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copied!<p>Wow, that's actually several questions in one :-). So one after the other:</p> <blockquote> <p>it's been cited that the value of GetHashCode should never change over the lifetime of the object. How does that work if the fields that it's based on are mutable? </p> </blockquote> <p>This common advice is meant for the case where you want to use your object as a key in a HashTable/dictionary etc. . HashTables usually require the hash not to change, because they use it to decide how to store & retrieve the key. If the hash changes, the HashTable will probably no longer find your object. </p> <p>To cite the docs of Java's <em>Map</em> interface:</p> <p><em>Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map.</em></p> <p>In general it's a bad idea to use <em>any</em> kind of mutable object as a key in a hash table: It's not even clear what should happen if a key changes after it's been added to the hash table. Should the hash table return the stored object via the old key, or via the new key, or via both? </p> <p>So the real advice is: Only use immutable objects as keys, and make sure their hashcode never changes either (which is usually automatic if the object is immutable).</p> <blockquote> <p>Also what if I do want dictionary lookups etc to be based on reference equality not my overridden Equals?</p> </blockquote> <p>Well, find a dictionary implementation that works like that. But the standard library dictionaries use the hashcode&Equals, and there's no way to change that.</p> <blockquote> <p>I'm primarily overriding Equals for the ease of unit testing my serialization code which I assume serializing and deserializing (to XML in my case) kills the reference equality so I want to make sure at least it's correct by value equality. Is this bad practice to override Equals in this case?</p> </blockquote> <p>No, I'd find that perfectly acceptable. However, you should not use such objects as keys in a dictionary/hashtable, as they're mutable. See above.</p>

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