using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

public class Program

{

    public static void Main()

    {

        IList<Foo> list = new List<Foo>() { 

            new Foo { Id = 1}, 

            new Foo { Id = 2}

        };

        IReadOnlyCollection<Foo> readOnlyList = new ReadOnlyCollection<Foo>(list);

        Process(readOnlyList);

    }

    public static void Process(IEnumerable<Foo> list)

    {       

        foreach(Foo foo in list)

        {

            Console.WriteLine(foo.Id);

        }

    }

}

​

public class Foo 

{

    public int Id {get; set;}

}