Conditional linq operators, C#

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//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var list = new List<Test>()
            {
                new Test(1, 2),
                new Test(1, 5),
                new Test(1, 3),
                new Test(1, 4),
                new Test(1, 1),
                new Test(1, 3),
                new Test(2, 2),
                new Test(2, 5),
                new Test(2, 3),
                new Test(2, 4),
                new Test(2, 1),
                new Test(2, 3)
            };
                
            Console.WriteLine("Orderer by one, then by two");
            list.OrderBy(f => f.One).ThenByIf(true, f => f.Two).ToList().ForEach(f => Console.WriteLine(f));
            
            Console.WriteLine("Orderer by one only");
            list.OrderBy(f => f.One).ThenByIf(false, f => f.Two).ToList().ForEach(f => Console.WriteLine(f));
        }
    }
    
    public class Test
    {
        public int One {get; set;}
        public int Two {get; set;}
        
        public Test(int one, int two)
        {
            One = one;
            Two = two;
        }
        public override string ToString()
        {
            return string.Format("One: {0}, Two: {1}", One, Two);
        }
    }
    
    public static class Ext
    {
        public static IOrderedEnumerable<T> ThenByIf<T, TKey>(this IOrderedEnumerable<T> list, bool predicate, Func<T, TKey> sel)
		{
			if(predicate)
				return list.ThenBy(f => sel(f));
			else
				return list;
		}
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0.48 sec, absolute running time: 0.19 sec, cpu time: 0.14 sec, average memory usage: 24 Mb, average nr of threads: 5