Get key from value - Dictionary>

Question

I am having trouble getting the key by specifying a value. What is the best way I can achieve this?

var st1= new List<string> { "NY", "CT", "ME" };
var st2= new List<string> { "KY", "TN", "SC" };
var st3= new List<string> { "TX", "OK", "MO" };
var statesToEmailDictionary = new Dictionary<string, List<string>>();
statesToEmailDictionary.Add("[email protected]", st1);
statesToEmailDictionary.Add("[email protected]", st2);
statesToEmailDictionary.Add("[email protected]", st3);

var emailAdd = statesToEmailDictionary.FirstOrDefault(x => x.Value.Where(y => y.Contains(state))).Key;

p.s.w.g · Accepted Answer · 2013-05-31T15:35:53.397

19

The return value from FirstOrDefault will be a KeyValuePair<string, List<string>>, so to get the key, simply use the Key property. Like this:

var emailAdd = statesToEmailDictionary
    .FirstOrDefault(x => x.Value.Contains(state))
    .Key;

Alternatively, here's the equivalent in query syntax:

var emailAdd = 
    (from p in statesToEmailDictionary
     where p.Value.Contains(state)
     select p.Key)
    .FirstOrDefault();

edited May 31 '13 at 15:35

answered May 31 '13 at 15:20

p.s.w.g

146,324
30
291
331

Isn't `FirstOrDefault(...).Key` dangerous, as in unexpected behavior when "OrDefault" occurs here? – Chris Marisic Jul 24 '15 at 16:22
2

@ChrisMarisic The 'OrDefault' case returns the default value of the type you're iterating over. For reference types, that's `null`, but for value types, it will be a new instance of that type. Since `KeyValuePair` is a value type (i.e. a `struct`), the 'OrDefault' case will not result in a null-reference exception. Since `TKey` here is `string`, the `.Key` of that new instance will be `null`. – p.s.w.g Jul 24 '15 at 19:23

score 2 · Answer 2 · answered May 31 '13 at 15:28

2

I think you want:

var emailAdd = statesToEmailDictionary.FirstOrDefault(x => x.Value.Any(y => y.Contains(state))).Key;

answered May 31 '13 at 15:28

Adam

741
6
4

score 2 · Answer 3 · answered Oct 16 '14 at 21:20

What everyone in this thread failed to mention is that the FirstOrDefault method is only available through Linq:

using System;
using System.Collections.Generic;
// FirstOrDefault is part of the Linq API
using System.Linq;

namespace Foo {
    class Program {
        static void main (string [] args) {
            var d = new Dictionary<string, string> () {
                { "one", "first" },
                { "two", "second" },
                { "three", "third" }
            };
            Console.WriteLine (d.FirstOrDefault (x => x.Value == "second").Key);
        }
    }
}

Joe Enos · Answer 4 · 2013-05-31T15:28:28.820

1

var emailAdd = statesToEmailDictionary
    .FirstOrDefault(x => x.Value != null && x.Value.Contains(state))
    .Key;

But if you're looking for performance, I'd suggest reversing your dictionary and creating a dictionary of <state, email> to do what you're looking for.

// To handle when it's not in the results
string emailAdd2 = null;
foreach (var kvp in statesToEmailDictionary)
{
    if (kvp.Value != null && kvp.Value.Contains(state))
    {
        emailAdd2 = kvp.Key;
        break;
    }
}

edited May 31 '13 at 15:28

answered May 31 '13 at 15:23

Joe Enos

39,478
11
80
136

any way to handle `NullReferenceException`? – Ilya Ivanov May 31 '13 at 15:23
I don't think so, in a one-liner at least. Edited with a non-LINQ check. It does the same thing as the LINQ one does - loops through each value until it finds one, then breaks out of the loop. If it never finds one, the original value of null is kept. – Joe Enos May 31 '13 at 15:29
1

Actually, if you were getting a NullReferenceException, then the list itself must have been null. You can handle that inside the lambda. Updated. – Joe Enos May 31 '13 at 15:32

score 1 · Answer 5 · edited Dec 16 '14 at 07:52

1

var emailAdd = statesToEmailDictionary.First(x=>x.Value.Contains(state)).Key;

edited Dec 16 '14 at 07:52

Andrey Korneyev

26,353
15
70
71

answered Dec 16 '14 at 07:29

G.J

31
1
4

score 0 · Answer 6 · edited Apr 27 '15 at 22:55

0

Simple Linq to do just that

Dim mKP = (From mType As KeyValuePair(Of <Key type>, <Value type>) In <Dictionary>
           Where mType.Value = <value seeked> Select mType).ToList

If mKP.Count > 0 then
    Dim value as <value type> = mKP.First.Value
    Dim key as <Key type> = mKP.First.Key 
End if

Of course if there are duplicate values this will return more than one KeyValuePair

edited Apr 27 '15 at 22:55

p.s.w.g

146,324
30
291
331

answered Nov 10 '14 at 16:08

Christian Alain Ouellet

1
1

score -1 · Answer 7 · answered May 31 '13 at 15:23

-1

var temp = statesToEmailDictionary.Where( x => x.Value.Contains(state)).FirstOrDefault();
var emailAdd = temp != null ? temp.Key : string.Empty;

answered May 31 '13 at 15:23

SamiHuutoniemi

1,576
2
16
35

1

`KeyValuePair` is a struct, so `temp` will never be null. – p.s.w.g May 31 '13 at 15:26
I have the same use case where I have to get the value when key = [email protected] tired below which is not giving expected output. var emailAdd = statesToEmailDictionary.FirstOrDefault(x => x.Key == "[email protected]").Value; – Blossom Nov 02 '17 at 19:26

Get key from value - Dictionary>

7 Answers7

Linked