The Collection classes are a group of classes designed specifically for grouping together objects and performing tasks on them.
In the .NET framework, there are several classes available for performing these operations. Some of the classes are as follows:
- List
- Dictionary
- HashSet
- Queue
A List<T> in C# is a dynamic array, which means that it can grow or shrink as elements are added or removed. Lists have a number of useful methods and properties for working with collections of data, such as Add, Remove, Contains, Count, and IndexOf.
A HashSet<T> is a collection that does not allow duplicate elements. It is implemented as a hash table, which means that it provides fast lookups, adds, and removes. The HashSet<T> class has similar methods to List<T> such as Add, Remove, and Contains, but it does not have an IndexOf method or a Count property.
In terms of performance, a HashSet<T> is generally faster than a List<T> for operations that involve looking up an element, such as checking if an element is contained in the collection. This is because a HashSet<T> uses a hash table, which has an average time complexity of O(1) for these operations. In contrast, a List<T> has an average time complexity of O(n) for these operations, where n is the number of elements in the list.
However, a List<T> is generally better suited for operations that involve traversing the collection in order, such as iterating through the elements or finding the index of an element. This is because a List<T> is implemented as an ordered collection, whereas a HashSet<T> is not.
In summary, if you need a collection that does not allow duplicate elements and that you will be frequently looking up elements in, then a HashSet<T> is the better choice. If you need a collection that allows duplicate elements and that you will be frequently traversing in order, then a List<T> is the better choice.
Another important consideration when choosing between a List<T> and a HashSet<T> is the type of elements that you will be storing in the collection. If your elements are simple types, such as integers or strings, then both a List<T> and a HashSet<T> will work well. However, if your elements are complex types, such as custom classes or structs, then you will need to decide which collection is more appropriate based on the specific requirements of your application.
For example, if your custom class or struct has a natural ordering (i.e. it implements IComparable<T> or IComparable) and you want to maintain this ordering, then a List<T> would be the better choice. On the other hand, if your custom class or struct does not have a natural ordering, or if you want to store multiple instances of the same object in the collection, then a HashSet<T> would be more appropriate.
Another thing to keep in mind is that a HashSet<T> uses the EqualityComparer<T> to determine equality of elements, if you have custom complex types and you want to use a specific way of comparison(not the default way) you should create an IEqualityComparer<T> and pass it as an argument in the constructor of HashSet<T>.
In conclusion, the choice between a List<T> and a HashSet<T> depends on the specific requirements of your application, such as the type of elements that you will be storing, the operations that you will be performing on the collection, and the performance characteristics that you need. Both collections have their own strengths and weaknesses, so it’s important to understand the trade-offs before making a decision.
What is the difference between HashSet<T> and List<T>
Unlike a List<> …
- A HashSet is a List with no duplicate members.
- Because a HashSet is constrained to contain only unique entries, the internal structure is optimised for searching (compared with a list) – it is considerably faster
- Adding to a HashSet returns a boolean – false if addition fails due to already existing in Set .) Can perform mathematical set operations against a Set: Union/Intersection/IsSubsetOf etc.
- HashSet doesn’t implement IList only ICollection
- You cannot use indices with a HashSet, only enumerators.
C# HashSet vs List Example
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class Program { static void Main(string[] args) { Console.Write("\n********************************"); Console.Write("\n** HashSet usage example ** \n"); Console.Write("********************************\n\n"); //Define a numbers List List<int> numbers = new List<int>() { 10, 25, 45, 78, 12, 10, 78, 45, 100, 100, 25 }; Console.WriteLine("Get Count(List): " + numbers.Count); Console.WriteLine("The Data int List"); //Output in List foreach (var n in numbers) { Console.Write(n+"\t"); } Console.Write("\n********************************\n\n"); //Define a numbers HashSet HashSet<int> hSet = new HashSet<int>(numbers); Console.WriteLine("Get Count(HashSet): " + hSet.Count); //Output in Hashset Console.WriteLine("The Data int HashSet"); foreach (var n in hSet) { Console.Write(n + "\t"); } Console.ReadLine(); } } |
Output:
Sources:
https://stackoverflow.com/questions/150750/hashset-vs-list-performance
https://stackoverflow.com/questions/6391738/what-is-the-difference-between-hashsett-and-listt?noredirect=1&lq=1
http://www.dotnetcurry.com/csharp/1362/hashset-csharp-with-examples
