encoding.Unicode.GetBytes(listenUpFR); Console.WriteLine("UTF-16"); Console.WriteLine("-----"); foreach (var encodedByte in utf16Bytes) { Console.Write(string.Format("{0:X2}", encodedByte)); Console.Write(" "); } } Console.ReadKey();
If we run again, we now see our bytes written out in hex format:
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