CODEWITHSUNDEEP
c#classobjectequals
Albert Cortada
Albert Cortada

Reputation: 747

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

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

Upvotes: 17

Views: 41378

Answers (9)

Edgar Gomez
Edgar Gomez

Reputation: 164

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));
    }
}

Upvotes: 0

Ali Mahmoodi
Ali Mahmoodi

Reputation: 1194

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.

Upvotes: 3

xa0082249956
xa0082249956

Reputation: 463

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

Upvotes: 3

Tim Schmelter
Tim Schmelter

Reputation: 460058

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 (==)

Upvotes: 2

Abbas
Abbas

Reputation: 14432

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:

Upvotes: 39

James
James

Reputation: 82096

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 (==).

Upvotes: 2

Shaharyar
Shaharyar

Reputation: 12439

object.Equals(object2);

in your case:

a.Equals(b);

Upvotes: 0

Suvasis
Suvasis

Reputation: 1481

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;
    }

}

Upvotes: 0

Kumod Singh
Kumod Singh

Reputation: 2163

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);

Upvotes: -2

Related Questions