Map two lists into a dictionary in C#

Question

Given two IEnumerables of the same size, how can I convert it to a Dictionary using Linq?

IEnumerable<string> keys = new List<string>() { "A", "B", "C" };
IEnumerable<string> values = new List<string>() { "Val A", "Val B", "Val C" };

var dictionary = /* Linq ? */;

And the expected output is:

A: Val A
B: Val B
C: Val C

I wonder if there is some simple way to achieve it.

And should I be worried about performance? What if I have large collections?

---

I don't if there is an easier way to do it, currently I'm doing like this:

I have an Extension method that will loop the IEnumerable providing me the element and the index number.

public static class Ext
{
    public static void Each<T>(this IEnumerable els, Action<T, int> a)
    {
        int i = 0;
        foreach (T e in els)
        {
            a(e, i++);
        }
    }
}

And I have a method that will loop one of the Enumerables and with the index retrieve the equivalent element on the other Enumerable.

public static Dictionary<TKey, TValue> Merge<TKey, TValue>(IEnumerable<TKey> keys, IEnumerable<TValue> values)
{
    var dic = new Dictionary<TKey, TValue>();

    keys.Each<TKey>((x, i) =>
    {
        dic.Add(x, values.ElementAt(i));
    });

    return dic;
}

Then I use it like:

IEnumerable<string> keys = new List<string>() { "A", "B", "C" };
IEnumerable<string> values = new List<string>() { "Val A", "Val B", "Val C" };

var dic = Util.Merge(keys, values);

And the output is correct:

A: Val A
B: Val B
C: Val C

Disrespectable

Answer 1

With .NET 4.0 (or the 3.5 version of System.Interactive from Rx), you can use Zip():

var dic = keys.Zip(values, (k, v) => new { k, v })
              .ToDictionary(x => x.k, x => x.v);

Feuillant

Thumbs up for .Zip. Glad it's made it into the BCL. I've had my own version for quite some time. – Titlark

@Titlark - Even if you made your own, i would take it from Eric Lipperts Blog – Diseuse

Pity there is a need for a Zip method. If only more statically typed languages would support generic variadic parameters, Select would handle this (like map in Scheme). – Stites

In 2022 it can be shorter as var dic = keys.Zip(values).ToDictionary(x => x.First, x => x.Second); – Pressley

Just for reference, @Romulus' method in VB.NET would be: Dim dic = keys.Zip(values).ToDictionary(Function(x) x.First, Function(x) x.Second). – Kaikaia

Answer 2

Or based on your idea, LINQ includes an overload of Select() that provides the index. Combined with the fact that values supports access by index, one could do the following:

var dic = keys.Select((k, i) => new { k, v = values[i] })
              .ToDictionary(x => x.k, x => x.v);

(If values is kept as List<string>, that is...)

Feuillant

Answer 3

I like this approach:

var dict =
   Enumerable.Range(0, keys.Length).ToDictionary(i => keys[i], i => values[i]);

Mcardle

I didn't realize it at first but both Enumerable and Zip are features of Linq. Food for thought. – Emmen

Answer 4

If you use MoreLINQ, you can also utilize it's ToDictionary extension method on previously created KeyValuePairs:

var dict = Enumerable
    .Zip(keys, values, (key, value) => KeyValuePair.Create(key, value))
    .ToDictionary();

It also should be noted that using Zip extension method is safe against input collections of different lengths.

Kingdom

Answer 5

Starting from .NET Core 3.0, you can just use the improved Zip() :

var dictionary = keys.Zip(values).ToDictionary(x => x.First, x => x.Second);

Define an extension method for this case, and you're as good as you can get :)

Corporative