<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 &lt;= 0)
	{
		return null;
	}

var dict = new Dictionary<int>();
int listVal = 0;
	for (int i = 0; i &lt; list.Count; i++)
	{
		listVal = list[i];
		if (!dict.ContainsKey(listVal))
		{
			dict.Add(listVal,1);
			continue;
		}
		
		dict[listVal]++;
	}
		 
	 return dict.OrderBy(c =&gt; c.Value).FirstOrDefault().Key;
  }
}

</int></int></int></p>