FieldOffsetsInMemory (from https://github.com/kevin-montrose/Jil/blob/519a0c552e9fb93a4df94eed0b2f9804271f2fef/Jil/Serialize/Utils.cs#L320)

gistfile1.cs

public static Dictionary<FieldInfo, int> FieldOffsetsInMemory(Type t)
{
	try
	{
		var fields = t.GetFields(BindingFlags.Public | BindingFlags.Instance | BindingFlags.NonPublic);

		var emit = Emit<Func<object, ulong[]>>.NewDynamicMethod("_GetOffsetsInMemory" + t.FullName);
		var retLoc = emit.DeclareLocal<ulong[]>("ret");

		emit.LoadConstant(fields.Length);	// ulong
		emit.NewArray(typeof(ulong));		// ulong[]
		emit.StoreLocal(retLoc);			// --empty--

		for (var i = 0; i < fields.Length; i++)
		{
			var field = fields[i];

			emit.LoadLocal(retLoc);			// ulong[]
			emit.LoadConstant(i);			// ulong[] ulong

			emit.LoadArgument(0);			// ulong[] ulong param#0
			emit.CastClass(t);	            // ulong[] ulong param#0

			emit.LoadFieldAddress(field);	// ulong[] ulong field&
			emit.Convert<ulong>();			// ulong[] ulong ulong

			emit.StoreElement<ulong>();		// --empty--
		}

		emit.LoadLocal(retLoc);			// ulong[]
		emit.Return();					// --empty--

		var getAddrs = emit.CreateDelegate();

		var cons = t.GetConstructors(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance).OrderBy(p => p.GetParameters().Count()).FirstOrDefault();
		var consParameters = cons != null ? cons.GetParameters().Select(p => p.ParameterType.DefaultValue()).ToArray() : null;

		object obj;
		if (cons != null)
		{
			obj = cons.Invoke(consParameters);
		}
		else
		{
			obj = Activator.CreateInstance(t);
		}

		var addrs = getAddrs(obj);

		if (addrs.Length == 0)
		{
			return new Dictionary<FieldInfo, int>();
		}

		var min = addrs.Min();

		var ret = new Dictionary<FieldInfo, int>();

		for (var i = 0; i < fields.Length; i++)
		{
			var field = fields[i];

			var addr = addrs[i];
			var offset = addr - min;

			ret[field] = (int)offset;
		}

		return ret;
	}
	catch
	{
		// A lot can go wrong during this, and the common response is just to bail
		// This catch is much simpler than trying (and probably failing) to enumerate
		//    all the exceptional cases
		return new Dictionary<FieldInfo, int>();
	}
}