using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Text;
 
public class Test
{
	[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Size = 24)]
    struct Valid
    {
        public uint Length;
        public uint Version;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
        public byte[] MachineId;
    }
 
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Size = 24)]
    struct Invalid
    {
        public uint Length;
        public uint Version;
        [MarshalAs(UnmanagedType.LPStr, SizeConst = 16)] public string MachineId;
    }
 
	public static void Main()
	{
		var bytes = new byte[]
        {
            0x58, 0, 0, 0,
            0, 0, 0, 0,
            0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4A, 0x4B, 0x4C, 0x4D, 0x4E, 0x4F, 0
        };
 
        var pinnedBuffer = GCHandle.Alloc(bytes, GCHandleType.Pinned);
        var ptr = pinnedBuffer.AddrOfPinnedObject();
 
        // Works!
        Debug.Assert(bytes.Length == Marshal.SizeOf<Valid>());
 
        var trackerData1 = Marshal.PtrToStructure<Valid>(ptr);
 
        Console.WriteLine("Length: {0}", trackerData1.Length);
        Console.WriteLine("Version: {0}", trackerData1.Version);
        Console.WriteLine("MachineId: {0}", Encoding.ASCII.GetString(trackerData1.MachineId));
 
        // Doesnt work!
        Debug.Assert(bytes.Length == Marshal.SizeOf<Invalid>());
 
        // Throws System.AccessViolationException
        var trackerData2 = Marshal.PtrToStructure<Invalid>(ptr);
 
        Console.WriteLine("Length: {0}", trackerData2.Length);
        Console.WriteLine("Version: {0}", trackerData2.Version);
        Console.WriteLine("MachineId: {0}", trackerData2.MachineId);
 
        pinnedBuffer.Free();
 
        Console.In.ReadLine();
	}
}

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