How do I check if an object is equal to a new object of the same class?

Question

If I have a object like:

public class Person
{
    public int id {get;set;}
    public string name {get;set;}
}

And I want the behavior:

Person a = new Person();
Person b = new Person();

a == b;

and that a == b returns true, do I have to override the Object.Equals() method? or is there some other way of doing it without overriding the Equals method?

EDIT

I want to compare data, as I want to know if a external method that I call returns a new object or a object with different data than a new object

score 37 · Accepted Answer · edited Aug 29 '18 at 18:45

There are a couple of ways you can do this. By default Equals() and == check for reference equality, meaning:

Person a = new Person();
Person b = a:

a.Equals(b); //true
a == b; //true

And therefore, the objects are not compared for value equality, meaning:

Person a = new Person { id = 1, name = "person1" };
Person b = new Person { id = 1, name = "person1" };

a.Equals(b); //false
a == b; //false

To compare objects for their values you can override the Equals() and GetHashcode() methods, like this:

public override bool Equals(System.Object obj)
{
    if (obj == null)
        return false;

    Person p = obj as Person;
    if ((System.Object)p == null)
        return false;

    return (id == p.id) && (name == p.name);
}

public bool Equals(Person p)
{
    if ((object)p == null)
        return false;

    return (id == p.id) && (name == p.name);
}

public override int GetHashCode()
{
    return id.GetHashCode() ^ name.GetHashCode();
}

Now you will see other results when comparing:

Person a = new Person { id = 1, name = "person1" };
Person b = new Person { id = 1, name = "person1" };
Person c = a;

a == b; //false
a == c; //true
a.Equals(b); //true
a.Equals(c); //true

The == operator is not overridden and therefore still does reference comparison. This can be solved by overloading it as well as the != operator:

public static bool operator ==(Person a, Person b)
{
    if (System.Object.ReferenceEquals(a, b))
        return true;

    if ((object)a == null || (object)b == null)
        return false;

    return a.id == b.id && a.name == b.name;
}

public static bool operator !=(Person a, Person b)
{
    return !(a == b);
}

Now running the checks results in following:

Person a = new Person { id = 1, name = "person1" };
Person b = new Person { id = 1, name = "person1" };
Person c = a;

a == b; //true
a == c; //true
a.Equals(b); //true
a.Equals(c); //true

More reading:

Guidelines for Implementing Equals and the Equality Operator (==)

See http://stackoverflow.com/questions/263400/what-is-the-best-algorithm-for-an-overridden-system-object-gethashcode for a better way of implementing a custom GetHashCode implementation. Also, why not use ReferenceEquals when comparing to null and avoid the cast? And should the name comparison be case sensitive? I'm guessing not because case is not usually important when it comes to names. — tvanfosson, Dec 20 '13 at 21:05

score 3 · Answer 2 · answered Aug 10 '21 at 10:53

3

If you want you can write an extention methode like this (make a new class like this) :

public static class Extensions
{
    public static bool IsEqual<T>(this T objA, T objB)
        {
            foreach (var item in objA.GetType().GetProperties())
            {
                if (item.GetValue(objA).ToString() != item.GetValue(objB).ToString()) 
                    return false;
            }

            return true;
        }
    }
}

Then you can use this extention methode something like this :

Person a = new Person { id = 1, name = "person1" };
Person b = new Person { id = 1, name = "person1" };
....
if ( a.IsEqual<Person>(b)) // this returns true ;) 
{
     do something ...
}
else 
{
    do something else ...
}
....

Enjoy it.

answered Aug 10 '21 at 10:53

Ali Mahmoodi

858
10
14

This got me super close but I had to make one small adjustment. if (item.GetValue(objA)?.ToString() != item.GetValue(objB)?.ToString()) because I was wanting to see if a object was equal to a new instance of an object and my property values will be null. Great work! – BondAddict Aug 03 '22 at 22:14
Please consider to ?. in item.GetValue(objA)?.ToString() that means if the value is 'null' it returns 'null' and you compare it with the reurn value of 'item.GetValue(objB)?.ToString()' in the same way . – Ali Mahmoodi Aug 04 '22 at 05:41

score 2 · Answer 3 · edited Aug 29 '18 at 14:08

This all depends on what you are trying to compare, by default Equals will compare by reference therefore

a == b

in your example will always be false. However, if you did something like

Person a = new Person();
Person b = a;

Then a == b would be true because both a and b are using the same reference.

do i have to override the Object.Equals() method?

