using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    private static readonly int[] ExcludedItems = { 21 };

    private static int Check(int i)

    {

        Console.WriteLine("Checking " + i); 

        return i;

    }

    private static void ProcessItems(IEnumerable<int> items)

    {

        Console.WriteLine("ProcessItems() start");

        foreach(var i in items.Except(ExcludedItems)) {}

        Console.WriteLine("ProcessItems() end\n");

    }

    private static void ProcessReadOnlyItems(IReadOnlyList<int> items)

    {

        Console.WriteLine("ProcessReadOnlyItems() start");

        foreach(var i in items.Except(ExcludedItems)) {}

        Console.WriteLine("ProcessReadOnlyItems() end\n");

    }

    public static void Main()

    {

        var l = new List<int>() { 1, 7, 11, 21, 314, 535, 999 };        

        var items = l.Where(i => Check(i) > 0);

        ProcessItems(items);

        ProcessReadOnlyItems((IReadOnlyList<int>)items.Except(ExcludedItems).ToList());

    }

}