ILookup<TKey, TElement> Interface

Interface ILookup<TKey, TElement>

Represents a collection of elements that are indexed by a specified key type. Each key in the lookup can be associated with zero or more elements.

Summary

• Defines a read-only collection of groups, where each group contains a key and a collection of elements associated with that key.
• Provides efficient querying of elements based on their associated keys.
• Typically used after a grouping operation (e.g., using LINQ's GroupBy method).

Syntax

Type Parameters

TKey: The type of the keys in the lookup.

TElement: The type of the elements in the lookup.

Members

Remarks

The ILookup<TKey, TElement> interface is an important part of the Language Integrated Query (LINQ) framework in .NET. It provides a convenient and efficient way to work with data that has been grouped by a common key.

Unlike a Dictionary<TKey, TValue>, where each key can map to only one value, a Lookup<TKey, TElement> (the concrete implementation of ILookup<TKey, TElement>) allows a key to be associated with multiple elements. The elements for a given key are stored in an IEnumerable<TElement>.

The ILookup<TKey, TElement> interface is read-only. Once created, you cannot add or remove keys or elements.

Example

The following example demonstrates how to create and use an ILookup<int, string> from a list of strings.

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

public class LookupExample
{
    public static void Main(string[] args)
    {
        var words = new List<string> { "apple", "banana", "apricot", "blueberry", "cherry", "avocado" };

        // Group words by their first letter
        ILookup<char, string> groupedByFirstLetter = words.ToLookup(word => word[0]);

        Console.WriteLine($"Number of keys: {groupedByFirstLetter.Count}"); // Output: Number of keys: 3

        // Accessing elements for a specific key
        Console.WriteLine("\nWords starting with 'a':");
        if (groupedByFirstLetter.Contains('a'))
        {
            foreach (var word in groupedByFirstLetter['a'])
            {
                Console.WriteLine($"- {word}");
            }
        }
        // Output:
        // - apple
        // - apricot
        // - avocado

        Console.WriteLine("\nWords starting with 'b':");
        if (groupedByFirstLetter.Contains('b'))
        {
            foreach (var word in groupedByFirstLetter['b'])
            {
                Console.WriteLine($"- {word}");
            }
        }
        // Output:
        // - banana
        // - blueberry

        // Iterating through all groups
        Console.WriteLine("\nAll groups:");
        foreach (var group in groupedByFirstLetter)
        {
            Console.WriteLine($"Key '{group.Key}':");
            foreach (var element in group)
            {
                Console.Write($"  {element}");
            }
            Console.WriteLine();
        }
        // Output:
        // Key 'a':
        //   apple apricot avocado
        // Key 'b':
        //   banana blueberry
        // Key 'c':
        //   cherry
    }
}