using System;

​

public class Program

{

    public static void Main()

    {

        var first = new string[]

        {

        "one", "two", "three"

        }

​

        ;

        var second = new string[]

        {

        "four", "five"

        }

​

        ;

        // concat 2 arrays

        var result1 = string.Join(",", first.ArrayConcat(second));

        Console.WriteLine(result1);

        // pass single argument to add to initial array

        var result2 = string.Join(",", first.ArrayConcat("FOUR"));

        Console.WriteLine(result2);

        // pass multiple arguments to add to initial array

        var result3 = string.Join(",", first.ArrayConcat("4", "5", "6"));

        Console.WriteLine(result3);

    }

}

​

public static class ArrayExtensions

{

    public static T[] ArrayConcat<T>(this T[] array1, params T[] array2)

    {

        T[] returnValue = new T[array1.Length + array2.Length];

        Array.Copy(array1, 0, returnValue, 0, array1.Length);

        Array.Copy(array2, 0, returnValue, array1.Length, array2.Length);

        return returnValue;

    }

}