c# - with - dictionary get key from value python

get dictionary key by value (5)

Below Code only works if It contain Unique Value Data

public string getKey(string Value)
    if (dictionary.ContainsValue(Value))
        var ListValueData=new List<string>();
        var ListKeyData = new List<string>();

        var Values = dictionary.Values;
        var Keys = dictionary.Keys;

        foreach (var item in Values)

        var ValueIndex = ListValueData.IndexOf(Value);
        foreach (var item in Keys)

        return  ListKeyData[ValueIndex];

    return string.Empty;

How do I get a Dictionary key by value in C#?

Dictionary<string, string> types = new Dictionary<string, string>()
            {"1", "one"},
            {"2", "two"},
            {"3", "three"}

I want something like this:

getByValueKey(string value);

getByValueKey("one") must be return "1".

What is the best way do this? Maybe HashTable, SortedLists?

I have very simple way to do this. It worked out perfect for me.

Dictionary<string, string> types = new Dictionary<string, string>();

types.Add("1", "one");
types.Add("2", "two");
types.Add("3", "three");

Console.WriteLine("Please type a key to show its value: ");
string rLine = Console.ReadLine();

    string value_For_Key = types[rLine];
    Console.WriteLine("Value for " + rLine + " is" + value_For_Key);

Values not necessarily have to be unique so you have to do a lookup. You can do something like this:

var myKey = types.FirstOrDefault(x => x.Value == "one").Key;

If values are unique and are inserted less frequently than read, then create an inverse dictionary where values are keys and keys are values.

You could do that:

  1. By looping through all the KeyValuePair<TKey, TValue>'s in the dictionary (which will be a sizable performance hit if you have a number of entries in the dictionary)
  2. Use two dictionaries, one for value-to-key mapping and one for key-to-value mapping (which would take up twice as much space in memory).

Use Method 1 if performance is not a consideration, use Method 2 if memory is not a consideration.

Also, all keys must be unique, but the values are not required to be unique. You may have more than one key with the specified value.

Is there any reason you can't reverse the key-value relationship?


Values.ToList() converts your dictionary values into a List of objects. IndexOf("one") searches your new List looking for "one" and returns the Index which would match the index of the Key/Value pair in the dictionary.

This method does not care about the dictionary keys, it simply returns the index of the value that you are looking for.

Keep in mind there may be more than one "one" value in your dictionary. And that is the reason there is no "get key" method.