var list1 = new [] { 1, 4, 5};
var list2 = new [] { 3, 4, 8};

var query = list1.Except(list2).Concat(list2.Except(list1));

ET>var list1 = new [] { 1, 4, 5};
ET>var list2 = new [] { 3, 4, 8};

ET>var query = list1.Except(list2).Concat(list2.Except(list1));
ET>

using System;
using System.Linq;
                    
public class Program
{
    public static void Main()
    {
        int[] num1 = { 1, 2, 3, 4, 3, 55, 23, 2 };           
        int[] num2 = { 55, 23, 45, 50, 80 };
 
        var all = num1.Union(num2).ToArray();
        Array.ForEach(all, n => Console.WriteLine(n));
    }
}

var set = new HashSet<int>(list1);
set.SymmetricExceptWith(list2);