Encoding.UTF8.GetString doesn't take into account the Preamble/BOM

user_17772 0
Viewed 12841

In .NET, I'm trying to use Encoding.UTF8.GetString method, which takes a byte array and converts it to a string.

It looks like this method ignores the BOM (Byte Order Mark), which might be a part of a legitimate binary representation of a UTF8 string, and takes it as a character.

I know I can use a TextReader to digest the BOM as needed, but I thought that the GetString method should be some kind of a macro that makes our code shorter.

Am I missing something? Is this like so intentionally?

Here's a reproduction code:

static void Main(string[] args)
{
    string s1 = "abc";
    byte[] abcWithBom;
    using (var ms = new MemoryStream())
    using (var sw = new StreamWriter(ms, new UTF8Encoding(true)))
    {
        sw.Write(s1);
        sw.Flush();
        abcWithBom = ms.ToArray();
        Console.WriteLine(FormatArray(abcWithBom)); // ef, bb, bf, 61, 62, 63
    }

    byte[] abcWithoutBom;
    using (var ms = new MemoryStream())
    using (var sw = new StreamWriter(ms, new UTF8Encoding(false)))
    {
        sw.Write(s1);
        sw.Flush();
        abcWithoutBom = ms.ToArray();
        Console.WriteLine(FormatArray(abcWithoutBom)); // 61, 62, 63
    }

    var restore1 = Encoding.UTF8.GetString(abcWithoutBom);
    Console.WriteLine(restore1.Length); // 3
    Console.WriteLine(restore1); // abc

    var restore2 = Encoding.UTF8.GetString(abcWithBom);
    Console.WriteLine(restore2.Length); // 4 (!)
    Console.WriteLine(restore2); // ?abc
}

private static string FormatArray(byte[] bytes1)
{
    return string.Join(", ", from b in bytes1 select b.ToString("x"));
}
unicode character-encoding byte-order-mark .net
4 Answers

Based on the answer by Jon Skeet (thanks!), this is how I just did it:

var memoryStream = new MemoryStream(byteArray);
var s = new StreamReader(memoryStream).ReadToEnd();

Note that this will probably only work reliably if there is a BOM in the byte array you are reading from. If not, you might want to look into another StreamReader constructor overload which takes an Encoding parameter so you can tell it what the byte array contains.

user_2277790

for those who do not want to use streams I found a quite simple solution using Linq:

public static string GetStringExcludeBOMPreamble(this Encoding encoding, byte[] bytes)
{
    var preamble = encoding.GetPreamble();
    if (preamble?.Length > 0 && bytes.Length >= preamble.Length && bytes.Take(preamble.Length).SequenceEqual(preamble))
    {
        return encoding.GetString(bytes, preamble.Length, bytes.Length - preamble.Length);
    }
    else
    {
        return encoding.GetString(bytes);
    }
}
user_26452070
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