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
Console.WriteLine(arg);
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 pastie.org.

1 comment:

configurator said...

I'm pretty sure if you define the operator in IL you could still call it from C#, resulting in code similar to Ruby's << operator:
var list = new MyList { 1, 2, 3 };
list = list << 4 << 5;
Console.WriteLine(list); // 1,2,3,4,5