How to Merge Two Lists in C#: Simple Methods Explained

To merge two lists in C#, you can use the AddRange method to add all elements from one list to another, or use LINQ's Concat method to create a new combined list. Both ways combine the elements, but AddRange modifies the original list while Concat returns a new list.

Syntax

Here are two common ways to merge lists in C#:

• AddRange: Adds all elements from one list to the end of another list.
• Concat: Creates a new sequence by joining two lists without changing the originals.

csharp

list1.AddRange(list2);

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

Example

This example shows how to merge two lists using both AddRange and Concat. It prints the merged lists to the console.

csharp

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

class Program
{
    static void Main()
    {
        List<int> list1 = new List<int> { 1, 2, 3 };
        List<int> list2 = new List<int> { 4, 5, 6 };

        // Using AddRange (modifies list1)
        List<int> list1Copy = new List<int>(list1); // copy to keep original
        list1Copy.AddRange(list2);
        Console.WriteLine("Merged with AddRange:");
        foreach (var item in list1Copy)
        {
            Console.Write(item + " ");
        }
        Console.WriteLine();

        // Using Concat (creates new list)
        var mergedList = list1.Concat(list2).ToList();
        Console.WriteLine("Merged with Concat:");
        foreach (var item in mergedList)
        {
            Console.Write(item + " ");
        }
        Console.WriteLine();
    }
}

Output

Merged with AddRange:
1 2 3 4 5 6 
Merged with Concat:
1 2 3 4 5 6 

Common Pitfalls

Modifying original lists unintentionally: Using AddRange changes the first list, which might not be what you want if you need to keep it unchanged.

Forgetting to convert after Concat: Concat returns an IEnumerable, so you must call ToList() to get a list.

csharp

List<int> list1 = new List<int> { 1, 2 };
List<int> list2 = new List<int> { 3, 4 };

// Wrong: This changes list1 permanently
list1.AddRange(list2);

// Right: Create a new merged list without changing originals
var merged = list1.Concat(list2).ToList();

Quick Reference

• AddRange: Use to add all elements from one list into another. Modifies the original list.
• Concat + ToList: Use to create a new merged list without changing originals.
• Remember to using System.Linq; for LINQ methods like Concat.

Key Takeaways

Use AddRange to merge lists when you want to modify the first list.

Use Concat with ToList to merge lists without changing the originals.

Always call ToList after Concat to get a List instead of IEnumerable.

Be careful not to unintentionally change your original lists.

Include System.Linq namespace to use LINQ methods like Concat.