C# program to concat two or more Lists

In C#, you can concatenate multiple List<T> objects using the Concat() method from LINQ. This method creates a new sequence that contains all elements from the original lists in order without modifying the original lists.

Syntax

Following is the syntax for concatenating lists using the Concat() method −

var result = list1.Concat(list2);

For multiple lists, you can chain the Concat() calls −

var result = list1.Concat(list2).Concat(list3);

Using Concat() to Merge Two Lists

Example

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

public class Demo {
   public static void Main() {
      var list1 = new List<int>{3, 4};
      var list2 = new List<int>{1, 2, 3};

      var result = list1.Concat(list2);

      Console.WriteLine("Concatenated result:");
      foreach(int i in result) {
         Console.WriteLine(i);
      }
   }
}

The output of the above code is −

Concatenated result:
3
4
1
2
3

Using Concat() to Merge Multiple Lists

Example

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

public class Demo {
   public static void Main() {
      var list1 = new List<int>{3, 4};
      var list2 = new List<int>{1, 2, 3};
      var list3 = new List<int>{2, 5, 6};

      var result = list1.Concat(list2).Concat(list3);

      Console.WriteLine("Result after concatenating three lists:");
      foreach(int i in result) {
         Console.WriteLine(i);
      }
   }
}

The output of the above code is −

Result after concatenating three lists:
3
4
1
2
3
2
5
6

Using AddRange() Alternative Method

Another approach to concatenate lists is using the AddRange() method, which modifies the original list −

Example

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main() {
      var list1 = new List<string>{"Apple", "Banana"};
      var list2 = new List<string>{"Orange", "Grape"};
      var list3 = new List<string>{"Mango"};

      list1.AddRange(list2);
      list1.AddRange(list3);

      Console.WriteLine("Result using AddRange():");
      foreach(string fruit in list1) {
         Console.WriteLine(fruit);
      }
   }
}

The output of the above code is −

Result using AddRange():
Apple
Banana
Orange
Grape
Mango

Comparison

Conclusion

The Concat() method is ideal when you want to combine lists without modifying the original collections, while AddRange() is better when you want to merge lists into an existing collection. Both methods preserve the order of elements from the source lists.