Union Method in C#

The Union method in C# is a LINQ extension method that returns the unique elements from both collections. It combines two sequences and automatically removes duplicate values, ensuring each element appears only once in the result.

Syntax

Following is the syntax for the Union method −

public static IEnumerable<TSource> Union<TSource>(
    this IEnumerable<TSource> first,
    IEnumerable<TSource> second
)

Parameters

• first − The first sequence to merge.

• second − The second sequence to merge.

Return Value

Returns an IEnumerable<T> containing the unique elements from both input sequences.

Using Union with Integer Lists

Let us set two lists with some common elements −

using System;
using System.Collections.Generic;
using System.Linq;

public class Demo {
   public static void Main() {
      // two lists with duplicate value 65
      var list1 = new List<int>{12, 65, 88, 45};
      var list2 = new List<int>{40, 34, 65};

      // finding union (removes duplicate 65)
      var res = list1.Union(list2);

      Console.WriteLine("Union result:");
      foreach(int i in res) {
         Console.WriteLine(i);
      }
   }
}

The output of the above code is −

Union result:
12
65
88
45
40
34

Using Union with String Collections

The Union method works with any type, including strings −

using System;
using System.Collections.Generic;
using System.Linq;

public class Demo {
   public static void Main() {
      var fruits1 = new List<string>{"apple", "banana", "cherry"};
      var fruits2 = new List<string>{"banana", "date", "elderberry"};

      var allFruits = fruits1.Union(fruits2);

      Console.WriteLine("All unique fruits:");
      foreach(string fruit in allFruits) {
         Console.WriteLine(fruit);
      }
   }
}

The output of the above code is −

All unique fruits:
apple
banana
cherry
date
elderberry

Union vs Concat Comparison

Conclusion

The Union method in C# provides an efficient way to combine two collections while automatically eliminating duplicates. It preserves the order of elements from the first collection and appends unique elements from the second collection, making it ideal for merging datasets without redundancy.