Exploding Kitten's Blog

How to implement IAlternateEqualityComparer

2 min read

Intro

Hello there!

It’s a small note about the custom implementation of the IAlternateEqualityComparer<TAlternative, T> interface and the GetAlternateLookup<T> method for Hashset<T> and Dictionary<TKey, TValue>.

In .NET 9, the .NET team introduced a new feature with an alternative lookup value for hashsets and dictionaries. Usually, it was advertised for string keys and ReadOnlySpan<char>s. For example, you have a dictionary where the key is a string and you want to look for a value by using ReadOnlySpan<char>. So, while your program is doing its stuff, you can reduce the memory allocation a little bit:

var d = new Dictionary<string, object>
{
    { "key1", ... },
    { "key2", ... },
    { "key3", ... },
};
var lookup = d.GetAlternateLookup<ReadOnlySpan<char>>();
 
// zero allocations
foreach (var range in names.AsSpan().Split(...))
{
   var key = names.AsSpan(range);
   var value = lookup[key];
   Console.WriteLine(value);
}

But it is just one use case. You can implement your own version of IAlternateEqualityComparer for any other type combination.

Custom Implementation

The custom implementation is provided through existing constructors with IEqualityComparer<T>. So, this IEqualityComparer<T> should implement IAlternateEqualityComparer<TAlternative, T> also. Something like this:

class Cmp : IEqualityComparer<string>, IAlternateEqualityComparer<int, string>
{
    public bool Equals(string? x, string? y)
        => x == y;
 
    public int GetHashCode(string obj)
        => obj.GetHashCode();
 
    public bool Equals(int alternate, string other)
        => alternate.ToString() == other;
 
    public int GetHashCode(int alternate)
        => GetHashCode(alternate.ToString());
 
    public string Create(int alternate)
        => alternate.ToString();
}

Note: the important point here is both GetHashCode methods should return the same value for the same object. So, basically follow the “common” rule.

Then you will be able to get a lookup value by a new type. You just need to use the GetAlternateLookup<T> method and provide a new type for a lookup. For example:

var d = new Dictionary<string, object>(new Cmp())
{
    { "1", new object() },
    { "2", new object() },
    { "3", new object() },
    { "4", new object() },
    { "5", new object() },
};
var lookup = d.GetAlternateLookup<int>();
 
if (lookup.TryGetValue(5, out var value))
    Console.WriteLine(value);
else
    Console.WriteLine("Value not found");

In this snippet, we can get a value by a string or a number.

Recent Notes