Distinct method doesn't work for class with overriden Equals

Question

I created class with overriden Equals. The problem is that Distinct method doesn't work for my class.

class MyClass
{
    public int Item1 { get; private set; }
    public int Item2 { get; private set; }

    public MyClass(int item1, int item2)=>(Item1,Item2)=(item1,item2); 


    public override bool Equals(object obj)
    {
        var other = obj as MyClass;

        if (other == null)
        {
            return false;
        }

        return (this.Item1 == other.Item1 && this.Item2 == other.Item2);
    }
}

class Program
{
    static void Main(string[] args)
    {
        MyClass x = new MyClass(1, 0);
        MyClass y = new MyClass(1, 0);
        var list = new List<MyClass>();
        list.Add(x);
        list.Add(y);


        bool b = x.Equals(y));  //True
        var distincts = list.Distinct(); //Doesn't work, contains both
    }
}

How can I fix that and why it doesn't use my Equals in Distinct?

`MyClass` should also override `GetHashCode` since it overrides `Equals`. There's even a compiler warning for it. — Dirk, Jun 07 '17 at 08:31
You should also override the `GetHashCode` method like this: https://stackoverflow.com/a/38434457/2946329 — Salah Akbari, Jun 07 '17 at 08:31
All set methods need `GetHashCode`(before `Equals` is used). But in general you should always implement `GetHashCode` **and** `Equals` together — Tim Schmelter, Jun 07 '17 at 08:32

score 3 · Answer 1 · answered Jun 07 '17 at 08:35

Distinct docs:

Returns distinct elements from a sequence by using the default equality comparer to compare values.

Let's see what the default equality comparer does:

The Default property checks whether type T implements the System.IEquatable<T> interface and, if so, returns an EqualityComparer<T> that uses that implementation. Otherwise, it returns an EqualityComparer<T> that uses the overrides of Object.Equals and Object.GetHashCode provided by T.

So basically, to make this work, you either:

implement GetHashCode as well
implement IEquatable<T>
Call the overload of Distinct that accepts a custom equality comparer.

If I were you, I would choose the second one because you need to change the least of your code.

class MyClass: IEquatable<MyClass> {
    ...

    public bool Equals(MyClass obj)
    {
        if (obj == null)
        {
            return false;
        }

        return (this.Item1 == obj.Item1 && this.Item2 == obj.Item2);
    }
}

adjan · Answer 2 · 2017-06-07T08:37:10.937

2

You have to override GetHashCode as well:

public override int GetHashCode()
{
    return Item1; // or something
}

Distinct first compares the hashcodes, which should be computed faster than the actual Equals. Equals is only further evaulated if the hashcodes are equal for two instances.

edited Jun 07 '17 at 08:37

answered Jun 07 '17 at 08:31

adjan

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score 1 · Accepted Answer · answered Jun 07 '17 at 08:33

You need to implement IEquatable<MyClass> in MyClass and provide your own implementation of GetHashCode and Equals method.

see this for more information.

class MyClass
{
    public int Item1 { get; private set; }
    public int Item2 { get; private set; }

    public MyClass(int item1, int item2)=>(Item1,Item2)=(item1,item2); 


    public override bool Equals(object obj)
    {
        var other = obj as MyClass;

        if (other == null)
        {
            return false;
        }

        return (this.Item1 == other.Item1 && this.Item2 == other.Item2);
    }

    public override int GetHashCode()
    {

        return this.Item1;
    }
}

Distinct method doesn't work for class with overriden Equals

3 Answers3