using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var orders = new List<Order>();

        var order = new Order

        {

            OrderId = 1,

            Index = 1,

        };

        orders.Add(order);

        var index = orders.Where(x => x.OrderId == 2).DefaultIfEmpty().Max(x => x.Index) + 1;

        Console.Write(index);

    }

}

​

public class Order {

    public int OrderId {get; set;}

    public int Index {get; set;}

}