Define What is a HashSet

Question

HashSet The C  HashSet data structure was introduced in the  NET Framework 3 5  A full list of the implemented members can be found at the HashSet MSDN page    Where is it used  Why would you want to use it

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Simply said and without revealing the kitchen secrets  a set in general  is a collection that contains no duplicate elements  and whose elements are in no particular order  So  A HashSet lt T gt  is similar to a generic List lt T gt   but is optimized for fast lookups  via hashtables  as the name implies  at the cost of losing order

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A HashSet holds a set of objects  but in a way that it allows you to easily and quickly determine whether an object is already in the set or not  It does so by internally managing an array and storing the object using an index which is calculated from the hashcode of the object  Take a look here HashSet is an unordered collection containing unique elements  It has the standard collection operations Add  Remove  Contains  but since it uses a hash-based implementation  these operations are O 1    As opposed to List for example  which is O n  for Contains and Remove   HashSet also provides standard set operations such as union  intersection  and symmetric difference  Take a look here  There are different implementations of Sets  Some make insertion and lookup operations super fast by hashing elements  However  that means that the order in which the elements were added is lost  Other implementations preserve the added order at the cost of slower running times    The HashSet class in C  goes for the first approach  thus not preserving the order of elements  It is much faster than a regular List  Some basic benchmarks showed that HashSet is decently faster when dealing with primary types  int  double  bool  etc    It is a lot faster when working with class objects  So that point is that HashSet is fast   The only catch of HashSet is that there is no access by indices  To access elements you can either use an enumerator or use the built-in function to convert the HashSet into a List and iterate through that  Take a look here

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From application perspective  if one needs only to avoid duplicates then HashSet is what you are looking for since it s Lookup  Insert and Remove complexities are O 1  - constant  What this means it does not matter how many elements HashSet has it will take same amount of time to check if there s such element or not  plus since you are inserting elements at O 1  too it makes it perfect for this sort of thing

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A HashSet has an internal structure  hash   where items can be searched and identified quickly   The downside is that iterating through a HashSet  or getting an item by index  is rather slow     So why would someone want be able to know if an entry already exists in a set     One situation where a HashSet is useful is in getting distinct values from a list where duplicates may exist  Once an item is added to the HashSet it is quick to determine if the item exists  Contains operator     Other advantages of the HashSet are the Set operations   IntersectWith  IsSubsetOf  IsSupersetOf  Overlaps  SymmetricExceptWith  UnionWith   If you are familiar with the object constraint language then you will identify these set operations   You will also see that it is one step closer to an implementation of executable UML

[c#] Define: What is a HashSet?

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