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PO
 

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    <p>Most <code>Equals</code> implementations I've seen check the types of the objects being compared, if they aren't the same then the method returns false.</p> <p>This neatly avoids the problem of a sub-type being compared against it's parent type, thereby negating the need for sealing a class.</p> <p>An obvious example of this would be trying to compare a 2D point (A) with a 3D point (B): for a 2D the x and y values of a 3D point might be equal, but for a 3D point, the z value will most likely be different.</p> <p>This means that <code>A == B</code> would be true, but <code>B == A</code> would be false. Most people like the <code>Equals</code> operators to be commutative, to checking types is clearly a good idea in this case.</p> <p>But what if you subclass and you don't add any new properties? Well, that's a bit harder to answer, and possibly depends on your situation.</p>
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    1. POShould IEquatable<T>, IComparable<T> be implemented on non-sealed classes?
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    1. USninj
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      1. VTUpMod
    1. COninj, I'm not sure I get you, checking that the other object's type isn't sufficient - what if Equals(A obj) is called on type B? - perfectly possible when for example you have a List<A> since the collection can contain both A and subtype B. And, since you haven't sealed A you can't know what would make sense for Equals(A obj) when called on any subtype (incl. B). You're going to be forced to provide a virtual member so that subtypes can override behavior as appropriate. An alternative might be for Equals(A object) to check this.GetType() == typeof(A) and throw an exception if false. YUCK!
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      1. USPhil
    2. COIMO, It keeps coming back to the contract for IEquatable<T> is to compare objects of type T, *not* subtypes. To guarantee this, T needs to be sealed. If alternatively, you don't agree with that, and want T to remain unsealed, you need to design the IEquatable<T> implementation to be overridable by a subtype, so you have to introduce an additional virtual member - but given system.object already has virtual object.Equals(object obj) would be there really be *any* point in implementing IEquatable<T>?
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    3. COOh, whoops. Good point, IEquatable<T> shouldn't really have to handle checking types (yes, it's icky to have to do so.) I was implementing a helper class for Equals and IEquatable a while ago, and eventually I took the chicken way out - I narrowed the purpose of IEquatable<T> from "type safe equals" to "what makes things like generic dictionaries work". No need to seal the types if you take this way out...
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