https://github.com/dotnet/coreclr/issues/10449 StructLayout

Program.cs

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

class Program
{
    static unsafe void Main()
    {
        Console.WriteLine($"Sequential: {sizeof(SequentialS)}");
        Console.WriteLine($"Auto: {sizeof(AutoS)}");

        Console.WriteLine(ToTuple(new AutoS()));
        Console.WriteLine(ToTuple(new SequentialS()));
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    static (short, short, short, DateTime, short) ToTuple(SequentialS s) =>
        (s._a, s._b, s._c, s._dateTimeOffset._dateTime, s._dateTimeOffset._offsetMinutes);

    [MethodImpl(MethodImplOptions.NoInlining)]
    static (short, short, short, DateTime, short) ToTuple(AutoS s) =>
        (s._a, s._b, s._c, s._dateTimeOffset._dateTime, s._dateTimeOffset._offsetMinutes);
}

[StructLayout(LayoutKind.Auto)]
struct MyDateTimeOffset
{
    public DateTime _dateTime;
    public short _offsetMinutes;
}

[StructLayout(LayoutKind.Sequential)]
struct SequentialS
{
    public short _a;
    public short _b;
    public short _c;
    public MyDateTimeOffset _dateTimeOffset;
}

[StructLayout(LayoutKind.Auto)]
struct AutoS
{
    public short _a;
    public short _b;
    public short _c;
    public MyDateTimeOffset _dateTimeOffset;
}