Introducing HashSet<T> [Kim Hamilton]

Introducing HashSet<T> [Kim Hamilton]

  • Comments 31

HashSet<T> is in our latest CTP, and you can find it in the System.Collections.Generic namespace. The naming discussion over the last month has motivated me to recap some naming highlights for HashSet, so hang in til the end if you’re interested.

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 operation are O(1). (As opposed to List<T> for example, which is O(n) for Contains and Remove.) HashSet also provides standard set operations such as union, intersection, and symmetric difference.

    HashSet<int> theSet1 = new HashSet<int>();
    // theSet1 contains 1,2

    HashSet<int> theSet2 = new HashSet<int>();
    // theSet2 contains 1,3,4

    // theSet1 contains 1,2,3,4

HashSet’s default Add operation returns a bool letting you know whether the item was added, so in the code sample above, you could check the return type to check whether the item was already in the set.

    bool added = theSet1.Add(2); // added is true
    added = theSet1.Add(2); // added is false

If you’re familiar with our ICollection<T> interface, notice that this means ICollection<T>.Add (returning void) has an explicit implementation, allowing HashSet<T> to introduce its own Add.

A note on uniqueness: HashSet determines equality according to the EqualityComparer you specify, or the default EqualityComparer for the type (if you didn’t specify). In the above example we didn’t specify an EqualityComparer so it will use the default for Int32. In the next example, we’ll use an OddEvenComparer, which considers items equal if they are both even or both odd.

    class OddEvenComparer : IEqualityComparer<int> {
        public OddEvenComparer() {}
        public bool Equals(int x, int y) {
            return (x & 1) == (y & 1);

        public int GetHashCode(int x) {
            return (x & 1);


    // Now use the comparer
    HashSet<int> oddEvenSet = new HashSet<int>(new OddEvenComparer());
    // oddEventSet contains 1,4; it considered 1 and 3 equal.

Notice the name UnionWith in the first example. UnionWith, as with the other set operations, modifies the set it’s called on and doesn’t create a new set. This distinction is important because the Linq operations Union, Intersect, etc on IEnumerable create a new set. So HashSet’s methods aren’t duplicating Linq; they’re provided in case you want to avoid creating a new set, and they’re distinguished by the With suffix.

Now for some naming fun, which will demonstrate some other framework guidelines. We would have liked to name this feature Set. This is because it’s preferred to use a common name rather than one that reveals details about the implementation. To borrow an example from Krzysztof Cwalina and Brad Abram’s book Framework Design Guidelines, a type used to submit print jobs to a print queue should be named Printer, and not PrintQueue. Applying this guideline to this class – HashSet, while more technically precise, isn’t as recognizable at Set. You can see this guideline in other class names in the System.Collections.Generic namespace: List<T> instead of ArrayList<T>, Dictionary<T> instead of Hashtable<T>.

This brings up the question of whether naming it Set would have been a bad idea if we add other sets in the future, such as an OrderedSet. However, a hash-based unordered set can reasonably be considered the “go-to” set because of its good performance, so distinguishing it with the name Set would still be acceptable.

Any guesses as to why we didn’t go with the name Set?

  • Kim,

    By HashList I was referring to an IList<T> implementation which would be O(1) for Add, Remove and Contains. Isn't it one of the main matters of HashSet ?

  • What I don't get is the reason you change the semantics of Add so lightly.

    I would prefer that you were more explicit. My suggestion:

    Create a public bool TryAdd() and then a void Add() that throws on duplicates as usual. Of course, you can implement Add() as a wrapper around TryAdd() if convinient.

  • Hi Diegov,

    Earlier versions of our design used Add/TryAdd, but we removed it to be consistent with the set interpretation of add. Even though a set is maintained such that it doesn't contain duplicate items, attempting to add a duplicate isn't exceptional. We provided the Add that returns bool for cases where the user may want different control flow based on whether the item was already present in the set, but this shouldn't be achieved through exceptions (with their associated perf cost, etc).

    So in fact, this isn't a change of semantics of Add. You can think of it this way -- if the items are equal according to the equality comparer, then it's an implementation choice whether the original item is replaced by the new duplicate item. After Add is finished, the item is present in the set.

    I should also mention that we came to this decision only after much debate. Using Add/TryAdd was a common suggestion, but more than 2/3rds of people reviewing the API thought that having Add throw on duplicates would be surprising for a set.

    Some readers may be wondering why IDictionary throws on duplicate adds. That's because it takes both a key and a value, and although the key may be the same, the values may differ.



  • Hi Sebastien,

    >> Isn't it one of the main matters of HashSet ?

    Not exactly; HashSet is unordered and doesn't contain duplicates, but an IList is ordered and may contain duplicates.



  • Well, I guess the majority rules :) Anyway, the use of IEqualityComparer<T>.GetHashCode() gives the slight feeling that this is also a dictionary, only the key is function of the value. I guess then, it is not completely possible to abstract of the fact that HashSet is implemented as with a hash. Which is another reason not to call it Set :)

  • System.Collections.Generic.HashSet

  • Незабаром в нас буде нова версія .Net Framework 3.5, яка схоже, як і 3.0, буде спиратися на базовий .Net

  • Since it's coming up on 2 years since I complained that the BCL didn't have Set&lt;T&gt; I figured I

  • While installing VisualStudio 2008 Beta2 I was surprised that the NET framework 2.0 installation got

  • Generic function to removeDuplicates from Generic List

  • Visual Studio (VS) 2008 and .Net 3.5, code named “Orcas”, will be released in the near future. This release

  • Mein TechTalk ist nun zu Ende. Meine letzte Station heute in Berlin war Lustig und Amüsant, ich hoffe

  • I recently wrote an MSDN Flash article on the hidden gems of System.Core.dll of .NET Framework 3.5 which

  • I recently wrote an MSDN Flash article on the hidden gems of System.Core.dll of .NET Framework 3.5 which

  • .NET Framework 3.5 and Visual Studio 2008 have officially shipped! Soma has the announcement on his blog

Page 2 of 3 (31 items) 123