using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    static IEnumerable<List<MyObj>> Get()

    {

        yield return new List<MyObj>();

        yield return new List<MyObj>();

    }

​

    static void Main()

    {

        IEnumerable<List<MyObj>> myObjGroups = Get();

        var result = myObjGroups.Cast<IEnumerable<IMyInterface>>();

        foreach(var val in result)

            Console.WriteLine(val.Count());

    }

}

​

public interface IMyInterface

{

    void Foo();

}

​

public class MyObj : IMyInterface

{

    public void Foo()

    {

        throw new NotImplementedException();

    }

}