Advantages of a HashSet over a List in C#

Efficiency and Search Performance:
A HashSet is optimized for searching and provides faster lookup times compared to a List.
When you need to check whether an element exists in the collection, a HashSet performs better due to its hash-based approach.
The time complexity for adding, removing, and searching in a HashSet is O(1) (constant time), making it efficient even with large amounts of data1.
Uniqueness:
A HashSet ensures that each element appears only once. It automatically handles duplicates by rejecting them.
In contrast, a List allows duplicate elements, and you need to manually check for duplicates before adding an item.
Set Operations:
A HashSet supports set operations like union, intersection, and set difference.
For example, you can combine two HashSets using UnionWith, find common elements with IntersectWith, or remove elements with ExceptWith2.
No Indexing:
Unlike a List, a HashSet does not provide indexing capabilities. You cannot access elements by index.
If you need to retrieve elements by position, a List is more suitable.
Ordering:
A List maintains the order of elements as they are inserted.
A HashSet is an unordered collection and does not guarantee any specific sequence3.
In summary, use a HashSet when you prioritize uniqueness, fast lookups, and set operations. If you need indexing or maintain insertion order, consider using a List. Keep in mind that the choice depends on your specific use case and requirements!