Attemping to add a value to a HashSet doesn't change the amount of values in it

Question

I have a HashSet and when I use the Add method of the collection, nothing is added. The output is still 2, 3, 5, 7, 11, 13 and the output from .Count is 6.

Is this a bug or am I doing something wrong here?

namespace AllerDiz
{
    class MainClass
        {
            public static void Main (string[] args)
            {
                HashSet<int> smallPrimeNumbers = new HashSet<int> { 2, 3, 5, 7, 11, 13 };
                smallPrimeNumbers.Add (3);
                smallPrimeNumbers.Add (5);
                smallPrimeNumbers.Add (7);
                Console.WriteLine ("{0}", smallPrimeNumbers.Count);
                foreach(int val in smallPrimeNumbers)
                {
                    Console.WriteLine ("HashSet Value= {0}", val);
                }
            }
      }
}

Answer 1

No, this is not a bug. This is precisely how a HashSet is supposed to work.

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.

So, if the item you are trying to add already exists in the set, the set is not modified.

If you want a collection which does allow duplicates, look at List<T>.

Answer 2

From HashSet<T> Class

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

From HashSet<T>.Add Method

Return Value true if the element is added to the HashSet object; false if the element is already present.

HashSet is a kind of optimized collection. Its constructor eliminates the non-unique elements.

For example;

string[] array = new string[] {"one", "one", "two", "two", "three", "three"};
HashSet<string> hash = new HashSet<string>(array);
string[] array2 = hash.ToArray();

array2 will be {"one", "two", "three"}

If you want to add duplicate values, List<int> collection allows you to add duplicate values.