如何在字典中查找内部哈希集并在 C# 中返回键值？

Question

I am trying to store signalR groups in Dictionary>

But i do not know how to lookup a string value in HashSet and return key string?

    var dict = new Dictionary<string, HashSet<string>>();
    dict["GroupA"] = ["user1","user2","user3"];
    dict["GroupB"] = ["user3", "user4"];
    dict["GroupC"] = ["user4","user5","user6"];

I want to lookup for "user3" and it should returns ["GroupA","GroupB"]. I do not know about that how to use dictionaries in c#. I have around ~20k groups and many of users in it. Does it handle too much groups and users with singleton pattern (store in memory)?

Hope someone help me with an working example. I am beginner here. Thanks!

Answer 1

You need to iterate over the dictionary, and get all the keys that point to a hashset that contains the value you're looking for:

var matches = dict.Where(kvp => kvp.Value.Contains("user3"));

Explanation:
You're asking for all the key-value pairs, where the Value (which we know is of type Hashset) contain the string you're looking for.

Update : to get just the keys from the key-value pairs, I believe you can do this:

var matches = dict.Where(kvp => kvp.Value.Contains("user3")).Select(kvp => kvp.Key);

Further musing: if your use cases will always prioritize finding by user, perhaps you should invert the dictionary: have the user name be the key, and the value would be the groups the user belongs to. That way, finding all groups for a user will be O(1).

Answer 2

Answer will depend on the situation:

• If SQL is an option, then do this in SQL before loading. It is way more efficient in general.
• If you always need to search by user, flip the dictionary, listing groups by member.
• If the lookup happens server-side, I would go with two dictionaries: 1 member by group, 1 group by member. That doubles memory usage, but if you can make these singletons serving all clients from same instances that should be a limited impact.
• If the lookup happens client-side than only load a mini list of groups for each user that they are a member of (either from SQL or from server-side singleton) No need to search any groups for other members in that case I would imagine.