<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);
}

using System.Collections.Generic;
using System.Linq;

public class Solution : ICodeRush
{
  private readonly Dictionary<int countandkey> _fDict = new Dictionary<int countandkey>();

  private class CountAndKey : IComparable<countandkey>
  {
    public int Repetitions;
    public int Key;

    public int CompareTo(CountAndKey other)
    {
      return Repetitions - other.Repetitions;
    }
  }

  public int? FindLeastFrequentNumber(IList<int> list)
  {
    // If list is null or empty, return null
    if (list == null || list.Count == 0) return null;

    // Build up a frequency dictionary
    _fDict.Clear();
    foreach (var key in list)
    {
      CountAndKey value;
      if (_fDict.TryGetValue(key, out value))
      {
        value.Repetitions++;
      }
      else
      {
        _fDict[key] = new CountAndKey { Key = key };
      }
    }

    // Return the first instance of the minimum frequency dictionary key
    return _fDict.Values.Min().Key;
  }
}

</int></countandkey></int></int></int></p>