using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Scenario1();

        Scenario2();

    }

    private static void Scenario1()

    {

        var dog = new Dog();

        var cat = new Cat();

​

        List<PetInterface> petInterfaceList = new List<PetInterface>();

​

        petInterfaceList.Add(dog);

        petInterfaceList.Add(cat);

    }

    private static void Scenario2()

    {

        var dog = new Dog();

        var cat = new Cat();

        List<Pet> petList = new List<Pet> { dog, cat };

        List<PetInterface> petInterfaceList = petList;

    }

}

​

public interface PetInterface

{

    public int petId { get; set; }

    public String name { get; set; }

}

​

public class Pet

{

    public int petId { get; set; }

    public String name { get; set; }

}

​

public class Dog : Pet, PetInterface

{

}

​

public class Cat : Pet, PetInterface

{

}

​