<p>using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Xml;
using System.Text.RegularExpressions;

public interface ICodeRush
{
  int? FindLeastFrequentNumber(IList<int> list);
}

public class Solution : ICodeRush
{
  public int? FindLeastFrequentNumber(IList<int> list)
  {
            if (list != null &amp;&amp; list.Any())
            {
                var dict = new System.Collections.Concurrent.ConcurrentDictionary<int int>();
                Parallel.ForEach(list, i =&gt; dict.AddOrUpdate(i, 1, (key, oldValue) =&gt; oldValue + 1));
                var min = dict.Values.Min();
                return dict.First(pair =&gt; pair.Value == min).Key;
            }
            return null;
  }
}

</int></int></int></p>