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
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 (==)
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 (==).
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 (==)
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;
}
}
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);
