using System;

using System.Linq;

using System.Collections.Generic;

​

class A {

    public A(string name) {Name = name;}

    public string Name {get;set;}

}

​

class B {

    public B(string name) {Name = name;}

    public string Name {get;set;}

}

​

public class Program

{

    public static void Main(string[] args)

    {

        var fields = new object[] 

        {

            new A("a"),

            new B("b"),

            new A("A"),

            new B("B")

        };

        var fieldNames = new List<string>();

        fieldNames.AddRange(fields.Select(obj => (obj as A)?.Name ?? (obj as B)?.Name));

        Console.WriteLine(string.Join("\n", fieldNames));

    }

}