Checking if an 8 bit character is alphanumeric

Question

This C# code would return an unexpected result:

char x = Convert.ToChar(0xff); Console.WriteLine( Char.IsLetterOrDigit(x));

It prints True were I was hoping for a False. I assume this is because IsLetterOrDigit is expecting a Unicode character as input versus the extended ascii value I convert from.

How could I make this work? I am reading a continuous binary string from a serial port where problematic characters needs to be removed for reporting.

As a side note, you could throw a lookup table at this, create an array of 255 booleans and use the value you put into toChar right now as the index. i don't know how much performance is a factor here, if you're processing huge gobs of data it might shave off some processing time in the long run. — Gimby
– Gimby, Commented Sep 8, 2021 at 11:44
0xff is the unicode code-point ÿ, which absolutely is a letter, according the the unicode specification; so: why are you expecting it not to be? what code-point were you expecting? note that "extended ascii" is ambiguous; you need to talk about a specific code-page or encoding for that to make sense — Marc Gravell
– Marc Gravell, Commented Sep 8, 2021 at 11:50

György Kőszeg · Accepted Answer · 2021-09-08 11:53:56Z

Char always represents a Unicode UTF-16 character.

You need to specify which 8 bit code page you use. For example, the OEM US 437 Encoding has a non-letter/digit character at code point #255:

int codePage = 437; var encoding = Encoding.GetEncoding(codePage); char x = encoding.GetChars(new byte[] { 0xFF })[0]; Console.WriteLine(Char.IsLetterOrDigit(x)); // False

Code page 437 (CCSID 437) is the character set of the original IBM PC (personal computer). This is exactly I was referring to - The original IBM PC.

Gerhard · Accepted Answer · 2021-09-08 13:46:28Z

ASCIIEncoding ascii = new ASCIIEncoding(); char x = ascii.GetString( new Byte[] { 0xff })[0]; Console.WriteLine(Char.IsLetterOrDigit(x));

With Rune manipulation

ASCIIEncoding ascii = new ASCIIEncoding(); var x = ascii.GetString(new Byte[] { 0xff }); foreach(var r in x.EnumerateRunes()) { Console.WriteLine(Rune.IsLetterOrDigit(r)); }

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