Using collections as keys allows for complex functional constructs to be expressed in C# more easily. This code provides two classes for doing ordered or unordered equality comparisons on collections so that they may be used as keys in dictionaries and sets.
Introduction
Sometimes when expressing complicated algorithms in C# you may find yourself needing to look up some information based on a set of items rather than a single item. Stock .NET provides a little bit of functionality for doing this via LINQ but it's not generalized such that Dictionary<TKey,TValue>
for example can use it.
This little library aims to provide two styles of comparison so that these types of operations are easy.
Using this Mess
A Cautionary Note
Keys should not be mutable, but because of how collections work these ones are mutable when they shouldn't be. Do not modify collections you are using as keys! It will cause you nasty bugs.
This library exposes two classes, OrderedCollectionEqualityComparer<T>
and UnorderedCollectionEqualityComparer<T>
, which provides ordered and unordered comparisons respectively. They work on anything implementing IList<T>
, ICollection<T>
, or ISet<T>
. Unordered comparisons can be expensive, but using HashSet<T>
for your collections can speed that up some. You can use these classes to provide Dictionary<TKey, TValue>
and HashSet<TKey, TValue>
with a way to compare collections instead of single items. The demo code indicates how to use them:
var set = new HashSet<int[]>(OrderedCollectionEqualityComparer<int>.Default);
set.Add(new int[] { 1, 2, 3, 4, 5 });
Console.WriteLine("Ordered cmp - contains 1-5: " + set.Contains(new int[] { 1, 2, 3, 4, 5 }));
Console.WriteLine("Ordered cmp contains 5-1: " + set.Contains(new int[] { 5, 4, 3, 2, 1 }));
set = new HashSet<int[]>(UnorderedCollectionEqualityComparer<int>.Default);
set.Add(new int[] { 1, 2, 3, 4, 5 });
Console.WriteLine("Unordered cmp - contains 1-5: " + set.Contains(new int[] { 1, 2, 3, 4, 5 }));
Console.WriteLine("Unordered cmp contains 5-1: " + set.Contains(new int[] { 5, 4, 3, 2, 1 }));
History
- 9th February, 2020 - Initial submission