using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var nums= (new List<int> (){1,2,3,25,4,5,6,7,8, 12, 15,21,22,23}).AsQueryable();

​

        nums

          .OrderBy(x => x)

          .GroupJoin(nums, n=> n + 1, ni => ni, (o,i)=> new Class1 {O=o, I=i})

          .Where(o =>  !(o.I is IGrouping<int, int>)).Dump();

    }

}

​

public class Class1 {

    public int O {get; set;}

    public IEnumerable<int> I {get; set;}

}