using System;

using System.Collections.Generic;

using System.Linq;

​

public class MainClass

{

   public ClassA someProp { get; set; }

   public ClassA someProp2 { get; set; }

}

​

public class ClassA

{

   public virtual Type Types => Type.None;

}

​

public class ClassB:ClassA

{

   public override Type Types => Type.Default;

   public string FieldName { get; set; }

}

​

public class ClassC:ClassA

{

   public override Type Types => Type.Value;

   public int FieldValue { get; set; }

}

​

public enum Type

{

    None = -1,

    Default = 0,

    Value = 1

}

​

public class Program

{

    public static void Main()

    {

        List<MainClass> entities = new()

        {

            new() { someProp = new ClassC() { FieldValue = 4 }, someProp2 = new ClassB() { FieldName = "Flower" } },

            new() { someProp = new ClassA() { }, someProp2 = new ClassC() { FieldValue = 7 } }

        };

        IEnumerable<string> values = entities

            .SelectMany(ent => new[] { ent.someProp, ent.someProp2 })

            .Aggregate(new List<string>(),

                ( fields, property ) => 

                {

                    if (property is ClassB)

                    {

                        fields.Add(((ClassB)property).FieldName);

                    }

                    else if (property is ClassC)

                    {

                        fields.Add(((ClassC)property).FieldValue.ToString());

                    }

                    return fields;

                });

        Console.WriteLine(string.Join(Environment.NewLine, values));

    }

}