Load Icon Data From File. Sideline of my other program using UpdateResource.

IconLoader.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Runtime.InteropServices;

// https://stackoverflow.com/a/36795367
// http://schima.hatenablog.com/entry/20090512/1242139542
// https://blogs.msdn.microsoft.com/oldnewthing/20120720-00/?p=7083

namespace IconLoadToStruct {
    class IconLoader {

        List<IconStore> LoadIcons(string iconFilename) {

            var icons = new List<IconStore>();
            var stream = File.OpenRead(iconFilename);

            using (var br = new BinaryReader(stream)) {
                var header = LoadToValueClass<IconFileHeader>(br);
                if (header.Type != 1) {
                    throw new Exception($"Invalid type. expected 1 but {header.Type}");
                }

                for (var i = 0; i < header.NumOfImages; i++) {
                    var dest = LoadToValueClass<IconBasicInfo>(br);
                    Console.WriteLine("bps = {0}, w:{1} h:{2}, offset:{3}", dest.BitsPerPixel, dest.Width == 0 ? 256 : dest.Width, dest.Height == 0 ? 256 : dest.Height, dest.Offset);
                    icons.Add(new IconStore {
                        Info = dest,
                    });
                }

                foreach (var icon in icons) {
                    if (br.BaseStream.Position < icon.Info.Offset) {
                        // skip to offset pos
                        br.ReadBytes((int)(icon.Info.Offset - br.BaseStream.Position));
                    }
                    icon.ImageData = br.ReadBytes((int)icon.Info.DataSize);
                }
            }
            return icons;
        }

        T LoadToValueClass<T>(BinaryReader br) where T : new() {
            var dest = new T();
            var size = Marshal.SizeOf(dest);

            IntPtr pBytes = Marshal.AllocHGlobal(size);
            Marshal.Copy(br.ReadBytes(size), 0, pBytes, size);

            Marshal.PtrToStructure<T>(pBytes, dest);
            Marshal.FreeHGlobal(pBytes);

            return dest;
        }

    }

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public class IconFileHeader {
        public ushort Reserved0;
        public ushort Type; // .ico 0, .cur 2
        public ushort NumOfImages;
    }

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public class IconBasicInfo {
        public byte Width;
        public byte Height;
        public byte Colors;
        public byte Reserved0;
        public ushort ColorPlanes;
        public ushort BitsPerPixel;
        public uint DataSize;
        public uint Offset;
    }

    public class IconStore {
        public IconBasicInfo Info;
        public byte[] ImageData;
    }


    public static class IconStoreListExt {
        public static byte[] ToGroupData(this List<IconStore> list, int startNId = 1) {
            using (var ms = new MemoryStream())
            using (var writer = new BinaryWriter(ms)) {
                var i = 0;

                writer.Write((ushort)0);  // reserved
                writer.Write((ushort)1);  // icon
                writer.Write((ushort)list.Count);  // number of icons

                foreach (var icon in list) {
                    writer.Write(icon.Info.Width);
                    writer.Write(icon.Info.Height);
                    writer.Write(icon.Info.Colors);
                    writer.Write((byte)0); // reserved, must be 0
                    writer.Write(icon.Info.ColorPlanes);
                    writer.Write(icon.Info.BitsPerPixel);
                    writer.Write(icon.Info.DataSize);

                    writer.Write((ushort)(startNId + i)); // nId
                    i++;
                }

                return ms.ToArray();
            }

            //typedef struct GRPICONDIR {
            //    WORD idReserved;
            //    WORD idType;
            //    WORD idCount;
            //    GRPICONDIRENTRY idEntries[];
            //} GRPICONDIR;

            //typedef struct GRPICONDIRENTRY {
            //    BYTE bWidth;
            //    BYTE bHeight;
            //    BYTE bColorCount;
            //    BYTE bReserved;
            //    WORD wPlanes;
            //    WORD wBitCount;
            //    DWORD dwBytesInRes;
            //    WORD nId;
            //} GRPICONDIRENTRY;

