IndexedList

This library provides a specialized collection for faster list lookups by creating dictionaries for specified properties of the list's elements. It's inspired by this article by Joel Champagne.

Example

Filtering a list by some property is a common usecase. This example will use an entity that looks like this:

public class Book
{
    public string Name {get; set;}
    public int Pages {get; set;}
}

If you want to retrieve all books from a list that have a specific amount of pages you could it solve it using LINQ's Where operation:

foreach(Book book in books.Where(b => b.Pages == 50)) 
{
    Console.WriteLine(book.Name);    
}

This operation will iterate over every element in the list and compare the property's value.

To speed this up one could create a Dictionary that maps the amount of pages of a book to a list of all books with that amount.

The IndexedList provided by this library will automatically do that for you. We will start by creating a new IndexedList:

var indexedList = new IndexedList<Book>(b => b.Pages, b => b.Name);

In the constructor's parameters you need to specify all properties that should be indexed.

In order to retrieve the books using the created index you need to use WhereIndexed:

foreach(Book book in indexedList.WhereIndexed(b => b.Pages, 50)) 
{
    Console.WriteLine(book.Name);    
}

Benchmark

|               Method |      Mean |     Error |    StdDev |    Median |  Gen 0 | Allocated |
|--------------------- |----------:|----------:|----------:|----------:|-------:|----------:|
|        BenchmarkList | 75.107 us | 1.4833 us | 3.6940 us | 73.917 us |      - |      72 B |
| BenchmarkIndexedList |  1.942 us | 0.0325 us | 0.0543 us |  1.928 us | 0.2174 |     688 B |

As you can see the IndexedList provides a roughly ~40x speedup as compared to a regular list and LINQ's Where operation. This comes at the cost of memory allocations which are the result of boxing operations and the underlying dictionaries.

The benchmarks were conducted using the following code:

[MemoryDiagnoser()]
public class Benchmarks
{
    private IndexedList<Book> indexlist;
    private List<Book> list;

    [GlobalSetup]
    public void Setup()
    {
        indexlist = new IndexedList<Book>(p => p.Pages);
        list = new List<Book>();

        Random random = new Random();
        for (int i = 0; i < 10000; i++)
        {
            var book = new Book()
            {
                Pages = random.Next(100),
                Name = random.Next().ToString()
            };
            
            indexlist.Add(book);
            list.Add(book);
        }
    }

    [Benchmark]
    public int BenchmarkList()
    {
        int totalPages = 0;
        
        foreach (var book in list.Where(p => p.Pages == 50))
        {
            totalPages += book.Pages;
        }

        return totalPages;
    }
    
    [Benchmark]
    public int BenchmarkIndexedList()
    {
        int totalPages = 0;
        
        foreach (var person in indexlist.WhereIndexed(p => p.Pages, 50))
        {
            totalPages += person.Pages;
        }

        return totalPages;
    }
}

Installation

You can use this library in your project by adding the following NuGet package:

Install-Package IndexedList