Get key by value in hash table c#

Question

I want to get the key of my value but this does not Possible in Hashtable
Is there data stretcher to do this ??

    Hashtable x = new Hashtable();
    x.Add("1", "10");
    x.Add("2", "20");
    x.Add("3", "30");

    x.GetKey(20);//equal 2

You can do that with a dictionary. See more here: http://stackoverflow.com/questions/255341/getting-key-of-value-of-a-generic-dictionary — Variant, Aug 29 '12 at 12:13
This completely defeats the purpose of the `Hashtable`. Why would you do that? BTW: Since .NET 2.0 you should be using `Dictionary` instead. — Daniel Hilgarth, Aug 29 '12 at 12:13

score 9 · Accepted Answer · edited Mar 29 '13 at 08:48

9

If all your keys are strings.

var key = x.Keys.OfType<String>().FirstOrDefault(s => x[s] == "20")

or better use a Dictionary instead:

Dictionary<string, string> x = new Dictionary<string, string>();
x.Add("1", "10");
x.Add("2", "20");
x.Add("3", "30");

string result = x.Keys.FirstOrDefault(s => x[s] == "20");

If you know that your value will always have just one distinct key, use Single instead of FirstOrDefault.

edited Mar 29 '13 at 08:48

Matthew Strawbridge

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answered Aug 29 '12 at 12:12

sloth

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Thank you Dictionary is better – motaz99 Aug 29 '12 at 12:19

score 3 · Answer 2 · answered Aug 29 '12 at 12:13

3

If you looking for O(1) solution, then you may need to implement the reversed Hashtable as well where the value of this table will be key and key becomes corresponding value.

answered Aug 29 '12 at 12:13

sundar

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score 2 · Answer 3 · answered Aug 29 '12 at 12:13

2

You can use Linq operators

x.Keys.OfType<String>().FirstOrDefault(a => x[a] == "20")

You can iterate with foreach

answered Aug 29 '12 at 12:13

Aghilas Yakoub

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score 1 · Answer 4 · answered Aug 29 '12 at 12:18

I strongly recommend to use two Dictionary<string, string> but this would mean that their is a 1-1 relationship between the items:

var dict1 = new Dictionary<string, string>();
var dict2 = new Dictionary<string, string>();

dict1.Add("1", "10");
dict2.Add("10", "1");

dict1.Add("2", "20");
dict2.Add("20", "2");

dict1.Add("3", "30");
dict2.Add("30", "3");

var valueOfDictOne = dict1["1"];
var valueOfDictTwo = dict2["10"];

Get key by value in hash table c#

4 Answers4