CODEWITHSUNDEEP
c#equalshashcode
Amichai Simcha
Amichai Simcha

Reputation: 21

Equal and hashcode collision in c#

i wrote the following code in c#:

public class Person
{
    private string idNumber;
    private string personName;
    public Person(string name, string id)
    {
        this.personName= name;
        this.idNumber= id;
    }
    public override bool Equals(Objectobj)
    {
        Person personObj= obj as Person;
        if(personObj== null) 
            return false;
        else 
            return idNumber.Equals(personObj.idNumber);
    }
    public override int GetHashCode()
    { 
        return this.idNumber.GetHashCode(); 
    }
}

public static void Main()
{
    Person p1 = new Person("John", "63412895");
    Person p2 = new Person("Jack", "63412895");
    Console.WriteLine(p1.Equals(p2));
    Console.WriteLine(Object.Equals(p1, p2));
}

i don't understand why the second Console.WriteLine() returns true??? the first returns true since i override equal method. but the second one related to equal in object class. please explain.

Upvotes: 1

Views: 304

Answers (5)

Kapol
Kapol

Reputation: 6463

Quote from MSDN documentation on Object.Equals(Object, Object)

If the two objects do not represent the same object reference and neither is null, it calls objA.Equals(objB) and returns the result. This means that if objA overrides the Object.Equals(Object) method, this override is called.

I believe that's exactly what happens in your scenario. Since you have overridden the Equals method, it is being called internally by the Object's static method.

Upvotes: 2

Jakub Lortz
Jakub Lortz

Reputation: 14894

The static object.Equals method is defined as

public static bool Equals(Object objA, Object objB) 
{
    if (objA==objB) {
        return true;
    }
    if (objA==null || objB==null) {
        return false;
    }
    return objA.Equals(objB);
}

So internally it uses the overriden Equals after null checks.

To compare references, use object.ReferenceEquals or cast to object and use == operator.

object.ReferenceEquals(p1, p2)
(object)p1 == (object)p2

Upvotes: 5

csharpwinphonexaml
csharpwinphonexaml

Reputation: 3683

The second line of code internally checks if p1 is null and calls the first line.

So you can't expect to have diferent results calling almost the same method. 

p1.Equals(p2);
Object.Equals(p1, p2);

Upvotes: 1

Sql Surfer
Sql Surfer

Reputation: 1422

You overrode GetHashCode. That is what the object equality is using. The classic example is in .Net the two objects objStringA ="MyVal" and ObjStringB = 'MyVal" will return equal because the string overrides the get hash code.

It is like you overrode it twice. I am not sure you needed the Equals operator to be overridden in your case.

Upvotes: -1

Festyk
Festyk

Reputation: 307

I think it is related to the fact that you also override GetHashcode which is used by Object.Equals. That's my guess.

Upvotes: -1

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