using System;
using System.Buffers;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

public class Program
{
    public static void Main()
    {
        MemoryManager<byte> memoryManager = new CustomMemoryManager<byte>(4);
        ReadOnlySequence<byte> ros = new ReadOnlySequence<byte>(memoryManager.Memory.Slice(1));
        Console.WriteLine(ros.Length); // prints 2 instead of 3

        foreach (var item in ros)
        {
            Console.WriteLine(item.Length); // prints 2 instead of 3
        }

        memoryManager = new CustomMemoryManager<byte>(3);
        ros = new ReadOnlySequence<byte>(memoryManager.Memory.Slice(2));
        Console.WriteLine(ros.Length); // prints -1 instead of 1

        foreach (var item in ros) // throws System.ArgumentOutOfRangeException
        {
            Console.WriteLine(item.Length); // never gets printed
        }
    }
}

public unsafe class CustomMemoryManager<T> : MemoryManager<T>
{
    private readonly T[] _buffer;

    public CustomMemoryManager(int size) => _buffer = new T[size];

    public unsafe override Span<T> GetSpan() => _buffer;

    public override unsafe MemoryHandle Pin(int elementIndex = 0)
    {
        if ((uint)elementIndex > (uint)_buffer.Length)
        {
            throw new ArgumentOutOfRangeException(nameof(elementIndex));
        }

        var handle = GCHandle.Alloc(_buffer, GCHandleType.Pinned);
        return new MemoryHandle(Unsafe.Add<T>((void*)handle.AddrOfPinnedObject(), elementIndex), handle, this);
    }

    public override void Unpin() { }

    protected override void Dispose(bool disposing) { }
}