Overriding Equals and GetHashCode is the recommended approach, however, (for arguments sake) it's not the only way. You could, for example, override the == operator and do your comparison in there. However, there are limitations with going down that route alone.

Most comparison checks, if not all, will use Equals internally which is why it's the preferred approach. See Guidelines for Implementing Equals and the Equality Operator (==).

I was trying to avoid the override route, but in the end it seems that is the solution to my problem, thanks. — Albert Cortada, Dec 20 '13 at 10:43

score 2 · Answer 4 · edited Aug 29 '18 at 21:05

You want to overload the == operator. Therefore you should also override Equals first. If you override Equals you should always also override GetHashCode. If you overload the == operator you must also overload the != operator:

public class Person
{
    public int id {get;set;}
    public string name {get;set;}

    public override bool Equals(object obj)
    {
        Person p2 = obj as Person;
        if (object.ReferenceEquals(null, p2)) return false;
        return id == p2.id;
    }

    public static bool operator ==(Person p1, Person p2)
    {
        if (object.ReferenceEquals(null, p1))
            return object.ReferenceEquals(null, p2);
        else if (object.ReferenceEquals(null, p2))
            return false;
        return p1.Equals(p2);
    }

    public static bool operator !=(Person p1, Person p2)
    {
        if (object.ReferenceEquals(null, p1))
            return !object.ReferenceEquals(null, p2);
        else if (object.ReferenceEquals(null, p2))
            return true;
        return !p1.Equals(p2);
    }

    public override int  GetHashCode()
    {
        return id ;
    }
}

Now this compares values (the id) instead of only references:

Person p1 = new Person { id = 1, name = "Jeff" };
Person p2 = new Person { id = 2, name = "Tim" };
bool equalPersons = p1 == p2; // false
Person p3 = new Person { id = 1, name = "Jeff 2.0" }; // same id -> equal
equalPersons = p1 == p3; // true

MSDN: Guidelines for Implementing Equals and the Equality Operator (==)

score 2 · Answer 5 · answered Oct 02 '20 at 10:58

In C# 9.0 and above, there is a new type called record whose equality is value-based. So you can define it like:

public record Person
{
    public string LastName { get; }
    public string FirstName { get; }

    public Person(string first, string last) => (FirstName, LastName) = (first, last);
}

Then you can use

var bill1 = new Person("Bill", "Wagner");
var bill2 = new Person("Bill", "Wagner");

Console.WriteLine(bill1 == bill2); // true

Reference: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9#record-types

Shaharyar · Answer 6 · 2013-12-21T06:31:52.643

0

object.Equals(object2);

in your case:

a.Equals(b);

edited Dec 21 '13 at 06:31

answered Dec 20 '13 at 10:29

Shaharyar

12,254
4
46
66

1

This only compares references, not data (that may be what the OP wants, however, they haven't specified). – James Dec 20 '13 at 10:30

score 0 · Answer 7 · answered Dec 20 '13 at 10:44

Yes. You want to compare to objects of Person, you need to override equals and hashcode method from Object class as by default reference check (==) is done through equals method.

Assuming two Persons with same name and id can only be considered equal use both of these properties in equals and hashcode method.

Generating equals and hashcode has beocme easier with the Java IDE's provided. U can try below code.

public class Person {

    private int id;
    private String name;

    // id getters and setters

    // name getters and setters 

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + id;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Person other = (Person) obj;
        if (id != other.id)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }

}

Edgar Gomez · Answer 8 · 2023-03-18T03:44:57.837

You can compere using JsonSerializer, where all the values types and the values types holding by refrences types are equals

this is as a method

using System.Text.Json;

public bool Compare<T> (T element1, T element2)
{
    return (JsonSerializer.Serialize(element1) == JsonSerializer.Serialize(element2));
}

this is as extension

using System.Text.Json;

static class Extension
{
    static bool Compare<T>(this T element1, T element2)
    {
        return (JsonSerializer.Serialize(element1) == JsonSerializer.Serialize(element2));
    }
}

score -2 · Answer 9 · answered Dec 20 '13 at 10:34

-2

Suppose Test is Class

Test a = new Test() { Num = 1, Str = "Hi" };
Test b = new Test() { Num = 1, Str = "Hi" };
 bool areEqual = System.Object.ReferenceEquals(a, b);

answered Dec 20 '13 at 10:34

Kumod Singh

2,113
1
16
18

This would return false, because they are not the same reference to the same object in memory. – Daniel Feb 04 '20 at 16:41

How do I check if an object is equal to a new object of the same class?

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