Question

How can I get the length in characters (not bytes) of a stream / file? Let’s assume the encoding for the file / stream is known (at runtime).

I’d rather not load the whole stream in memory, so I’m against using TextReader.ReadToEnd()

Answer 1

Unless the encoding is a fixed-width one (the same number of bytes for every character – e.g. ASCII but not UTF-8), you’ll need to read the whole file – but it doesn’t need to be in memory. For example:

public long CountCharacters(TextReader reader)
{
    char[] buffer = new char[8 * 1024]; 
    long total = 0;
    int charsRead;
    while ((charsRead = reader.Read(buffer, 0, buffer.Length)) > 0)
    {
        total += charsRead;
    }
    return total;
}

Used like this:

using (var reader = File.OpenText("file.txt", Encoding.UTF8))
{
    Console.WriteLine(CountCharacters(reader));
}

Note that this will count UTF-16 code units, which isn’t quite the same as Unicode characters or displayable glyphs, but in most cases it will be good enough. (Consider cases such as combining characters and surrogate pairs.)