<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 System.Collections.Concurrent.ConcurrentDictionary<int>();   
	
	System.Threading.Tasks.Parallel.ForEach(list, i =&gt;	
	{	         
	dict.AddOrUpdate(i, 1, (k,v) =&gt; v += 1);
	});	
	
	return dict.OrderBy(c =&gt; c.Value).FirstOrDefault().Key;
    
  }
}

</int></int></int></p>