StreamWriter and UTF-8 Byte Order Marks

Question

I'm having an issue with StreamWriter and Byte Order Marks. The documentation seems to state that the Encoding.UTF8 encoding has byte order marks enabled but when files are being written some have the marks while other don't.

I'm creating the stream writer in the following way:

this.Writer = new StreamWriter(this.Stream, System.Text.Encoding.UTF8);

Any ideas on what could be happening would be appreciated.

Answer 1

The only time I've seen that constructor not add the UTF-8 BOM is if the stream is not at position 0 when you call it. For example, in the code below, the BOM isn't written:

using (var s = File.Create("test2.txt"))
{
    s.WriteByte(32);
    using (var sw = new StreamWriter(s, Encoding.UTF8))
    {
        sw.WriteLine("hello, world");
    }
}

As others have said, if you're using the StreamWriter(stream) constructor, without specifying the encoding, then you won't see the BOM.

Answer 2

Seems that if the file already existed and didn't contain BOM, then it won't contain BOM when overwritten, in other words StreamWriter preserves BOM (or it's absence) when overwriting a file.