Example ICNS encoder using ImageSharp

ICNSEncoder.cs

using System;
using System.Linq;
using System.IO;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Processing;
using SixLabors.ImageSharp.PixelFormats;

namespace EQL.IconAutosizer.ImageEncoders
{
    //https://en.wikipedia.org/wiki/Apple_Icon_Image_format

    public static class ICNSEncoder
    {
        private struct IconType
        {
            public string OsType { get; }
            public int Size { get; }

            public IconType(string osType, int size)
            {
                OsType = osType;
                Size = size;
            }
        }

        private static void ResizeImageToPngBinaryWriter(BinaryWriter binaryWriter, Image<Bgra32> image, int size, out int imageSize)
        {
            using (var sizedImage = image.Clone(i => i.Resize(size, size)))
            {
                var position = binaryWriter.BaseStream.Position;
                sizedImage.SaveAsPng(binaryWriter.BaseStream);
                imageSize = (int)(binaryWriter.BaseStream.Position - position);
            }
        }

        private static byte[] IntToBigEndian(int value)
        {
            byte[] bytes = BitConverter.GetBytes(value);
            if (BitConverter.IsLittleEndian)
            {
                Array.Reverse(bytes);
            }
            return bytes;
        }

        public static void Encode(string filePath, Image<Bgra32> image)
        {
            using (var fileStream = File.Create(filePath))
            {
                Encode(fileStream, image);
            }
        }

        private static IconType[] GetIconTypes()
        {
            return new IconType[]
            {
                new IconType("icp4", 16),
                new IconType("icp5", 32),
                new IconType("icp6", 64),
                new IconType("ic07", 128),
                new IconType("ic08", 256),
                new IconType("ic09", 512),
                new IconType("ic10", 1024),
                new IconType("ic11", 32),
                new IconType("ic12", 64),
                new IconType("ic13", 256),
                new IconType("ic14", 512)
            };
        }

        public static void Encode(Stream stream, Image<Bgra32> image)
        {
            if (!stream.CanSeek)
            {
                throw new Exception("Seekable stream is required");
            }

            if (image.Width != image.Height || image.Width < 16)
            {
                throw new Exception("Image should be square and at least 16px in size");
            }

            var iconTypes = GetIconTypes().Where(i => i.Size < image.Width).ToArray();

            var totalSize = 8;

            using (var binaryWriter = new BinaryWriter(stream))
            {
                binaryWriter.Write("icns".ToCharArray());
                binaryWriter.Write(0);
                foreach (var iconType in iconTypes)
                {
                    stream.Seek(0, SeekOrigin.End);
                    binaryWriter.Write(iconType.OsType.ToCharArray());
                    binaryWriter.Write(0);
                    ResizeImageToPngBinaryWriter(binaryWriter, image, iconType.Size, out var imageSize);
                    stream.Position = totalSize + 4;
                    var size = imageSize + 8;
                    binaryWriter.Write(IntToBigEndian(size));
                    totalSize += size;
                }
                stream.Position = 4;
                binaryWriter.Write(IntToBigEndian(totalSize));
                binaryWriter.Flush();
            }
        }
    }
}