Intersect two lists in C#

In C#, you can find common elements between two lists using the Intersect() method from LINQ. This method returns an IEnumerable<T> containing elements that appear in both lists, automatically removing duplicates.

Syntax

Following is the syntax for using the Intersect() method −

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

The method can also accept a custom equality comparer −

IEnumerable<T> result = list1.Intersect(list2, comparer);

Using Intersect() with Integer Lists

The most common use case is finding intersection between two integer lists −

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

class Demo {
   static void Main() {
      List<int> val1 = new List<int> { 25, 30, 40, 60, 80, 95, 110 };
      List<int> val2 = new List<int> { 27, 35, 40, 75, 95, 100, 110 };
      
      IEnumerable<int> res = val1.Intersect(val2);
      
      Console.WriteLine("Intersection of both the lists...");
      foreach (int a in res) {
         Console.WriteLine(a);
      }
   }
}

The output of the above code is −

Intersection of both the lists...
40
95
110

Using Intersect() with String Lists

The Intersect() method also works with string collections −

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

class Program {
   static void Main() {
      List<string> fruits1 = new List<string> { "apple", "banana", "orange", "grape" };
      List<string> fruits2 = new List<string> { "banana", "kiwi", "grape", "mango" };
      
      var commonFruits = fruits1.Intersect(fruits2);
      
      Console.WriteLine("Common fruits:");
      foreach (string fruit in commonFruits) {
         Console.WriteLine(fruit);
      }
   }
}

The output of the above code is −

Common fruits:
banana
grape

Using Intersect() with Custom Objects

For custom objects, you can override Equals() and GetHashCode() methods or use a custom comparer −

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

class Student {
   public int Id { get; set; }
   public string Name { get; set; }
   
   public override bool Equals(object obj) {
      if (obj is Student student) {
         return this.Id == student.Id;
      }
      return false;
   }
   
   public override int GetHashCode() {
      return Id.GetHashCode();
   }
   
   public override string ToString() {
      return $"Id: {Id}, Name: {Name}";
   }
}

class Program {
   static void Main() {
      List<Student> class1 = new List<Student> {
         new Student { Id = 1, Name = "John" },
         new Student { Id = 2, Name = "Alice" },
         new Student { Id = 3, Name = "Bob" }
      };
      
      List<Student> class2 = new List<Student> {
         new Student { Id = 2, Name = "Alice" },
         new Student { Id = 4, Name = "Carol" },
         new Student { Id = 3, Name = "Bob" }
      };
      
      var commonStudents = class1.Intersect(class2);
      
      Console.WriteLine("Students in both classes:");
      foreach (var student in commonStudents) {
         Console.WriteLine(student);
      }
   }
}

The output of the above code is −

Students in both classes:
Id: 2, Name: Alice
Id: 3, Name: Bob

Key Features

• The Intersect() method automatically removes duplicates from the result.

• The order of elements in the result follows the order of the first list.

• It uses the default equality comparer unless a custom comparer is provided.

• The method is part of LINQ and requires using System.Linq;.

Conclusion

The Intersect() method in C# provides an efficient way to find common elements between two lists. It works with any type of collection and automatically handles duplicate removal, making it a powerful tool for set operations in LINQ.