List sum

using System;
using System.IO;
using System.Collections.Generic;

namespace ConsoleApp3
{
    internal class Program
    {
        static void Main(string[] args)
        {
            int[] array1 = { 1, 2, 3, 5 };
            int[] array2 = { 1, 2, 3, 4, 5 };
            List<int> sumArray = new List<int>();

            for (int i = 0; i < array1.Length; i++)
            {
                sumArray.Add(array1[i]);
            }
            for (int i = 0; i < array2.Length; i++)
            {
                bool isAdd = true;

                foreach (var element in sumArray)
                {
                    if (element == array2[i])
                    {
                        isAdd = false;
                    }
                }
                if (isAdd)
                {
                    sumArray.Add(array2[i]);
                }
            }
            foreach (var elemnt in sumArray)
            {
                Console.Write(elemnt + " ");
            }
        }
    }
}