using System;

using System.Text.RegularExpressions;

using System.Linq;

using System.Collections.Generic;

​

public interface IHasId

{

    int Id { get; }

}

​

public class Subfoo : IHasId {

    public int Id { get; set;}

    public Subfoo(int id)

    {

        Id = id;

    }

}

​

public class Foo<T>

{

    public List<T> Children {get; internal set;}

    public Foo()

    {

        Children = new List<T>();   

    }

    public TItem GetById<TItem>(int id)

        where TItem : T, IHasId

    {

        return Children.Where(t => t is TItem).Cast<TItem>().FirstOrDefault(t => t.Id == id);

    }

}

​

public class Program

{

    public static void Main()

    {

        var foo = new Foo<Subfoo>();

        foo.Children.Add(new Subfoo(23));

        Console.WriteLine(foo.GetById<Subfoo>(23) == null ? "Null" : "Found");

    }

}