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    <p>A hash table is a data structure. More information can be found <a href="http://en.wikipedia.org/wiki/Hash_table" rel="nofollow noreferrer">when looking for more general information</a>.</p> <p>1) A default search in lists are linear (all elements need to be traversed). Perfect hashing (no collisions) allows for constant time lookups in the worst case. More collisions result in a slower lookup.</p> <p>2) Hash collisions are practically unavoidable when hashing a random subset of a large set of possible keys. Therefore, most hash table implementations have some collision resolution strategy to handle such events. .NET's Hashtable implementation seems to use <a href="http://en.wikipedia.org/wiki/Double_hashing" rel="nofollow noreferrer">double hashing</a>.</p> <p>3) This is something you shouldn't worry about, as long as you provide proper hash codes. When interested, read the wiki article about hash tables, which explains several techniques.</p> <p>UPDATE: There is <a href="https://stackoverflow.com/questions/1089132/net-hashtable-vs-dictionary-can-the-dictionary-be-as-fast">a difference</a> in the implementation of Hashtable and Dictionaries in collision handling. Apparently Hashtable is obsolete and <a href="http://msdn.microsoft.com/en-us/library/xfhwa508.aspx" rel="nofollow noreferrer">Dictionary</a> or <a href="http://msdn.microsoft.com/en-us/library/bb359438.aspx" rel="nofollow noreferrer">HashSet</a> is preferred.</p> <p>As Jim Mischel mentions, you should override GetHashCode as well as Equals. Inserting items which are equal isn't possible, but items with the same hashcode are handled by the collection type you choose.</p>
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    1. POInterview Questions relating to Hashtable & Dictionary
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    1. USSteven Jeuris
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    1. COMany Thanks for your answer. Realistically if I base my GetHashCode() on a primary key field retrieved from DB, wouldn't I bring the changes of a collision to Zero? But in case the hash could be duplicated after all, isn't it .NET taking care of rehashing/double hashing the values in case of a collision automatically? In the interview it sounded as if it was my responsibility to do something about it myself. :) Maybe they just wanted to hear about the double hashing being used internally, which I didn't say.
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    2. COIf the DB pk type is int and the dictionary only contains objects of that type than yes. (You would just return the pk field in the GetHAshCode function). But most of the time you need a good hashing function, see http://stackoverflow.com/questions/263400/what-is-the-best-algorithm-for-an-overridden-system-object-gethashcode/3880895#3880895
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    3. COI agree with @Magnus. As for taking care of collisions, it is just something to be aware of so you understand why a unique as possible hashing function is important.
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