Naive OLE file header parser based on inline AAF spec comments from olefile

OleReader.cs

using System;
using System.IO;
using System.Runtime.InteropServices;

public class OleReader
{
    public OleReader(string filename, long offset = 0L)
    {
        if (string.IsNullOrWhiteSpace(filename))
        {
            throw new ArgumentNullException("filename");
        }
        if (!File.Exists(filename))
        {
            throw new IOException("File '" + filename + "' not found");
        }
        if (offset < 0L)
        {
            throw new ArgumentOutOfRangeException("offset");
        }
        this.reader = new BinaryReader(File.OpenRead(filename));
        this.offset = offset;
    }

    public OleHeader ParseHeader()
    {
        byte[] readBuffer = this.reader.ReadBytes(512);
        OleHeader result;
        GCHandle handle;
        try
        {
            handle = GCHandle.Alloc(readBuffer, GCHandleType.Pinned);
            result = (OleHeader)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(OleHeader));
        }
        finally
        {
            handle.Free();
        }
        return result;
    }

    private BinaryReader reader;

    private long offset;
}

[StructLayout(LayoutKind.Sequential, Pack = 1, Size = 4)]
public struct FATEntry
{
    [MarshalAs(UnmanagedType.I1)]
    public byte A;

    [MarshalAs(UnmanagedType.I1)]
    public byte B;

    [MarshalAs(UnmanagedType.I1)]
    public byte C;

    [MarshalAs(UnmanagedType.I1)]
    public byte D;
}

[StructLayout(LayoutKind.Explicit)]
public struct OleHeader
{
    [FieldOffset(0)]
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
    public byte[] MagicBytes;

    [FieldOffset(8)]
    [MarshalAs(UnmanagedType.Struct)]
    public Guid CLSID;

    [FieldOffset(24)]
    [MarshalAs(UnmanagedType.U2)]
    public ushort MinorVersion;

    [FieldOffset(26)]
    [MarshalAs(UnmanagedType.U2)]
    public ushort DllVersion;

    [FieldOffset(28)]
    [MarshalAs(UnmanagedType.U2)]
    public ushort ByteOrder;

    [FieldOffset(30)]
    [MarshalAs(UnmanagedType.U2)]
    public ushort SectorShift;

    [FieldOffset(32)]
    [MarshalAs(UnmanagedType.U2)]
    public ushort MiniSectorShift;

    [FieldOffset(34)]
    [MarshalAs(UnmanagedType.U2)]
    public ushort Reserved;

    [FieldOffset(36)]
    [MarshalAs(UnmanagedType.U4)]
    public int Reserved1;

    [FieldOffset(40)]
    [MarshalAs(UnmanagedType.U4)]
    public int SectDir;

    [FieldOffset(44)]
    [MarshalAs(UnmanagedType.U4)]
    public int SectCount;

    [FieldOffset(48)]
    [MarshalAs(UnmanagedType.U4)]
    public int SectDirStart;

    [FieldOffset(52)]
    [MarshalAs(UnmanagedType.U4)]
    public int Signature;

    [FieldOffset(56)]
    [MarshalAs(UnmanagedType.U4)]
    public int MiniSectorCutoff;

    [FieldOffset(60)]
    [MarshalAs(UnmanagedType.U4)]
    public int SectMiniFatStart;

    [FieldOffset(64)]
    [MarshalAs(UnmanagedType.U4)]
    public int SectMiniFat;

    [FieldOffset(68)]
    [MarshalAs(UnmanagedType.U4)]
    public int SectDifStart;

    [FieldOffset(72)]
    [MarshalAs(UnmanagedType.U4)]
    public int SectDif;

    [FieldOffset(76)]
    [MarshalAs(UnmanagedType.Struct)]
    public FATEntry FATHead;

    [FieldOffset(80)]
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 432)]
    public byte[] FATs;
}