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ASCIIEncoding Class

Represents an ASCII character encoding of Unicode characters.

Namespace: System.Text
Assembly: mscorlib (in mscorlib.dll)

[SerializableAttribute] 
[ComVisibleAttribute(true)] 
public class ASCIIEncoding : Encoding
/** @attribute SerializableAttribute() */ 
/** @attribute ComVisibleAttribute(true) */ 
public class ASCIIEncoding extends Encoding
SerializableAttribute 
ComVisibleAttribute(true) 
public class ASCIIEncoding extends Encoding
Not applicable.

Encoding is the process of transforming a set of Unicode characters into a sequence of bytes. Decoding is the process of transforming a sequence of encoded bytes into a set of Unicode characters.

ASCIIEncoding corresponds to the Windows code page 20127. ASCII characters are limited to the lowest 128 Unicode characters, from U+0000 to U+007F. For more information about the encodings supported by System.Text, see Understanding Encodings and Using Unicode Encoding.

Caution noteCaution:

ASCIIEncoding does not provide error detection. For security reasons, your application is recommended to use UTF8Encoding, UnicodeEncoding, or UTF32Encoding and enable error detection.

NoteNote:

Since ASCIIEncoding supports only the Unicode character values between U+0000 and U+007F, UTF8Encoding, UnicodeEncoding, and UTF32Encoding are better suited for globalized applications.

The GetByteCount method determines how many bytes result in encoding a set of Unicode characters, and the GetBytes method performs the actual encoding.

Likewise, the GetCharCount method determines how many characters result in decoding a sequence of bytes, and the GetChars and GetString methods perform the actual decoding.

When selecting the ASCII encoding for your applications, consider the following:

  • The ASCII encoding is usually appropriate for protocols that require ASCII.

  • If your application requires 8-bit encoding, the UTF-8 encoding is recommended over the ASCII encoding. For the characters 0-7F, the results are identical, but use of UTF-8 avoids data loss by allowing representation of all Unicode characters that are representable. Note that the ASCII encoding has an 8th bit ambiguity that can allow malicious use, but the UTF-8 encoding removes ambiguity about the 8th bit.

  • Previous versions of .NET Framework allowed spoofing by merely ignoring the 8th bit. The current version has been changed so that non-ASCII code points fall back during the decoding of bytes.

Note that the default ASCIIEncoding constructor by itself might not have the appropriate behavior for your application. You might want to consider having your application set EncoderFallback or DecoderFallback to EncoderExceptionFallback or DecoderExceptionFallback to prevent sequences with the 8th bit set. Custom behavior might also be appropriate for these cases.

The following example demonstrates how to encode Unicode characters into ASCII. Notice the loss of data that occurs when your application uses ASCIIEncoding to encode Unicode characters outside of the ASCII range.

using System;
using System.Text;

class ASCIIEncodingExample {
    public static void Main() {
        // The encoding.
        ASCIIEncoding ascii = new ASCIIEncoding();
        
        // A Unicode string with two characters outside the ASCII code range.
        String unicodeString =
            "This Unicode string contains two characters " +
            "with codes outside the ASCII code range, " +
            "Pi (\u03a0) and Sigma (\u03a3).";
        Console.WriteLine("Original string:");
        Console.WriteLine(unicodeString);

        // Save positions of the special characters for later reference.
        int indexOfPi = unicodeString.IndexOf('\u03a0');
        int indexOfSigma = unicodeString.IndexOf('\u03a3');

        // Encode string.
        Byte[] encodedBytes = ascii.GetBytes(unicodeString);
        Console.WriteLine();
        Console.WriteLine("Encoded bytes:");
        foreach (Byte b in encodedBytes) {
            Console.Write("[{0}]", b);
        }
        Console.WriteLine();
        
        // Notice that the special characters have been replaced with
        // the value 63, which is the ASCII character code for '?'.
        Console.WriteLine();
        Console.WriteLine(
            "Value at position of Pi character: {0}",
            encodedBytes[indexOfPi]
        );
        Console.WriteLine(
            "Value at position of Sigma character: {0}",
            encodedBytes[indexOfSigma]
        );

        // Decode bytes back to string.
        // Notice missing Pi and Sigma characters.
        String decodedString = ascii.GetString(encodedBytes);
        Console.WriteLine();
        Console.WriteLine("Decoded bytes:");
        Console.WriteLine(decodedString);
    }
}

import System.*;
import System.Text.*;
import System.Byte;

class ASCIIEncodingExample
{
    public static void main(String[] args)
    {
        // The encoding.
        ASCIIEncoding ascii = new ASCIIEncoding();

        // A Unicode string with two characters outside the ASCII code range.
        String unicodeString = "This Unicode string contains two characters "
            + "with codes outside the ASCII code range, " 
            + "Pi (\u03a0) and Sigma (\u03a3).";
        Console.WriteLine("Original string:");
        Console.WriteLine(unicodeString);

        // Save positions of the special characters for later reference.
        int indexOfPi = unicodeString.IndexOf('\u03a0');
        int indexOfSigma = unicodeString.IndexOf('\u03a3');

        // Encode string.
        ubyte encodedBytes[] = ascii.GetBytes(unicodeString);
        Console.WriteLine();
        Console.WriteLine("Encoded bytes:");
        for(int iCtr = 0; iCtr < encodedBytes.length; iCtr++) {
            ubyte b = encodedBytes[iCtr];
            Console.Write("[{0}]", String.valueOf((Byte)b));
        }
        Console.WriteLine();

        // Notice that the special characters have been replaced with
        // the value 63, which is the ASCII character code for '?'.
        Console.WriteLine();
        Console.WriteLine("Value at position of Pi character: {0}", 
            encodedBytes.get_Item(indexOfPi));
        Console.WriteLine("Value at position of Sigma character: {0}", 
            encodedBytes.get_Item(indexOfSigma));

        // Decode bytes back to string.
        // Notice missing Pi and Sigma characters.
        String decodedString = ascii.GetString((ubyte[])encodedBytes);
        Console.WriteLine();
        Console.WriteLine("Decoded bytes:");
        Console.WriteLine(decodedString);
    } //main
} //ASCIIEncodingExample

System.Object
   System.Text.Encoding
    System.Text.ASCIIEncoding

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0

XNA Framework

Supported in: 1.0

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