StringReaderStream.cs

using System;
using System.IO;
using System.Text;

public class StringReaderStream : Stream
{
    private readonly string _input;
    private readonly Encoding _encoding;
    private readonly int _maxBytesPerChar;
    private readonly int _inputLength;
    private int _inputPosition;
    private long _position;

    public StringReaderStream(string input)
        : this(input, Encoding.UTF8)
    { }

    public StringReaderStream(string input, Encoding encoding)
    {
        ArgumentNullException.ThrowIfNull(input);
        ArgumentNullException.ThrowIfNull(encoding);

        _encoding = encoding;
        _input = input;
        _inputLength = input.Length;
        Length = encoding.GetByteCount(input);
        _maxBytesPerChar = encoding.GetMaxByteCount(1);
    }

    public override bool CanRead => true;

    public override bool CanSeek => false;

    public override bool CanWrite => false;

    public override long Length { get; }

    public override long Position
    {
        get => _position;
        set => throw new NotSupportedException();
    }

    public override void Flush()
    {
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        return Read(buffer.AsSpan(offset, count));
    }

    public override int Read(Span<byte> buffer)
    {
        if (_inputPosition >= _inputLength) return 0;
        if (buffer.Length < _maxBytesPerChar) throw new ArgumentException("Buffer is too small", nameof(buffer));

        var charCount = Math.Min(_inputLength - _inputPosition, buffer.Length / _maxBytesPerChar);
        var byteCount = _encoding.GetBytes(_input.AsSpan(_inputPosition, charCount), buffer);
        _inputPosition += charCount;
        _position += byteCount;
        return byteCount;
    }

    public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();
    public override void SetLength(long value)  => throw new NotSupportedException();
    public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
}