Recommended .NET Class for a collection of unique integers?

Question

What would you recommend for class that needs to keep a list of unique integers?

I'm going to want to Add() integers to the collection and also check for existence e.g. Contains().

Would be nice to also get them in a list as a string for display, ie. "1, 5, 10, 21".

Answer 1

HashSet:

The HashSet<T> class provides high-performance set operations. A set is a collection that contains no duplicate elements, and whose elements are in no particular order...

The capacity of a HashSet<T> object is the number of elements that the object can hold. A HashSet<T> object's capacity automatically increases as elements are added to the object.

The HashSet<T> class is based on the model of mathematical sets and provides high-performance set operations similar to accessing the keys of the Dictionary<TKey, TValue> or Hashtable collections. In simple terms, the HashSet<T> class can be thought of as a Dictionary<TKey, TValue> collection without values.

A HashSet<T> collection is not sorted and cannot contain duplicate elements...

Answer 2

In my testing, I have found that a Dictionary with a dummy value is faster than a HashSet, when dealing with very large sets of data (100,000+ in my case). I expect this is because the Dictionary allows you to set an initial capacity, but I don't really know. In the case you're describing, I would probably use the Dictionary if I was expecting a very large set of numbers, and then (or as I was adding to the Dictionary, depending on the intent) iterate over it using a string builder, to create the output string.