Task.Intersection and union

using System;
using System.Collections.Generic;
class IntersectionAndUnion
{
    static void Main()
    {
        int[] firstArray = {1,2,3};
        Console.Write("First array: ");
        PrintArray(firstArray);
        Console.Write("Second array: ");
        int[] secondArray = { 23, 34, 45, 3};
        PrintArray(secondArray);

        int[] union = CreateUnion(firstArray, secondArray);
        Console.WriteLine("Without repeate numbers: ");
        PrintArray(union);
        Console.WriteLine("Numbers occur in both arrays:");
        CreateIntersetion(firstArray, secondArray);//направо ги принтя в метода
    }

    static void CreateIntersetion(int[] firstArray, int[] secondArray)
    {
        Console.Write("{");
        foreach (int element in firstArray)
        {
            if(Array.IndexOf(secondArray, element) >= 0)
            {
                Console.Write(element + " ");
            }
        }
        Console.Write("}");
        Console.WriteLine();
    }

    static int[] CreateUnion(int[] firstArray, int[] secondArray)
    {
        List<int> union = new List<int>();
        union.AddRange(firstArray);

        foreach (int element in secondArray)
        {
            //if (Array.IndexOf(firstArray, element) == -1)//Може и така да се направи
            //{
            //    union.Add(element);
            //}
            if (!union.Contains(element))
            {
                union.Add(element);
            }
        }
        return union.ToArray();
    }

    private static void PrintArray(int[] firstArray)
    {
        Console.Write("{");

        foreach (int elements in firstArray)
        {
            Console.Write(elements + " ");
        }
        Console.Write("}");

        Console.WriteLine();
    }
}