using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var counter = new Dictionary<int, int>();

        var numbers = new int[] { 1, 9, 4, 5, 8, 5, 4, 1, 9 };

        int? singleOccurrence = null;

​

        foreach (var item in numbers)

        {

            if (!counter.TryAdd(item, 1))

            {

                counter[item]++;

            }

            else

            {

                singleOccurrence = item;

            }

        }

​

        Console.WriteLine($"Item that only appeared once is: {singleOccurrence}");

        Console.WriteLine($"Number of items in the original collection: {numbers.Length}");

        Console.WriteLine($"Number of items in the dictionary: {counter.Count}");

    }

}