覆盖Equals和GetHashCode-派生类中的默认实现

Question

I'm a little confused about the default behaviour of Equals and GetHashCode in C#. Say I have two classes, one deriving from the other:

public abstract class Question
    {
        public string QuestionText
        {
            get;
            set;
        }

        public override bool Equals(object obj)
        {
            if (obj is Question)
            {
                Question q = (Question)obj;
                return this.QuestionText.Equals(q.QuestionText);
            }
            else
            {
                return false;
            }
        }

        public override int GetHashCode()
        {
            int hash = 13; 
            hash = (hash * 7) + this.QuestionText.GetHashCode(); 
            return hash; 
        }
 }

public class QuestionTrueFalse : Question
    {
        public bool CorrectAnswer
        {
            get;
            set;
        }

        public override bool Equals(object obj)
        {
            return base.Equals(q);
        }

        public override int GetHashCode()
        {
            return base.GetHashCode(); 
        }
    } 

The derived class doesn't affect whether one item equals another, I still want that to be based simply upon the QuestionText property.

Do I need to override Equals and GetHashCode to reference the base implementation, as I have done here, or is that the default behaviour?

Answer 1

The base class behavior is inherited by the inheriting classes. You don't need to explicitly override Equals and GetHashCode unless you want to change their behavior.

Answer 2

1. You probably don't want this. Why would you want to to have separate but Equal instances of a question object?

2. You will at least have to add operator== and operator!= to prevent nasty suprises.

3. But no, you don't need to override in QuestionTrueFalse to call the base implementation. That is provided for.

The standard example (can't come up with a more PC one):

Q1: "Do you still beat your wife?" { true, false }
Q2: "Do you still beat your wife?" { true, false, N/A }

Are they really the same?

Answer 3

Having a class derived from a class which cares about Equals and GetHashCode is not that simple. There are many considerations omitted by just letting the base class do the job.

You can refer to this article for deeper analysis how purpose of Equals and GetHashCode methods changes once the class is derived: How to Override Equals and GetHashCode Methods in Base and Derived Classes