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

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

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

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...