ToDictionary

This C# extension method converts a collection into a Dictionary. It works on IEnumerable collections such as arrays and Lists.

We can use ToDictionary to optimize performance—while retaining short and clear code. This method simplifies the demands of the code.

First example

Building up dictionaries can require a significant amount of code. When we use ToDictionary, we can use less code to create a Dictionary.

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

class Program
{
    static void Main()
    {
        // Part 1: create example integer array.
        int[] values = new int[] { 1, 3, 5, 7 };

        // Part 2: call ToDictionary.
        // Part 3: specify lambda as argument.
        Dictionary<int, bool> dictionary =
            values.ToDictionary(v => v, v => true);

        // Display all keys and values.
        foreach (KeyValuePair<int, bool> pair in dictionary)
        {
            Console.WriteLine(pair);
        }
    }
}
[1, True]
[3, True]
[5, True]
[7, True]

String example

Dictionaries are most useful for strings and string lookups. This allows us to use a number (hash code) in place of a string, greatly speeding things up.

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

class Program
{
    static void Main()
    {
        // Example with strings and List.
        List<string> list = new List<string>() { "cat", "dog", "animal" };
        var animals = list.ToDictionary(x => x, x => true);
        if (animals.ContainsKey("dog"))
        {
            // This is in the Dictionary.
            Console.WriteLine("dog exists");
        }
    }
}
dog exists

IEqualityComparer

The ToDictionary method can receives a third argument, an IEqualityComparer. Here we use StringComparer.OrdinalIgnoreCase to create a case-insensitive dictionary.

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

class Program
{
    static void Main()
    {
        List<string> list = new List<string>() { "cat", "bird" };
        // Create case-insensitive dictionary.
        var pets = list.ToDictionary(x => x,
            x => true,
            StringComparer.OrdinalIgnoreCase);
        if (pets.ContainsKey("CAT"))
        {
            Console.WriteLine("CAT exists");
        }
    }
}
CAT exists

Notes, LINQ

ToDictionary allows us to use fewer lines of code to insert elements into a Dictionary. This method is elegant and fits well with other LINQ code.

We used ToDictionary to transform a collection (such as an array or List) into a Dictionary collection. This provides constant-time lookups.