Concat. This is a C# extension method. With it we take 2 IEnumerable collections. We then get a collection with all of the elements together.

This extension method effectively concatenates sequences. We can combine Lists or arrays (and even different IEnumerables) together.

Example. In this program, we include the System.Linq namespace to get access to the Concat method. Two arrays are created upon execution of the Main method.

using System;
using System.Linq;

int[] array1 = { 1, 3, 5 };
int[] array2 = { 0, 2, 4 };

// Concat array1 and array2.
var result1 = array1.Concat(array2);
foreach (int value in result1)
{
    Console.WriteLine(value);
}
Console.WriteLine();

// Concat array2 and then array1.
var result2 = array2.Concat(array1);
foreach (int value in result2)
{
    Console.WriteLine(value);
}
1
3
5
0
2
4

0
2
4
1
3
5

List example. To use Concat, we must have 2 collections that implement IEnumerable. This includes the List type or the array type.

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

List<string> list1 = new List<string>();
list1.Add("dot");
list1.Add("net");

List<string> list2 = new List<string>();
list2.Add("perls");
list2.Add("!");

var result = list1.Concat(list2);
List<string> resultList = result.ToList();
foreach (var entry in resultList)
{
    Console.WriteLine(entry);
}
dot
net
perls
!

Summary. If you have two Lists, you can use the Concat method to combine them. For optimal performance, however, using a method such as AddRange would be better if you need a List result.

Much like string.Concat concatenates strings, the Concat extension concatenates collections. Whenever we need to combine two arrays into a third array, we use the Concat extension method.