        }
    }

}

Native.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace IconResource {

    public class Native {
        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern int UpdateResource(IntPtr hUpdate, uint lpType, ushort lpName, ushort wLanguage, byte[] lpData, uint cbData);

        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern IntPtr BeginUpdateResource(string pFileName, [MarshalAs(UnmanagedType.Bool)]bool bDeleteExistingResources);

        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern bool EndUpdateResource(IntPtr hUpdate, bool fDiscard);

        public const uint RT_ICON = 3;
        public const uint RT_GROUP_ICON = 14;

        public const uint LOAD_LIBRARY_AS_DATAFILE = 0x00000002;


        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern IntPtr LoadLibraryEx(string lpFileName, IntPtr hFile, uint dwFlags);

        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern bool FreeLibrary(IntPtr hModule);



        [DllImport("kernel32.dll")]
        static extern bool EnumResourceNames(IntPtr hModule, string lpszType, EnumResNameProcDelegate lpEnumFunc, IntPtr lParam);

        [DllImport("kernel32.dll")]
        static extern bool EnumResourceNames(IntPtr hModule, int dwID, EnumResNameProcDelegate lpEnumFunc, IntPtr lParam);


        delegate bool EnumResNameProcDelegate(IntPtr hModule, IntPtr lpszType, IntPtr lpszName, IntPtr lParam);

        enum ResType {
            CURSOR = 1,
            BITMAP = 2,
            ICON = 3,
            MENU = 4,
            DIALOG = 5,
            STRING = 6,
            FONTDIR = 7,
            FONT = 8,
            ACCELERATOR = 9,
            RCDATA = 10,
            MESSAGETABLE = 11,
            GROUP_CURSOR = 12,
            GROUP_ICON = 14,
            VERSION = 16,
            DLGINCLUDE = 17,
            PLUGPLAY = 19,
            VXD = 20,
            ANICURSOR = 21,
            ANIICON = 22,
            HTML = 23,
            MANIFEST = 24
        }

    }
}

Program.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Runtime.InteropServices;

// https://stackoverflow.com/a/36795367
// http://schima.hatenablog.com/entry/20090512/1242139542
// https://blogs.msdn.microsoft.com/oldnewthing/20120720-00/?p=7083

namespace IconLoadToStruct {
    class Program {

        static void Main(string[] args) {
            var icons = new Program().Load("app.ico");
            Console.ReadKey();
        }

        List<IconStore> Load(string iconFilename) {

            var icons = new List<IconStore>();
            var stream = File.OpenRead(iconFilename);

            using (var br = new BinaryReader(stream)) {
                var header = LoadToValueClass<IconFileHeader>(br);
                if(header.Type != 1) {
                    throw new Exception($"Invalid type. expected 1 but {header.Type}");
                }

                for (var i = 0; i < header.NumOfImages; i++) {
                    var dest = LoadToValueClass<IconBasicInfo>(br);
                    Console.WriteLine("bps = {0}, w:{1} h:{2}, offset:{3}", dest.BitsPerPixel, dest.Width == 0 ? 256 : dest.Width, dest.Height == 0 ? 256 : dest.Height, dest.Offset);
                    icons.Add(new IconStore {
                        Info = dest,
                    });
                }

                foreach (var icon in icons) {
                    if (br.BaseStream.Position < icon.Info.Offset) {
                        // skip to offset pos
                        br.ReadBytes((int)(icon.Info.Offset - br.BaseStream.Position));
                    }
                    icon.ImageData = br.ReadBytes((int)icon.Info.DataSize);
                }
            }
            return icons;
        }

        T LoadToValueClass<T>(BinaryReader br) where T : new(){
            var dest = new T();
            var size = Marshal.SizeOf(dest);

            IntPtr pBytes = Marshal.AllocHGlobal(size);
            Marshal.Copy(br.ReadBytes(size), 0, pBytes, size);
            
            Marshal.PtrToStructure<T>(pBytes, dest);
            Marshal.FreeHGlobal(pBytes);

            return dest;
        }

    }

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public class IconFileHeader {
        public ushort Reserved0;
        public ushort Type; // .ico 0, .cur 2
        public ushort NumOfImages;
    }

