Question

I declared the dictionary obj.

Dictionary<string, string> aDict = new Dictionary<string, string>(); 
        aDict .Add("IP", "Host"); 

As I remembered,

The expression of aDict[IP] can return the value (Host).

Now if I go in the opposite direction.

How to get the Key from Value ? aDict[Host] ?

Does the Dictionary is a one-way street in C#, only running from Key to Value ? thanks.

Answer 1

Dictionaries do not work like this, nor are they intended to. How would you resolve the following:

key = "1", value = "x",
key = "2", value = "x"

You could do this:

var keys = dict.Where(kvp => kvp.Value == someValue).Select(kvp => kvp.Key);
foreach(var key in keys) {
    Console.WriteLine(key);
}

But if you really need to go back and forth between keys and values you should consider encapsulating the problem into a two-way map. Here’s a very simple implementation of this that you can tailor to your needs:

class TwoWayDictionary<TLeft, TRight> {
    IDictionary<TLeft, TRight> leftToRight = new Dictionary<TLeft, TRight>();
    IDictionary<TRight, TLeft> rightToLeft = new Dictionary<TRight, TLeft>();

    public void Add(TLeft left, TRight right) {
        if (leftToRight.ContainsKey(left)) {
            throw new InvalidOperationException("key left is duplicate");
        }
        if (rightToLeft.ContainsKey(right)) {
            throw new InvalidOperationException("key right is duplicate");
        }
        leftToRight.Add(left, right);
        rightToLeft.Add(right, left);
    }

    public bool TryGetRightByLeft(TLeft left, out TRight right) {
        return leftToRight.TryGetValue(left, out right);
    }

    public bool TryGetLeftByRight(out TLeft left, TRight right) {
        return rightToLeft.TryGetValue(right, out left);
    }
}

Note that this assumes that no key is ever duplicated.

Now you can say:

TwoWayDictionary<string, string> dict = new TwoWayDictionary<string, string>();
dict.Add("127.0.0.1", "localhost");

string host;
dict.TryGetRightByLeft("127.0.0.1", out host);
// host is "localhost"

string ip;
dict.TryGetLeftByRight("localhost", out ip);
// ip is "127.0.0.1"