How can I access the last object added to a HashSet?

Question

I have a HashSet that is a collection of client sockets. When a client connects, the socket is added to the HashSet. I then need to access that socket, but I don't know how to access it in the HashSet.

... 
clientSockets.Add(listenerSocket.EndAccept(async));
WaitForData(lastAddedSocket);
....

How could I determine what lastAddedSocket is?

I think you should actually use Stack generic. http://msdn.microsoft.com/en-us/library/3278tedw.aspx. HashSet is to make sure you won't add duplicated objects. I don't see how HashSet<> brings any value to you here. — Harvey Kwok, Feb 08 '11 at 02:21
@Harvey Kwok It depends on what the author intended. If they truly only want to know the last socket added (for the purpose of calling `WaitForData`), and subsequently treat all sockets as peers in a set, then it makes perfect sense to store them in a "set" data type (such as HashSet), and use a local variable to track the currently-being-inserted socket. It isn't just about preventing duplication; it's imposing a semantics which boldly states "the order of these sockets is not important". The semantics of a Stack implies that you will only *ever* refer to the most recently-added socket. — mgiuca, Feb 08 '11 at 03:00
@mgiuca true - perhaps, he is trying to just hold a reference to it by using `HashSet`. I don't know but it looks weird to me anyway :) — Harvey Kwok, Feb 08 '11 at 03:14
possible duplicate of [Does HashSet preserve insertion order?](http://stackoverflow.com/questions/657263/does-hashset-preserve-insertion-order) — Cody Gray - on strike, Feb 08 '11 at 06:44

score 7 · Accepted Answer · answered Feb 08 '11 at 02:06

There is no way to ask a HashSet "what was the last thing that was added to you?" The order is all jumbled up.

You should simply keep a separate variable called lastAddedSocket. Every time you add a socket to the HashSet, also assign it to lastAddedSocket. Then you will be able to look it up easily and in constant time.

score 2 · Answer 2 · edited May 23 '17 at 12:14

It is generally understood that a hashset's order is not guaranteed. While the Java documentation comes right out and says this the closest the MSDN comes is

A set is a collection that contains no duplicate elements, and whose elements are in no particular order.

Information thanks to: Does HashSet preserve insertion order?

That said, it should be fairly easy for you to preserve your last socket yourself.

score 0 · Answer 3 · answered Feb 08 '11 at 09:34

0

You will probably want your last to be previous-last when you delete the last one. I recommend using Queue<> in addition to the HashSet so you will be able always remember the sequence in which they arrived, in case that last goes away.

answered Feb 08 '11 at 09:34

Daniel Mošmondor

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How can I access the last object added to a HashSet?

3 Answers3