How to remove an item from a nested ConcurrentDictionary?

Question

What I'm trying to do is to keep online members of group chats in memory. I've defined a static nested dictionary like this:

private static ConcurrentDictionary<string, ConcurrentDictionary<string, ChatMember>> onlineGroupsMembers = new ConcurrentDictionary<string, ConcurrentDictionary<string, ChatMember>>();

Then when a new member arrives, I add it:

        onlineGroupsMembers.AddOrUpdate
            (chatKey,
            (k) => // add new
            {
                var dic = new ConcurrentDictionary<string, ChatMember>();
                dic[chatMember.Id] = chatMember;
                return dic;
            },
            (k, value) => // update
            {
                value[chatMember.Id] = chatMember;
                return value;
            });

Now the problem is how can I delete a member from the inner dictionary? also how to delete a dictionary from outer dictionary when it's empty?

Concurrent dictionary has TryRemove but it does not help and checking for ContainsKey then removing it is not atomic.

Thanks.

@BACON It's not even possible. it gets a key which in my case is group key, and removes the value which is a dictionary of that group members. I just want to remove a single member, not all. — Blendester
– Blendester, Commented Feb 12, 2019 at 2:40
Perhaps a nested concurrent dictionary isn't the best way to go? Either way, it seems that you need a TryGetValue to extract the inner dictionary, and then a TryRemove on the inner dictionary. — ProgrammingLlama
– ProgrammingLlama, Commented Feb 12, 2019 at 2:46
@John Thanks. and for the second part, is it possible to safely remove an inner dictionary if it's empty? — Blendester
– Blendester, Commented Feb 12, 2019 at 2:51
Couldn't you accomplish this with two dictionaries that aren't nested? One that maps from groups to either ChatMembers or member IDs, and another that maps from member IDs to ChatMembers. — Lance U. Matthews
– Lance U. Matthews, Commented Feb 12, 2019 at 2:56

Lance U. Matthews · Accepted Answer · 2019-02-12 03:55:06Z

1

To remove a ChatMember from a group you need to get the ConcurrentDictionary<> for that group with...

var groupDictionary = onlineGroupsMembers["groupID"];

...or...

var groupDictionary = onlineGroupsMembers.TryGetValue("groupID", out ConcurrentDictionary<string, ChatMember> group);

From groupDictionary you would then attempt to remove the member...

var wasMemberRemoved = groupDictionary.TryRemove("memberID", out ChatMember removedMember);

To completely remove a group from onlineGroupsMembers you would call TryRemove directly on that dictionary...

 var wasGroupRemoved = onlineGroupsMembers.TryRemove("groupID", out ConcurrentDictionary<string, ChatMember> removedGroup);

A less cumbersome way to implement this might be with two dictionaries that are not nested. One would map from a group ID to something like a ConcurrentBag<> or a concurrent HashSet<> (if it existed) of its ChatMembers...

ConcurrentDictionary<string, ConcurrentBag<ChatMember>> groupIdToMembers;

...or from a group ID to its member IDs...

ConcurrentDictionary<string, ConcurrentBag<string>> groupIdToMemberIds;

Note that ConcurrentBag<> allows duplicate values.

In the latter case, if you want a quick way to get the ChatMember for a given member ID, you could use another dictionary for that...

ConcurrentDictionary<string, ChatMember> memberIdToMember;

edited Feb 12, 2019 at 3:55

answered Feb 12, 2019 at 3:04

Lance U. Matthews

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4 Comments

Blendester Over a year ago

Thanks.. I want to remove a group only if it's empty though.

Blendester Over a year ago

Ahh, maybe after removing it, check the removed object again to see if it has got some values meanwhile, and add them back!

Lance U. Matthews Over a year ago

Ah, yes. The TValue of the group dictionary needs to be a collection type, or otherwise some kind of ChatGroup type that contains a collection of ChatMembers. That's the tricky part with wanting to remove the group when the last member leaves is ensuring there's not a race condition between removing the group and a new member joining; you can't check the Count of the nested dictionary and then remove it from onlineGroupsMembers atomically.

Blendester Over a year ago

using ConcurrentBag is still nested, and this approach adds a second list that should take care of and keep them synced. Thanks for your answer. maybe I should forget about the dictionaries..

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