using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<SomeBaseType> list = [new SomeSubType(), 

                                   new SomeSubTypeWP(){SomeProperty="One"},

                                   new SomeSubType(), 

                                   new SomeSubTypeWP(){SomeProperty="Two"},

                                   new SomeSubType(), 

                                   new SomeSubTypeWP(){SomeProperty="Three"}

                                  ];

        list.OfType<IHasSomeProperty>().Print();

    }

}

​

public interface IHasSomeProperty

{

    string SomeProperty {get; set;}

}

​

public abstract class SomeBaseType

{}

​

public sealed class SomeSubType : SomeBaseType

{}

​

public sealed class SomeSubTypeWP : SomeBaseType, IHasSomeProperty

{

    public string SomeProperty {get; set;}

}

​

public static class ListOfTypesWithSomePropertyExtensions

{

    public static void Print(this IEnumerable<IHasSomeProperty> list)

    {

        foreach(var item in list) Console.WriteLine(item.SomeProperty);

    }

}