using System;

using System.Collections.Generic;

using System.Linq;

​

public interface IMyString

{

    string Id {get;set;}

};

​

public class MyString : IMyString

{

    public string Id {get;set;}

}

​

public static class Extensions

{

    public static IEnumerable<IMyString> WithId(this IEnumerable<IMyString> source, string id)

    {

        return source.Where(x => x.Id == id);

    }

}

​

public class Program

{

    private static List<IMyString> GetMyStrings(string key, List<IMyString> input)

    {

        return input.WithId(key).ToList();

    }

​

    public static void Main()

    {

        var foo = new List<IMyString>{ new MyString { Id = "yes"}, new MyString { Id = "no" } };

        var result = GetMyStrings("yes", foo);

        var result2 = foo.WithId("no");

        Console.WriteLine(result2);

    }

}