using System;

using System.Text;

using System.IO;

​

public class ConstrainedStream : Stream

{

    private readonly Stream _baseStream;

​

    public long ConstrainedLength { get; set; }

​

    public override bool CanRead => _baseStream.CanRead;

    public override bool CanSeek => _baseStream.CanSeek;

    public override bool CanWrite => false;

    public override long Length => Math.Min(ConstrainedLength, _baseStream.Length);

    public override long Position

    {

        get => _baseStream.Position;

        set => _baseStream.Position = value;

    }

​

    public ConstrainedStream(Stream baseStream)

    {

        _baseStream = baseStream;

    }

​

    public override void Flush() => _baseStream.Flush();

​

    public override int Read(byte[] buffer, int offset, int count)

    {

        long maxLength = Math.Min(Length, ConstrainedLength) - Position;

        return _baseStream.Read(buffer, offset, (int)Math.Min(count, maxLength));

    }

​

    public override long Seek(long offset, SeekOrigin origin)

    {

        return _baseStream.Seek(offset, origin);

    }

​

    public override void SetLength(long value) => _baseStream.SetLength(value);

​

    public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();

}

public class Program

{

    public static void Main()

    {

        string content = """

            NRRD000X

            <field>: <desc>

            <field>: <desc>

            # <comment>

            ...

            <field>: <desc>

            <key>:=<value>

            <key>:=<value>

            <key>:=<value>

            # <comment>

​

            <data><data><data><data><data><data>...

            <data><data><data><data><data><data>...

            """;

        byte[] bytes = Encoding.ASCII.GetBytes(content);

        var file = new MemoryStream(bytes);

        var constrainedStream = new ConstrainedStream(file) { ConstrainedLength = 1 };

        var reader = new StreamReader(constrainedStream);

        long startPosition = 0;

        while (true)

        {

            constrainedStream.ConstrainedLength++;

            if (constrainedStream.ConstrainedLength > file.Length)

            {

                // Run out of file

                break; 

            }

            reader.DiscardBufferedData();

            file.Position = startPosition;

            string? line = reader.ReadLine();

            if (line != null && !reader.EndOfStream)

            {

                Console.WriteLine(line);

                startPosition = file.Position - 1;

                if (reader.ReadLine() == "")

                {

                    break;

                }

            }

        }

        Console.WriteLine("Body:");

        Console.WriteLine(new StreamReader(file).ReadToEnd());

    }

}