How to join two lists in C#?

To join two lists in C#, you can use several methods. The most common approaches are using the AddRange() method to modify an existing list, or using LINQ's Concat() method to create a new combined list without modifying the originals.

Syntax

Using AddRange() to modify the first list −

list1.AddRange(list2);

Using LINQ Concat() to create a new list −

var combinedList = list1.Concat(list2).ToList();

Using AddRange() Method

The AddRange() method adds all elements from the second list to the end of the first list, modifying the original list −

using System;
using System.Collections.Generic;

class Program {
    public static void Main() {
        var list1 = new List<string>();
        list1.Add("Keyboard");
        list1.Add("Mouse");

        Console.WriteLine("Our list1....");
        foreach(var item in list1) {
            Console.WriteLine(item);
        }

        var list2 = new List<string>();
        list2.Add("Hard Disk");
        list2.Add("Pen Drive");

        Console.WriteLine("Our list2....");
        foreach(var item in list2) {
            Console.WriteLine(item);
        }

        list1.AddRange(list2);

        Console.WriteLine("Concatenated list....");
        foreach(var item in list1) {
            Console.WriteLine(item);
        }
    }
}

The output of the above code is −

Our list1....
Keyboard
Mouse
Our list2....
Hard Disk
Pen Drive
Concatenated list....
Keyboard
Mouse
Hard Disk
Pen Drive

Using LINQ Concat() Method

The Concat() method creates a new list containing elements from both lists without modifying the original lists −

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

class Program {
    public static void Main() {
        var list1 = new List<string> { "Apple", "Banana" };
        var list2 = new List<string> { "Cherry", "Date" };

        Console.WriteLine("Original list1:");
        foreach(var item in list1) {
            Console.WriteLine(item);
        }

        Console.WriteLine("Original list2:");
        foreach(var item in list2) {
            Console.WriteLine(item);
        }

        var combinedList = list1.Concat(list2).ToList();

        Console.WriteLine("Combined list:");
        foreach(var item in combinedList) {
            Console.WriteLine(item);
        }

        Console.WriteLine("list1 remains unchanged: " + list1.Count + " items");
        Console.WriteLine("list2 remains unchanged: " + list2.Count + " items");
    }
}

The output of the above code is −

Original list1:
Apple
Banana
Original list2:
Cherry
Date
Combined list:
Apple
Banana
Cherry
Date
list1 remains unchanged: 2 items
list2 remains unchanged: 2 items

Comparison of Methods

Joining Lists with Different Types

You can also join lists of different but compatible types using LINQ −

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

class Program {
    public static void Main() {
        var numbers = new List<int> { 1, 2, 3 };
        var strings = new List<string> { "4", "5", "6" };

        var combined = numbers.Select(n => n.ToString()).Concat(strings).ToList();

        Console.WriteLine("Combined list:");
        foreach(var item in combined) {
            Console.WriteLine(item);
        }
    }
}

The output of the above code is −

Combined list:
1
2
3
4
5
6

Conclusion

To join two lists in C#, use AddRange() to modify an existing list or Concat() to create a new combined list. Choose AddRange() for better performance when you don't need to preserve the original lists, and Concat() when you want to keep the original lists unchanged.