using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var sut = new Test();

        var fields = sut.ReturnCustomFieldsOnly();

        Console.WriteLine("Before");

        foreach(var field in fields)

        {

            Console.WriteLine(field.Name);

        }

        Console.WriteLine("\nAfter");

        fields = sut.ReturnAllFields();     

        foreach(var field in fields)

        {

            Console.WriteLine(field.Name);

        }

    }

    public class Test

    {

        public IEnumerable<Type> BaseMethod()

        {

            // this is the emulation of the base method

            yield return typeof(field1);

            yield return typeof(field2);

            yield return typeof(field3);

        }

        public IEnumerable<Type> ReturnCustomFieldsOnly()

        {

            // we return our custom set of fields

            yield return typeof(field4);

            yield return typeof(field5);

            // base method is just called

            // its return object never used and never passed through

            // meaning field1, field2 and field 3 are never returning to the user of the method

            BaseMethod();

        }

​

        public IEnumerable<Type> ReturnAllFields()

        {

            // we return our custom set of fields

            yield return typeof(field4);

            yield return typeof(field5);

​

            // call base method and iterate through each field

            foreach (var baseField in BaseMethod())

            {

                // return each field that you got from the base method

                yield return baseField;

            }

        }

        public abstract class field1 {}

        public abstract class field2 {}

        public abstract class field3 {}

        public abstract class field4 {}

        public abstract class field5 {}

    }

}