Question

I have read that when you override Equals on an class/object you need to override GetHashCode.

 public class Person : IEquatable<Person>
    {
        public int PersonId { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public Person(int personId, string firstName, string lastName)
        {
            PersonId = personId;
            FirstName = firstName;
            LastName = lastName;

        }

        public bool Equals(Person obj)
        {
            Person p = obj as Person;

            if (ReferenceEquals(null, p)) 
                return false;
            if (ReferenceEquals(this, p)) 
                return true;

            return Equals(p.FirstName, FirstName) && 
                   Equals(p.LastName, LastName);
        }


    }

Now given the following:

 public static Dictionary<Person, Person> ObjDic= new Dictionary<Person, Person>();
 public static Dictionary<int, Person> PKDic = new Dictionary<int, Person>();

Will not overridding the GetHashCode affect both of the Dictionary’s above? What I am basically asking is how is GetHashCode generated? IF I still look for an object in PKDic will I be able to find it just based of the PK. If I wanted to override the GetHashCode how would one go about doing that?

Answer 1

You should always override GetHashCode.

A Dictionary<int, Person> will function without GetHashCode, but as soon as you call LINQ methods like Distinct or GroupBy, it will stop working.

Note, by the way, that you haven’t actually overridden Equals either.
The IEquatable.Equals method is not the same as the virtual bool Equals(object obj) inherited from Object. Although the default IEqualityComparer<T> will use the IEquatable<T> interface if the class implements it, you should still override Equals, because other code might not.

In your case, you should override Equals and GetHashCode like this:

public override bool Equals(object obj) { return Equals(obj as Person); }
public override int GetHashCode() {
    return FirstName.GetHashCode() ^ LastName.GetHashCode();
}