Union

The C# Union() method computes mathematical unions. This extension method, from System.Linq, acts upon 2 collections. It returns a new collection that contains the elements that are found.

So this method can be thought of as two actions: it combines the two collections and then uses Distinct() on them, removing duplicate elements.

Example

First, Union() is found in System.Linq. So you will want to include the appropriate using-directive. The Union() method will work with 2 collections of the same type of elements.

using System;
using System.Linq;

// Create two example arrays.
int[] array1 = { 1, 2, 3 };
int[] array2 = { 2, 3, 4 };
// Union the two arrays.
var result = array1.Union(array2);
// Enumerate the union.
foreach (int value in result)
{
    Console.WriteLine(value);
}
1
2
3
4

Example 2

Union() does not sort. The first example makes the result appear sorted, but this is due to already-sorted arrays. Here, I union two unsorted char arrays. The result is not sorted.

using System;
using System.Linq;

char[] values1 = { 'a', 'z', 'c' };
char[] values2 = { 'c', 's' };
// Take union.
var result = values1.Union(values2);
Console.WriteLine(string.Join(",", result));
a,z,c,s

Intersect

Please note the Intersect() method. This returns only the shared elements in both collections it is called upon. It is similar to Union() but with this important difference.

With Union, we combine collections. An imperative approach would involve a hash table or Dictionary. But the Union method can resolve duplicates automatically.