Getting a value from a ConcurrentDictionary

Question

If I have this ConcurrentDictionary:

public class User
{
    public string Context { get; set; }
    public bool Owner { get; set; }
}

protected static ConcurrentDictionary<User, string> OnlineUsers = new ConcurrentDictionary<User, string>();

Does anyone know how I would get the value of Owner if I already have the value of the Context? Basically I want to do a "find" using the Context. Thanks.

Answer 1

It sounds like you need a Dictionary<string, User> which, when given the context as a string will tell you which User it corresponds to. If you are going to need to perform this lookup several times, and there isn't a problem using the additional memory, it may be worth creating such a dictionary.

If you are only going to be doing the search once or twice, or the mappings will be changing so often that you can't keep the dictionary up to date, then you can simply do a linear search on the dictionary using a foreach loop, or using a LINQ method such as First or Where, as demonstrated in other answers.

Answer 2

Does anything stop you from using standard Linq FirstOrDefault() method like so:

var item = OnlineUsers.FirstOrDefault(kvp => kvp.Key.Context == myContext);

Answer 3

How obout something like

var ou = OnlineUsers.First(x => x.Key.Context == "TADA");

Answer 4

Here you go:

ConcurrentDictionary<User, string> dict = new ConcurrentDictionary<User, string>();

dict.TryAdd(new User() { Context = "a", Ownder = false }, "aa");
dict.TryAdd(new User() { Context = "b", Ownder = false }, "bb");
dict.TryAdd(new User() { Context = "c", Ownder = true }, "cc");
dict.TryAdd(new User() { Context = "d", Ownder = false }, "dd");

IEnumerable<User> list = dict.Keys.Where(p => p.Context == "a");