Implementing IEquatable in Base Class

Question

I am using ASP.NET MVC with EntityFramework code first.

I am attempting to create a base class for all objects that will be saved to my database (here called the Entity class).

Is there a good way of implementing IEquatable in the base class? In my implementation, I check the type and ID - is this a good way of doing it? Will this always return the ultimate derived type? What about intermediate parents?

Objects

public abstract class Entity : IEquatable<Entity>
{
    public int ID { get; set; }
    public string Name { get; set; }

    public Entity(string Name)
    {
        this.Name = Name;
    }

    public override string ToString() { return Name; }
    public override int GetHashCode() { return ID.GetHashCode(); }
    public override bool Equals(object obj) { return this == (Entity)obj; }
    public bool Equals(Entity other) { return this == other; }
    public static bool operator ==(Entity x, Entity y) { return Object.ReferenceEquals(x, y) ? true : ((object)x == null || (object)y == null) ? false : (x.GetType() == y.GetType() && x.ID.Equals(y.ID)); }
    public static bool operator !=(Entity x, Entity y) { return !(x == y); }
}

public abstract class Content : Entity
{
    public Content(string Name) : base(Name)
    {

    }
}

public class Page : Content
{
            public string Body { get; set; }
    public Page(string Name) : base(Name)
    {

    }
}

public class User : Entity
{
    public User(string Name) : base(Name)
    {

    }
}

Database

Each derived object type will normally exist in a separate database. However, there will be some objects with intermediate inherited parents which share primary keys.

Let's say that there are three tables with the following columns: Entity

Content
- ID
- Name
Page
- ID
- Body
User
- ID
- Name

Therefore, the Page and Content tables share a Primary Key.

You should only do this if ID is read only – paparazzo Mar 06 '18 at 16:36 — paparazzo, Mar 06 '18 at 16:36

score 1 · Answer 1 · answered Nov 22 '14 at 22:03

1

There's no need to implement IEquitable, it doesn't give any value (neither performance value, no readability value).
In your comparison, you rely on reference equality check only. You should check ID equality as well.
Your GetHashCode() implementation is sub-optimal, you should include entity's type as well.
Are you sure that all of your entities will have the Name property? In the base class, you should include properties that are presented in every entity of the bounded context.

Here I describe this topic in detail: Domain object base class.

answered Nov 22 '14 at 22:03

Vladimir

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_"There's no need to implement IEquitable"_ - Why do you say that? There are [several reasons](https://stackoverflow.com/a/2477000/585968) to. – Jun 24 '20 at 05:53
@MickyD it only makes sense when the type is a struct. Otherwise, there's no significant performance gains. Unless you work on a software with high performance requirements and need all micro-optimizations you can get, you won't notice the difference. – Vladimir Jun 24 '20 at 12:19
1

It's not done for performance reasons, but to help you write better code. If you have a class named Entity, and it has an Id property which is what identifies that entity in your program, and you want multiple instances of the Entity class that have the same Id, also be considered equal (for example when checking if a collection contains that object), the best way to do it would be to make the class implement IEquatable. – Sasino Jan 08 '21 at 19:18

score 0 · Answer 2 · answered Mar 23 '14 at 23:49

0

Your implementation is correct.

Be careful doing this though. If you could have objects that are partially loaded, then they will be considered equal but may not have all of the same values.

If you want to sometimes use default equals and sometimes use ID based equals, create a separate helper class implementing IEqualityComparer<Entity>.

answered Mar 23 '14 at 23:49

Samuel Neff

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What do you mean by partially loaded? – William Mar 24 '14 at 02:47
@William, with EF this normally would only come up with relationships. If a `Page` has `Content` then you can load a `Page` instance from the db and not load the referenced `Content`. Then you may separately load the same `Page` instance somewhere, but do load referenced `Content`. Now you have two `Page` instances that are different, one that has `Content` and one that doesn't. Are they equal? The answer can be yes and no depending on the needs of the particular circumstance. There are circumstances that make them equal and some that make them not. I have run into this problem specifically. – Samuel Neff Mar 24 '14 at 03:44

Implementing IEquatable in Base Class

Objects

Database

2 Answers2