Why is Marshal.DestroyStructure considered Pure?

Question

Marshal.DestroyStructure is marked with the Pure attribute in the .NET Framework but I'm unsure as to why when it clearly has an effect on the context calling it.

The state is modified (the pointer is freed) even if it doesn't directly modify the pointer instance itself.

Implied in the question is: Can a developer, in good faith, mark something as Pure even if she knows it modifies the context's state indirectly?

Where are you seeing this `Pure` attribute? It's not documented that way and using reflection tools don't show a `PureAttribute` on the method — Peter Ritchie, Oct 24 '12 at 13:20
@Peter, Contract annotations are not directly part of the BCL assemblies; they are applied after the fact through external contract assemblies. — Dan Bryant, Oct 24 '12 at 14:10
@DanBryant Then Code Contracts is making an assumption about Purity. — Peter Ritchie, Oct 24 '12 at 14:17
That's true. In VS2010 the IDE doesn't make any distinction. — Rushyo, Oct 24 '12 at 14:26

score 2 · Accepted Answer · answered Oct 24 '12 at 14:09

2

It's Pure in the sense that it has no effect on visible managed state, which means, for the purpose of code contracts, calling the method could not violate class invariants.

Granted, it's slightly misleading to think of the method as Pure, as it does have side effects, even if those side effects are not visible. They are observable (if you try to use the pointer after freeing it, you'll cause a failure), but not exactly visible (you can't tell without trying to use the pointer that something is wrong.) I'm not sure what the motivation was for marking the method Pure, since I can't see why it would have ever been used in a Contracts block, but I'm guessing there must have been some reason somewhere deep in the BCL contracts that required it.

answered Oct 24 '12 at 14:09

Dan Bryant

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Pure is _explicitly_ for avoiding _observable_ side effects, so the implication here is that they just plain broke their own rules. I'd really love to know why they'd do that. Given what Contracts are all about 'convenience' just doesn't strike me as the answer. – Rushyo Oct 24 '12 at 14:18
@Rushyo, I agree it's not a particularly satisfying rationale, but the only other reason I can think of is that it was a mistake. I do think there is a distinction between plainly visible vs indirectly observable side effects, which could explain why it was marked Pure. – Dan Bryant Oct 24 '12 at 15:06

Why is Marshal.DestroyStructure considered Pure?

1 Answers1