Continuing confusion regarding overring Equals for mutable objects that are used in data bound collections

Question

Background:

I've written a large scale WPF application using MVVM and it's been suffering from some intermittent problems. I initially asked the 'An item with the same key has already been added' Exception on selecting a ListBoxItem from code question here which explains the problem, but got no answers.

After some time, I managed to work out the cause of the Exceptions that I was getting and documented it in the What to return when overriding Object.GetHashCode() in classes with no immutable fields? question. Basically, it was because I had used mutable fields in the formula to return a value for GetHashCode.

From the very useful answers that I received for that question, I managed to deepen my understanding in that area. Here are three relevant rules:

1. If x equals y then the hash code of x must equal the hash code of y. Equivalently, if the hash code of x does not equal the hash code of y, then x and y must be unequal.
2. The hash code of x must remain stable while x is in a hash table.
3. The hash function should generate a random distribution among all integers for all inputs.

These rules affected the possible solutions that I had to my problem of not knowing what to return from the GetHashCode method:

• I couldn't return a constant because that would break the first and third rules above.
• I couldn't create an additional readonly field for each class, solely to be used in the GetHashCode method for the same reasons.

The solution that I eventually went with was to remove each item from its ObservableCollection before editing any of the properties used in the GetHashCode method and then to re-add it again afterwards. While this has worked Ok in a number of views so far, I've run into a further problem as my UI items are animated using custom Panels. When I re-add an item (even by inserting it back to its original index in the collection), it sets off the entry animation(s) again.

I had already added a number of base class methods such as AddWithoutAnimation, RemoveWithoutAnimation, which has helped fix some of these issues, but it doesn't affect any Storyboard animations, which still get triggered after re-adding. So finally, we come to the question:

Question:

First, I'd like to clearly state that I am not using any Dictionary objects in my code... the Dictionary that throws the Exception must be internal to an ObservableCollection<T>. This point seems to have been missed by most people in my last question. Therefore, I cannot chose to simply not use a Dictionary... if only I could.

So, my question is 'is there any other way that I can implement GetHashCode in mutable classes while not breaking the three rules above, or avoid implementing it in the first place?'

I received a comment on the previous question from @HansPassant that suggested that

A good starting point is to completely remove the Equals and GetHashCode overrides, the default implementations inherited from Object are excellent and guarantee object uniqueness.

Can anyone tell me how can I remove the Equals and GetHashCode overrides? On the IEquatable<T> Interface page on MSDN it says It should be implemented for any object that might be stored in a generic collection and then on the IEquatable<T>.Equals Method page it says If you implement Equals, you should also override the base class implementations of Object.Equals(Object) and GetHashCode so that their behaviour is consistent with that of the IEquatable<T>.

If this is possible, it would be my preferred solution.

---

UPDATE >>>

After downloading and installing dotPeek, I have been able to look inside the PresentationFramework namespace where the Exception is actually occurring. I have found the exact part that uses the Dictionary that is causing this problem. It is in the internal InternalSelectedItemsStorage class constructor:

  internal InternalSelectedItemsStorage(Selector.InternalSelectedItemsStorage collection, IEqualityComparer<ItemsControl.ItemInfo> equalityComparer = null)
  {
    this._equalityComparer = equalityComparer ?? collection._equalityComparer;
    this._list = new List<ItemsControl.ItemInfo>((IEnumerable<ItemsControl.ItemInfo>) collection._list);
    if (collection.UsesItemHashCodes)
      this._set = new Dictionary<ItemsControl.ItemInfo, ItemsControl.ItemInfo>((IDictionary<ItemsControl.ItemInfo, ItemsControl.ItemInfo>) collection._set, this._equalityComparer);
    this._resolvedCount = collection._resolvedCount;
    this._unresolvedCount = collection._unresolvedCount;
  }

This is used internally by the Selector class after the ListBoxItem.OnSelected method has been called, so I can only assume that this has something to do with when a selection is made on the Listbox.

Answer 1

Can anyone tell me how can I remove the Equals and GetHashCode overrides? On the IEquatable Interface page on MSDN it says It should be implemented for any object that might be stored in a generic collection and then on the IEquatable.Equals Method page it says If you implement Equals, you should also override the base class implementations of Object.Equals(Object) and GetHashCode so that their behaviour is consistent with that of the IEquatable.

Mutable objects are comparable by their identity while immutable or value objects by their values.

If you have a mutable object you need to figure out its identity (e.g. if it is a representation of an entity stored in the database the identity is the primary key of the identity; if it is just an 'ad hoc' mutable object created in memory, then its identity is reference of this object (i.e. the default implementation of Equals and GetHashCode)).

So if your object is not an entity you simply implement IEquatable.Equals(T x) { return this.Equals(x); }, i.e. you say that, yes you can compare objects of this class with objects of class T and you compare it by reference (Equals() method inherited from System.Object).

If your object is an entity and e.g. has a primary key PersonId, then you do comparison by PersonId and return PersonId.GetHashCode() from your GetHashCode() method.

Btw, in case of entities you usually use some OR mapper and Identity map pattern which ensures that within a given unit of work you don't have more than one instance of a given entity, i.e. whenever primary keys are equal the object references are equal too.