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copied!<p>I've been thinking about this question for a bit and after a bit of consideration I agree that implementing <code>IEquatable<T></code> and <code>IComparable<T></code> should only be done on sealed types. </p> <p>I went back and forth for a bit but then I thought of the following test. Under what circumstances should the following ever return false? IMHO, 2 objects are either equal or they are not. </p> <pre><code>public void EqualitySanityCheck<T>(T left, T right) where T : IEquatable<T> { var equals1 = left.Equals(right); var equals2 = ((IEqutable<T>)left).Equals(right); Assert.AreEqual(equals1,equals2); } </code></pre> <p>The result of <code>IEquatable<T></code> on a given object should have the same behavior as <code>Object.Equals</code> assuming the comparer is of the equivalent type. Implementing <code>IEquatable<T></code> twice in an object hierarchy allows for, and implies, there are 2 different ways of expressing equality in your system. It's easy to contrive any number of scenarios where <code>IEquatable<T></code> and <code>Object.Equals</code> would differ since there are multiple <code>IEquatable<T></code> implementations but only a single <code>Object.Equals</code>. Hence the above would fail and create a bit of confusion in your code. </p> <p>Some people may argue that implementing <code>IEquatable<T></code> at a higher point in the object hierarchy is valid because you want to compare a subset of the objects properties. In that case you should favor an <code>IEqualityComparer<T></code> which is specifically designed to compare those properties. </p>

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