Is the enumeration order of the Stack<T> collection well defined?

[theodorzoulias]

Hi! I used the Stack<T> collection recently to implement an algorithm, whose correctness now depends on the Stack<T> having a LIFO enumeration order. I checked the documentation to see if this enumeration order is well defined, but I can't find an explicit statement about this. I found though an old StackOverflow question, where the accepted answer (by Matthew Watson) includes this excerpt:

The Stack.GetEnumerator() strongly implies that LIFO order is used.

If you look at the example for Stack<T>.GetEnumerator() in Microsoft's documentation and inspect the stated output, you can see that it is in LIFO order.

This strongly suggests that Microsoft fully intend a Stack to be enumerated in LIFO order - but they forgot (or didn't bother to) to explicitly document this!

I would like to ask if the current LIFO behavior is just an implementation detail, or if it's a publicly documented guarantee on which algorithms can rely on.

Thanks!

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Issue Details

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Hi! I used the Stack<T> collection recently to implement an algorithm, whose correctness now depends on the Stack<T> having a LIFO enumeration order. I checked the documentation to see if this enumeration order is well defined, but I can't find an explicit statement about this. I found though an old StackOverflow question, where the accepted answer (by Matthew Watson) includes this excerpt:

The Stack.GetEnumerator() strongly implies that LIFO order is used.

If you look at the example for Stack<T>.GetEnumerator() in Microsoft's documentation and inspect the stated output, you can see that it is in LIFO order.

This strongly suggests that Microsoft fully intend a Stack to be enumerated in LIFO order - but they forgot (or didn't bother to) to explicitly document this!

I would like to ask if the current LIFO behavior is just an implementation detail, or if it's a publicly documented guarantee on which algorithms can rely on.

Thanks!

Author:	theodorzoulias
Assignees:	-
Labels:	area-System.Collections, untriaged
Milestone:	-

[Joe4evr]

The entire reason for Stack<T>'s existence is to be a LIFO collection, so yes, I'd say it's a publicly documented guarantee.

Represents a variable size last-in-first-out (LIFO) collection of instances of the same specified type.

[eiriktsarpalis]

I would say this is very much by design. The examples section in the Stack<T> documentation page implies this by showing the print output of the enumerated stacks.

[GSPP]

I believe that this question has been answered, but the answer does not follow from the documentation. It should be documented.

List, Queue and SortedSet are also not documented in this way.

[eiriktsarpalis]

Fair, I can transfer this issue to dotnet-api-docs. Obviously, we're open to PRs that can improve the current documentation.