Mono structure marshalling

using System;
using System.Linq;
using System.Runtime.InteropServices;

namespace explicit1
{
    class Program
    {

        public ImageNtHeaders32 NtHeaders { get; set; }

        public static T GetStructure<T>(byte[] data, int offset = 0)
        {
            var ptr = Marshal.AllocHGlobal(data.Length);
            Marshal.Copy(data, offset, ptr, data.Length - offset);
            var obj = (T)Marshal.PtrToStructure(ptr, typeof(T));
            Marshal.FreeHGlobal(ptr);

            return obj;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct ImageFileHeader32
        {
            public ushort Machine;
            public ushort NumberOfSections;
            public uint TimeDateStamp;
            public uint PointerToSymbolTable;
            public uint NumberOfSymbols;
            public ushort SizeOfOptionalHeader;
            public ushort Characteristics;
        }

        public enum MagicType : ushort
        {
            ImageNtOptionalHdr32Magic = 0x10B,
            ImageNtOptionalHdr64Magic = 0x20B
        }


        [StructLayout(LayoutKind.Explicit)]
        public struct ImageOptionalHeader32
        {
            [FieldOffset(0)]
            public MagicType Magic;

            [FieldOffset(2)]
            public byte MajorLinkerVersion;
        }


        [StructLayout(LayoutKind.Explicit)]
        public struct ImageNtHeaders32
        {
            [FieldOffset(0)]
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 4)]
            public char[] Signature;

            [FieldOffset(4)]
            public ImageFileHeader32 FileHeader;

            [FieldOffset(24)]
            public ImageOptionalHeader32 OptionalHeader;

            /// <summary>
            /// Gets if this structure is valid for a PE file.
            /// </summary>
            public bool IsValid => new string(this.Signature).Trim('\0') == "PE";
        }


        public void RunMe()
        {
            byte[] FileData = Enumerable.Repeat((byte)0x41 , 1000).ToArray();

            this.NtHeaders = new ImageNtHeaders32();
            this.NtHeaders = GetStructure<ImageNtHeaders32>(FileData, 320);

            Console.WriteLine("NtHeaders[0]= " + this.NtHeaders.Signature[1]);
        }

    }

    class trigger
    {
        static void Main(string[] args)
        {
            var inst = new Program();
            inst.RunMe();
        }
    }
}