Monday, 15 August 2011

Automatic serialization; what’s in a tuple?

I recently had a lot of reason (read: a serialization snafu) to think about tuples in the context of serialization. Historically, protobuf-net has focused on mutable types, which are convenient to manipulate while processing a protobuf stream (setting fields/properties on a per-field basis). However, if you think about it, tuples have an implicit obvious positional “contract”, so it makes a lot of sense to serialize them automatically. If I write:

var tuple = Tuple.Create(123, "abc");

then it doesn’t take a lot of initiative to think of tuple as an implicit contract with two fields:

  • field 1 is an integer with value 123
  • field 2 is a string with value “abc”

Since it is deeply immutable, at the moment we would need to either abuse reflection to mutate private fields, or write a mutable surrogate type for serialization, with conversion operators, and tell protobuf-net about the surrogate. Wouldn’t it be nice if protobuf-net could make this leap for us?

Well, after a Sunday-night hack it now (in the source code) does.

The rules are:

  • it must not already be marked as an explicit contract
  • only public fields / properties are considered
  • any public fields (spit) must be readonly
  • any public properties must have a get but not a set (on the public API, at least)
  • there must be exactly one interesting constructor, with parameters that are a case-insensitive match for each field/property in some order (i.e. there must be an obvious 1:1 mapping between members and constructor parameter names)

If all of the above conditions are met then it is now capable of behaving as you might hope and expect, deducing the contract and using the chosen constructor to rehydrate the objects. Which is nice! As a few side-benefits:

  • this completely removes the need for the existing KeyValuePairSurrogate<,>, which conveniently meets all of the above requirements
  • it also works for C# anonymous types if we want, since they too have an implicit positional contract (I am not convinced this is significant, but it may have uses)

This should make it into the next deploy, once I’m sure there are no obvious failures in my assumptions.

Just one more thing, sir…

While I’m on the subject of serialization (which, to be fair, I often am) – I have now also completed some changes to use RuntimeHelpers.GetHashCode()for reference-tracking (serialization). This lets met construct a reference-preserving hash-based lookup to minimise the cost of checking whether an object has already been seen (and if so, fetch the existing token). Wins all round.

Friday, 12 August 2011

Shifting expectations: non-integer shift operators in C#

In C#, it is quite nice that the shift operators (<< and >>) have had their second argument constrained to int arguments, avoiding the oft-confusing piped C++ usage:

cout << "abc" << "def";

But wait a minute! The above line is actually C#, in an all-C# project. OK, I cheated a little (I always do…) but genuinely! This little nugget comes from a stackoverflow post that really piqued my curiosity. All credit here goes to vcsjones, but indeed the line in the documentation about restricting to int (and the return type, etc) is about declaring the operator – not consuming it – so it is seemingly valid for the C# dynamic implementation to use it quite happily.

In fact, the main cheat I used here was simply hiding the assignment of the result, since that is still required. Here’s the full evil:

  using System;

using System.Dynamic;
using System.Linq.Expressions;

class Evil : DynamicObject {
static void Main() {
dynamic cout = new Evil();
var hacketyHackHack =
cout << "abc" << "def";
public override bool TryBinaryOperation(
BinaryOperationBinder binder,
object arg, out object result) {
switch(binder.Operation) {
case ExpressionType.LeftShift:
case ExpressionType.LeftShiftAssign:
// or whatever you want to do
result = this;
return true;
return base.TryBinaryOperation(
binder, arg, out result);

I've seen more evil things (subverting new() via ContextBoundObject is still my favorite evil), but quite dastardly, IMO!

Additional: grrr! I don't know why blogger hates me so much; here it is on