UnsafeAsRepro.cs

using System.Runtime.CompilerServices;

var repro = new Example(new object(), 1234, 5678);
PrintAsMinOpts(repro);
PrintAs(repro);
PrintBitcast(repro);

[MethodImpl(MethodImplOptions.AggressiveOptimization)]
static void PrintAsMinOpts(Example example)
{
  Console.WriteLine("--- MinOpts");
  Console.WriteLine($"Offset: {example.OffsetAsMinOpts}");
  Console.WriteLine($"Length: {example.LengthAsMinOpts}");
}

[MethodImpl(MethodImplOptions.AggressiveOptimization)]
static void PrintAs(Example example)
{
  Console.WriteLine("--- As");
  Console.WriteLine($"Offset: {example.OffsetAs}");
  Console.WriteLine($"Length: {example.LengthAs}");
}

[MethodImpl(MethodImplOptions.AggressiveOptimization)]
static void PrintBitcast(Example example)
{
  Console.WriteLine("--- Bitcast");
  Console.WriteLine($"Offset: {example.OffsetBitcast}");
  Console.WriteLine($"Length: {example.LengthBitcast}");
}

readonly struct Example
{
  public readonly object? Value;
  public readonly ulong Inner;

  struct ExampleInner
  {
    public int Offset;
    public int Length;
  }

  [MethodImpl(MethodImplOptions.AggressiveInlining)]
  public Example(object? value, int offset, int length)
  {
    var inner = new ExampleInner
    {
      Offset = offset,
      Length = length
    };

    Value = value;
    Inner = Unsafe.As<ExampleInner, ulong>(ref inner);
  }

  public int OffsetAsMinOpts
  {
    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.NoOptimization)]
    get
    {
      var inner = Inner;
      return Unsafe.As<ulong, ExampleInner>(ref inner).Offset;
    }
  }

  public int OffsetAs
  {
    [MethodImpl(MethodImplOptions.AggressiveOptimization | MethodImplOptions.NoInlining)]
    get
    {
      var inner = Inner;
      return Unsafe.As<ulong, ExampleInner>(ref inner).Offset;
    }
  }

  public int OffsetBitcast => Unsafe.BitCast<ulong, ExampleInner>(Inner).Offset;

  public int LengthAsMinOpts
  {
    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.NoOptimization)]
    get
    {
      var inner = Inner;
      return Unsafe.As<ulong, ExampleInner>(ref inner).Length;
    }
  }

  public int LengthAs
  {
    [MethodImpl(MethodImplOptions.AggressiveOptimization | MethodImplOptions.NoInlining)]
    get
    {
      var inner = Inner;
      return Unsafe.As<ulong, ExampleInner>(ref inner).Length;
    }
  }

  public int LengthBitcast => Unsafe.BitCast<ulong, ExampleInner>(Inner).Length;
}