System.Linq.Enumerable.ToLookup Method

System.Linq.Enumerable Class

Provides a number of methods by calling the methods of an iterator object.

ToLookup Method

public static ILookup<TKey, TElement> ToLookup<TSource, TKey, TElement>( IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector )

Parameters

source: IEnumerable<TSource>

An IEnumerable<TSource> to group elements from.

keySelector: Func<TSource, TKey>

A function to extract each element's key.

elementSelector: Func<TSource, TElement>

A function to map each source element to an element in the resulting ILookup<TKey, TElement>.

Return Value

ILookup<TKey, TElement>

An ILookup<TKey, TElement> that contains elements of the input collection keyed by keySelector.

Description

Creates a Lookup<TKey, TElement> by applying a selector function to each element of the source collection. This operation is deferred.

An ILookup<TKey, TElement> is a collection of elements that are grouped by a key. Each key in the ILookup<TKey, TElement> has an associated collection of elements that share that key.

Examples

Basic Usage

using System;
using System.Collections.Generic;
using System.Linq;

public class Example
{
    public static void Main()
    {
        var words = new[] {
            "apple", "banana", "cherry", "apricot", "blueberry", "grape"
        };

        // Group words by the first letter and create a lookup
        var lookupByFirstLetter = words.ToLookup(
            word => word[0],           // Key selector: the first character of the word
            word => word               // Element selector: the word itself
        );

        // Output the results
        foreach (var group in lookupByFirstLetter)
        {
            Console.Write($"Key: '{group.Key}' - ");
            Console.WriteLine(string.Join( ", ", group));
        }
    }
}

Output:

Key: 'a' - apple, apricot
Key: 'b' - banana, blueberry
Key: 'c' - cherry
Key: 'g' - grape

Using different element selector

using System;
using System.Collections.Generic;
using System.Linq;

public class Example
{
    public static void Main()
    {
        var numbers = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        // Group numbers by whether they are even or odd
        var lookupByParity = numbers.ToLookup(
            num => num % 2 == 0,     // Key selector: true for even, false for odd
            num => num.ToString()    // Element selector: the number as a string
        );

        // Output the results
        foreach (var group in lookupByParity)
        {
            if (group.Key)
            {
                Console.Write("Even numbers: ");
            }
            else
            {
                Console.Write("Odd numbers: ");
            }
            Console.WriteLine(string.Join( ", ", group));
        }
    }
}