BitmapFileHeader.cs

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

namespace SL3Reader;

[StructLayout(LayoutKind.Sequential, Size = Size, Pack = 2), // Pack is important!
 SkipLocalsInit] 
internal struct BitmapFileHeader
{
    internal const int Size = 14;

    private ushort Type;
    private uint FileSize;
    private ushort Reserved1;
    private ushort Reserved2;
    private uint OffsetOfImageData;

    internal BitmapFileHeader(int width, int height) => 
        Update(width, height);

    internal uint Update(int width, int height)
    {
        Type = 19778; // This is "BM" in ASCII.
        FileSize = (uint)(BitmapHelper.BitmapCombinedHeaderSize + height * width); // (width + (4 - width % 4))
        Reserved1 = 0; Reserved2 = 0;
        OffsetOfImageData = BitmapHelper.BitmapCombinedHeaderSize;
        return FileSize;
    }
}