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    <p>Firstly - there's no knowing if caching this result would actually improve <code>Dictionary&lt;string, ...&gt;</code> et al because they don't necessarily use String.GetHashCode, because it uses an IComparer to get the hashcode for a string.</p> <p>And if you follow the likely call chain for the StringComparer class, it ends up going through to the System.Globalization.CompareInfo class, which finally terminates at this method:</p> <pre><code>[SecurityCritical, SuppressUnmanagedCodeSecurity, DllImport("QCall", CharSet=CharSet.Unicode)] private static extern int InternalGetGlobalizedHashCode(IntPtr handle, string localeName, string source, int length, int dwFlags); </code></pre> <p>There's no knowing if that library - which appears to be a native method - doesn't use some form of internal caching based on the underlying .Net object data structure that we can't get at once inside the .Net runtime.</p> <p>However, the important thing to note with this is that one string can have <em>many different</em> hash codes based on how you chose to interpret the characters. Granted, this implementation is culture-inspecific - which is why it's unsuitable for these comparers.</p> <p>So, whilst the additional memory storage <em>could</em> be a factor, I actually think it's because to store a hash code along with an instance of the string misleads the caller, and indeed the .Net internal dev team(!), into thinking that the string only has one hash code, when in fact it entirely depends on how you're going to interpret it - as a series of bytes (which most of us do not), or as a series of printable characters.</p> <p>From a performance point of view, then, if we also accept that these comparers used by <code>Dictionary&lt;,&gt;</code> etc can't be using the internal implementation, not caching this result probably doesn't have much of an impact because, frankly, how often will this method actually get called in the real world: since most of the time a hashcode of a string is most likely calculated via some other mechanism.</p> <p><strong>EDIT</strong></p> <p>There's also the point made in Tim's answer(+1 there). If he's right, and I think he is, then there's no guarantee that a string is actually immutable after construction, therefore to cache the result would be wrong.</p> <p><strong>AN ADDITIONAL EDIT(!)</strong></p> <p>Dan makes the point that strings are meant to be immutable within the Net sphere and therefore that string should be free to cache it's own hashcode based on this. The problem here is that the .Net framework also provides a <strong><em>legitimate way to change the supposedly immutable string</em></strong> that does not involve privileged reflection or anything else. It's a fundamental problem with strings, it's a pointer to a buffer that you cannot control. Never mind in the C# world, what about in C++, where vectoring over and modifying memory buffers is common-place. Just because you ideally <em>shouldn't</em> do it doesn't mean that the framework should expect you not to.</p> <p>.Net happens to provide this functionality, and therefore if this was a design decision by the .Net team in response to the kind of binary thuggery suggested by Tim, then they were very wise to have taken it into account. Whether they did, or whether it is by fluke, is another matter entirely! :)</p>
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    1. POWhy might a System.String object not cache its hash code?
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    1. USAndras Zoltan
    1. USAndras Zoltan
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    1. COI knew there was something I just wasn't thinking of, and I think you nailed it here: the hashtable-based classes in .NET don't necessarily use `TKey.GetHashCode`; they use the `IComparer<TKey>.GetHashCode` of their internal comparers. That makes sense!
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      1. USDan Tao
    2. COIn response to your edit: Take a look at my update at the bottom of the question. (Not saying it isn't an interesting point, of course! I just don't think that could be part of the *reason* for the implementation of `GetHashCode`.)
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    3. CO@Dan Tao: Have updated again - while I think you have a point about 'shouldn't modify it', the point is that .Net framework gives the developer the ability to do so within legal bounds - and therefore the .Net team would have been right to have taken that into account.
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