using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static List<Child> GetChildren() {

        return new List<Child> { new Child() };

    }

    public static void Main()

    {

        Console.WriteLine("Hello World");

        List<Child> originalList = GetChildren();

// some lines of code

        List<IHasChildID> orderedList = ChildOrderHelper.OrderChildren(originalList).ToList(); // error

​

    }

​

​

public interface IHasChildID

{

    string GetChildID();

}

​

public class Child : IHasChildID

{

    public string ChildID { get; set; }

    public string GetChildID()

    {

        return ChildID;

    }

}

​

public class Parent : IHasChildID

{

    public Child Child { get; set; }

    public string GetChildID()

    {

        return Child.ChildID;

    }

}

​

public class OtherClassID 

{ 

    public int ID { get; set; }

    public string ChildID { get; set; }

}

​

public class SomeOtherClass : IHasChildID

{

    public OtherClassID ID { get; set; }

    public string GetChildID()

    {

        return ID.ChildID;

    }

}

​

​

public static class ChildOrderHelper

{

    public static IEnumerable<IHasChildID> OrderChildren(IEnumerable<IHasChildID> children)

    {

        var childrenList = children.ToList();

        //do some splitting, ordering and conatenation of lists

        return childrenList.OrderBy(c => c.GetChildID()).ToList();

​

    }

}

}