The ```HashSet<T>``` generic class in the ```System.Collections.Generic``` namespace provides the **```UnionWith()``` method**, which modifies the ```HashSet<T>``` such that it contains a union of current ```HashSet<T>``` and the specified collection provided as the input.

# Syntax
```
public void UnionWith (System.Collections.Generic.IEnumerable<T> other);
```
- This method takes a `Collection` `IEnumerable<T>` as input and changes the current ```HashSet<T>``` object to have all elements that are present in the ```HashSet<T>```, the input collection, or both.

# Things to note
- This method represents the mathematical set union operation.

- It is an $O(n)$ operation, where n is the count of elements in the passed collection of elements.

- It throws an exception if the input collection is `null`.

- This method changes the state of the ```HashSet<T>```.

# Code

In the following example, we create a HashSet of integers and add a few numbers to it. We also create a `List` of integers that have a few elements common with the HashSet, and some elements which are not present in the HashSet.

We call the ```UnionWith()``` method with the list of integers as input. It does a set union operation and modifies the `HashSet` to contain the union of itself and the specified list of integers.

We also print the elements of the HashSet before and after the ```UnionWith()``` operation, along with the list contents. 


using System;
using System.Collections.Generic;

class HashSetUnion
{
    static void Main()
    {
        HashSet<int> numSet = new HashSet<int>();
        numSet.Add(1);
        numSet.Add(5);
        numSet.Add(9);
        numSet.Add(11);
        numSet.Add(15);
        numSet.Add(6);

        Console.WriteLine("HashSet Elements : ");
        PrintSet(numSet);
        List<int> otherList = new List<int>{9,11,39};
        Console.WriteLine("Input List 1: {0}", string.Join(" ", otherList.ToArray()));
        numSet.UnionWith(otherList);
        Console.WriteLine("HashSet Elements after UnionWith() : ");
        PrintSet(numSet);
    }

    private static void PrintSet(HashSet<int> set)
    {
      Console.Write("{");
      foreach (int i in set)
      {
          Console.Write(" {0}", i);
      }
      Console.WriteLine(" }");
    }
}

How to create a union of a HashSet with a collection in C#

Use the UnionWith() method on HashSet<T> to modify it by including elements from a specified collection.