Concept

When working with immutable objects (usually record), each modification on an entity means that a new instance is created. While this offers lots of (great advantages)[https://en.wikipedia.org/wiki/Immutable_object] it makes more difficult to properly track (and animate) changes on an (immutable) item in a (immutable) collection.

The key equality is a standarized way to determine that 2 objects that are not Equals are however representing the same entity.

For instance:

public partial record Person(string Name, int Age);

var john1 = new Person("John Doe", 20);
var john2 = john1 with { Age=21 };

Console.WriteLine("Are the same : " + john1.Equals(john2));
Console.WriteLine("Are the same person : " + john1.KeyEquals(john2));

This would output:

Are the same : false
Are the same person : true

In this example the key is the Name, and any instance of Person that Name == "John Doe" is condered to represent the same person, no matter values of the other properties.

Then if you are dealing with list:

var list1 = ImmutableList.Create(john1);
var list2 = list1.Replace(john1, john2);

When comparing the list1 and the list2 the IKeyEquatable<T> allows you to detect that the item is only a newer version of the same entity, so visually we only need to update the current item and not animate the removal and then the add of the item.

The implementation of this concept is located in the Uno.Extensions.Equality namespace.

IKeyEquatable

This is like IEquatable<T> but specialized for the key comparison.

When you implement this, you should compare only the keys of your object.

Tip

You usually don't have to implement it by yourself, cf. (generation)[#Generation].

KeyEqualityComparer

This is like the EqualityComparer but which relies on the IKeyEquatable instead of IEquatable to check equality.

Tip

As all types are not necessarily implementing IKeyEquatable<T>, there is no equivalent to the EqualityComparer<T>.Default. You can however use the static KeyEqualityComparer.Find<T>() to dynamically get a key equality comparer, if the given T does implement IKeyEquatable<T>.

Generation

The IKeyEquatable<T> implementation is automatically generated for partial record that has a property named Id or Key.

How to configure keys?

You have several way to configure the keys:

1. Add the [Key] attribute on properties that shoudl be used for key equality.

public partial record MyItem(
	[property:Key] Guid EntityId,
	[property:Key] string SourceId,
	string Value);

Important

As soon as a property has been flagged with the [Key] attribute, the implicit keys are not used.

Note

This is the single way to have more than one property to compute key equality.

Note

You can use indifferently the Uno.Extensions.Equality.KeyAttribute or the System.ComponentModel.DataAnnotations.KeyAttribute

2. Add the [ImplicitKeyEquality] attribute on your record

[ImplicitKeyEquality("EntityId")]
public partial record MyItem(Guid EntityId, string SourceId, string Value);

3. Change the default keys for the whole project by setting the [ImplicitKeyEquality] on the assembly:

[assembly:ImplicitKeyEquality("Id", "Key", "EntityId")]

Important

The generation of IKeyEquatable<T> using implicit keys will use only one matching property. The properties a tested in the order in which they have been defined on the [ImplicitKeyEquality] attribute. This means that in teh example above, if a record have 2 properties Id and EntityId, only the property named Id will be used.

How to disable generation?

You can disable the generation on a given type by adding [ImplicitKeys(IsEnabled = false)] on it.

To disable the generation for the whole project you set that attibute directly on the assembly:

[assembly:ImplicitKeys(IsEnabled = false)]

![IMPORTANT] This disable only the generation based on implicit keys. If you have a record that has a property flagged with the [Key] attribute, the generator will still generate the IKeyEquatable<T> implementation for that type.