The Limber Lambda

So, you’ve used delegates with gay abandon and everything just seems to work.  In particular, this sort of thing works without a hitch:

	class Proggy
	{
		public static void Main(string[] args)
		{
			var p = new Proggy();
			Console.WriteLine(p.StuffThatNeedsDoing()());
			Console.ReadLine();
		}

		public Func<int> StuffThatNeedsDoing()
		{
			var ia = new int[1];
			ia[0] = 2;
			return (() => ia[0] * ia[0]);
		}
	}

I’m referring, of course, to the bit that returns a delegate that references a method variable. I’ve always been curious as to how that works, so I dug into the IL to reveal the magic.

The interesting bit is the implementation of StuffThatNeedsDoing():

Let’s take a closer look as to what is happening here:

• Behind the scenes, the compiler generates a nested class called <>c__DisplayClass1
  and encapsulates our int[] in it.
• L_0000 to L_000e news up an instance of <>c__DisplayClass1 and assigns the ia field therein to a new int[] of size 1.
• L_0013 to L_001b assigns the value 2 to element 0 of ia within the instance.
• L_001c to L_0028 news up a Func<int>, passing to the constructor the reference to <>c__DisplayClass1 and the address of a method on <>c__DisplayClass1 called <StuffThatNeedsDoing>b__0 (which turns out to be our delegate implementation).
• The new Func<int> is then effectively returned.

In short – it looks (unsurprisingly) like the local ia is no longer a local, and naively referencing it from within the delegate has caused it to be treated like a heap variable.  Of course our “local” ia now has a lifetime consistent with the lifetime of the delegate returned by StuffThatNeedsDoing—not a problem if we don’t mind, but what if we had referenced a “local” IDataReader from within our delegate?  The IDataReader, and associated unmanaged resources, would lurk around possibly for a very long time … something to be aware of.

So, as it turns out, the thing that binds a function to it’s “environment” (in this case, the local that we referenced) is known as a closure.