How do you read UTF-8 characters from an infinite byte stream - C#

user_3995 0
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Normally, to read characters from a byte stream you use a StreamReader. In this example I'm reading records delimited by '\r' from an infinite stream.

using(var reader = new StreamReader(stream, Encoding.UTF8))
{
    var messageBuilder = new StringBuilder();
    var nextChar = 'x';
    while (reader.Peek() >= 0)
    {
        nextChar = (char)reader.Read()
        messageBuilder.Append(nextChar);

        if (nextChar == '\r')
        {
            ProcessBuffer(messageBuilder.ToString());
            messageBuilder.Clear();
        }
    }
}

The problem is that the StreamReader has a small internal buffer, so if the code waiting for an 'end of record' delimiter ('\r' in this case) it has to wait until the StreamReader's internal buffer is flushed (usually because more bytes have arrived).

This alternative implementation works for single byte UTF-8 characters, but will fail on multibyte characters.

int byteAsInt = 0;
var messageBuilder = new StringBuilder();
while ((byteAsInt = stream.ReadByte()) != -1)
{
    var nextChar = Encoding.UTF8.GetChars(new[]{(byte) byteAsInt});
    Console.Write(nextChar[0]);
    messageBuilder.Append(nextChar);

    if (nextChar[0] == '\r')
    {
        ProcessBuffer(messageBuilder.ToString());
        messageBuilder.Clear();
    }
}

How can I modify this code so that it works with multi-byte characters?

stream c#
4 Answers

Mike, I found your solution perfect for my situation as well. But I noticed that sometimes it takes four GetChar() calls to determine the characters to be returned. This meant that charCount was 2, while my nextChar buffer size was 1. So I got error "The output character buffer is too small to contain the decoded characters, encoding Unicode fallback System.Text.DecoderReplacementFallback."

I changed my code to:

    // ...
    var nextChar = new char[4];  // 2 might suffice

    for (var i = startPos; i < bytesRead; i++)
    {
        int charCount;
        //...
        charCount = decoder.GetChars(buffer, i, 1, nextChar, 0);

        if (charCount == 0)
        {
            bytesSkipped++;
            continue;
        }

        for (int ic = 0; ic < charCount; ic++)
        {
            char c = nextChar[ic];
            charPos++;

            // Process character here...
        }
    }
user_104453230
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