Override the Object.GetHashCode method, why you should bother

When you design a class you sometimes want to override the Equals method in order to use some internal fields to determine if two instances of the class are equal.

For example let's assume that you have implemented a class Money that has two fields, one named _amount of type decimal and another one called _currency of type string. You decide that two objects of type Money are equal if and only if the amount and currency fields have the same values.

The implementation of Equals looks like this:

public override bool Equals(object obj)
{
  if (obj is Money other)
  {
    return _currency.Equals(other._currency, StringComparison.Ordinal)
        && _amount.Equals(other._amount);
  }
  return false;
}

You then notice that you get a warning during compilation,Warning CS0659 'Money' overrides Object.Equals(object o) but does not override Object.GetHashCode(). So what is this all about?

If you look into it a bit you will find that there is a rule saying that if two objects are equal, they must have the same hash code, and since you have just overridden the method that determines if two objects are equal, it is your responsibility to ensure that they also get the same hash. But why do two identical objects need to have the same hash?

The hash code is used when you build HashSets and Dictionaries. If you don't override the GetHashCode method the default method will be used, this will only compare the object's memory address and even though you consider them to be equal they will be treated as different in the HashSet or Dictionary. This breaks the use of HashSets and Dictionaries with your objects and you do not want that.

However, since the rule says that objects that are identical should have the same hash, but not that objects that are not identical cannot have the same hash, a quick fix is to implement a GetHashCode method that always returns the same integer value. This actually is a valid implementation. But, this also means that you will get hash collisions for instance of your class. If used as key-values in a Dictionary this will make addition and look-up to the dictionary much much slower.

The best thing you can do is to use the same data that you use to check if the two instances of the class are equal to also calculate the hash code. For the Money class the GetHashCode method can look like this:

public override int GetHashCode()
{
  var hashCode = _currency.GetHashCode() + _amount.GetHashCode();
  return hashCode;
}

It is worth putting some effort your hash code method!