StreamReader.ReadLine fails to return null when end-of-stream is reached

[tintoy]

Hi.

I've run into some puzzling behaviour with StreamReader.ReadLine that appears to deviate that described in the documentation. According to these docs, StreamReader.ReadLine should return null if the end of the input stream is reached (before a line-ending sequence is encountered, I assume, although that assumption may be the source of the problem). But that's not the behaviour I'm seeing.

A simple repro:

byte[] data = Encoding.ASCII.GetBytes("Hello world"); // No newline
using (MemoryStream dataStream = new MemoryStream(data))
using (StreamReader dataReader = new StreamReader(dataStream, Encoding.ASCII, detectEncodingFromByteOrderMarks: false, bufferSize: 5))
{
    string line = dataReader.ReadLine();
    if (line == null)
        Console.WriteLine("No line.");
    else
        Console.WriteLine($"Line: '{line}'.");
}

Am I reading the documentation correctly, or is the implication that ReadLine will return whatever it has even if it has never encountered a new-line sequence?

[danmoseley]

@tintoy my reading of the code is that it does not require a line ending at the end of the stream. Files often don't end in a newline.

https://github.com/dotnet/coreclr/blob/master/src/mscorlib/src/System/IO/StreamReader.cs#L704

I believe this is intended behavior -- perhaps the docs could be clearer, perhaps you might want to open an issue against the docs repo for that ? https://github.com/dotnet/docs

[tintoy]

Yeah now that you put it that way, it does sound like that's the correct behaviour. I'll open a PR for the docs though - maybe I can save someone else from running into the same confusion.

Thanks!

[danmoseley]

Thanks @tintoy !