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public class IconBasicInfo {
        public byte Width;
        public byte Height;
        public byte Colors;
        public byte Reserved0;
        public ushort ColorPlanes;
        public ushort BitsPerPixel;
        public uint DataSize;
        public uint Offset;
    }

    public class IconStore {
        public IconBasicInfo Info;
        public byte[] ImageData;
    }


    public static class IconStoreListExt {
        public static byte[] ToGroupData(this List<IconStore> list, int startNId = 1) {
            using (var ms = new MemoryStream())
            using (var writer = new BinaryWriter(ms)) {
                var i = 0;

                writer.Write((ushort)0);  // reserved
                writer.Write((ushort)1);  // icon
                writer.Write((ushort)list.Count);  // number of icons

                foreach (var icon in list) {
                    writer.Write(icon.Info.Width);
                    writer.Write(icon.Info.Height);
                    writer.Write(icon.Info.Colors);
                    writer.Write((byte)0); // reserved, must be 0
                    writer.Write(icon.Info.ColorPlanes);
                    writer.Write(icon.Info.BitsPerPixel);
                    writer.Write(icon.Info.DataSize);

                    writer.Write((ushort)(startNId + i)); // nId
                    i++;
                }

                return ms.ToArray();
            }

            //typedef struct GRPICONDIR {
            //    WORD idReserved;
            //    WORD idType;
            //    WORD idCount;
            //    GRPICONDIRENTRY idEntries[];
            //} GRPICONDIR;

            //typedef struct GRPICONDIRENTRY {
            //    BYTE bWidth;
            //    BYTE bHeight;
            //    BYTE bColorCount;
            //    BYTE bReserved;
            //    WORD wPlanes;
            //    WORD wBitCount;
            //    DWORD dwBytesInRes;
            //    WORD nId;
            //} GRPICONDIRENTRY;

        }
    }
    
}

Program2.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace IconResource {
    class Program {
        static void Main(string[] args) {

            IntPtr hModule = Native.LoadLibraryEx("app.exe", IntPtr.Zero, Native.LOAD_LIBRARY_AS_DATAFILE);
            if(hModule != IntPtr.Zero) {
                try {

                    Native.



                } finally {
                    Native.FreeLibrary(hModule);
                }
            }




        }


        public enum ICResult {
            Success,
            FailBegin,
            FailUpdate,
            FailEnd
        }


        //public ICResult ChangeIcon(string exeFilePath, string iconFilePath) {
        //    using (FileStream fs = new FileStream(iconFilePath, FileMode.Open, FileAccess.Read)) {
        //        var reader = new IconReader(fs);

        //        var iconChanger = new IconChanger();
        //        return iconChanger.ChangeIcon(exeFilePath, reader.Icons);
        //    }
        //}

        //public ICResult ChangeIcon(string exeFilePath, Icons icons) {
        //    // Load executable
        //    IntPtr handleExe = Native.BeginUpdateResource(exeFilePath, false);

        //    if (handleExe == null) return ICResult.FailBegin;

        //    ushort startindex = 1;
        //    ushort index = startindex;
        //    ICResult result = ICResult.Success;

        //    var ret = 1;

        //    foreach (var icon in icons) {
        //        // Replace the icon
        //        // todo :Improve the return value handling of UpdateResource
        //        ret = Native.UpdateResource(handleExe, Native.RT_ICON, index, 0, icon.Data, icon.Size);

        //        index++;
        //    }

        //    var groupdata = icons.ToGroupData();

        //    // todo :Improve the return value handling of UpdateResource
        //    ret = Native.UpdateResource(handleExe, Native.RT_GROUP_ICON, startindex, 0, groupdata, (uint)groupdata.Length);
        //    if (ret == 1) {
        //        if (EndUpdateResource(handleExe, false))
        //            result = ICResult.Success;
        //        else
        //            result = ICResult.FailEnd;
        //    } else
        //        result = ICResult.FailUpdate;

        //    return result;
        //}


    }
}