How to compare two lists and add the difference to a third list in C#?

When working with lists in C#, you often need to compare two lists and find the elements that exist in one list but not in another. This is commonly done using LINQ's Except method, which returns the set difference between two sequences.

Syntax

Following is the syntax for using the Except method to find differences between two lists −

IEnumerable<T> result = list1.Except(list2);

To store the result in a new list −

List<T> differenceList = list1.Except(list2).ToList();

Using Except Method to Find Differences

The Except method returns elements from the first list that are not present in the second list. This creates a new sequence containing the difference −

Example

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

public class Demo {
   public static void Main() {
      List<string> list1 = new List<string> {"A", "B", "C", "D"};
      List<string> list2 = new List<string> {"C", "D"};

      Console.WriteLine("First list...");
      foreach(string value in list1) {
         Console.WriteLine(value);
      }

      Console.WriteLine("Second list...");
      foreach(string value in list2) {
         Console.WriteLine(value);
      }

      Console.WriteLine("Difference (elements in list1 but not in list2)...");
      IEnumerable<string> list3 = list1.Except(list2);
      foreach(string value in list3) {
         Console.WriteLine(value);
      }
   }
}

The output of the above code is −

First list...
A
B
C
D
Second list...
C
D
Difference (elements in list1 but not in list2)...
A
B

Creating a Third List with Differences

Instead of using IEnumerable, you can convert the result to a concrete List using the ToList() method −

Example

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

public class ListComparison {
   public static void Main() {
      List<int> numbers1 = new List<int> {1, 2, 3, 4, 5, 6};
      List<int> numbers2 = new List<int> {3, 4, 5, 7, 8};

      // Create a third list containing the difference
      List<int> differenceList = numbers1.Except(numbers2).ToList();

      Console.WriteLine("Numbers in list1 but not in list2:");
      foreach(int number in differenceList) {
         Console.WriteLine(number);
      }

      Console.WriteLine($"Total unique elements: {differenceList.Count}");
   }
}

The output of the above code is −

Numbers in list1 but not in list2:
1
2
6
Total unique elements: 3

Finding Symmetric Difference

To find elements that are in either list but not in both (symmetric difference), you can combine both directions using Union −

Example

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

public class SymmetricDifference {
   public static void Main() {
      List<string> fruits1 = new List<string> {"Apple", "Banana", "Orange", "Mango"};
      List<string> fruits2 = new List<string> {"Orange", "Mango", "Grape", "Pineapple"};

      // Elements in fruits1 but not in fruits2
      var diff1 = fruits1.Except(fruits2);
      // Elements in fruits2 but not in fruits1
      var diff2 = fruits2.Except(fruits1);
      // Combine both differences
      List<string> symmetricDiff = diff1.Union(diff2).ToList();

      Console.WriteLine("Symmetric difference (elements in either list but not both):");
      foreach(string fruit in symmetricDiff) {
         Console.WriteLine(fruit);
      }
   }
}

The output of the above code is −

Symmetric difference (elements in either list but not both):
Apple
Banana
Grape
Pineapple

Comparison of Methods

Conclusion

The Except method in LINQ provides an efficient way to compare two lists and find the difference. Use ToList() to convert the result to a concrete list, and combine Except with Union to find symmetric differences between lists.