<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 || list.Count == 0) return null;
    Dictionary<int int> NumberCountDictionary = new Dictionary<int int>();
    foreach (int item in list)
    {
      if (!NumberCountDictionary.ContainsKey(item))
      {
        NumberCountDictionary.Add(item, 1);
      }
      else
      {
        NumberCountDictionary[item]++;
      }
    }
    for (int i = 0; i &lt; NumberCountDictionary.Count; i++)
    {
      if (NumberCountDictionary.ContainsValue(i))
      {
        foreach (KeyValuePair<int int> pair in NumberCountDictionary)
        {
          if (pair.Value == i)
          {
            return pair.Key;
          }
        }
      }
    }
    return null;
  }
}

</int></int></int></int></int></p>