using System;

using System.Collections.Generic;

using System.Linq;

​

#nullable enable

public class Program

{

    public static void Main()

    {

        var list = new List<string?> {

            null,

            "hello",

            null,

            "goodbye",

        };

        foreach (var word in list.OfType<string>()) {

            Console.WriteLine(word);

        }

    }

}