#nullable enable

using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public class Item

    {

        public string? MyString { get; init; }

    }

    public static void Main()

    {

        Item item1 = new () { MyString = "Item1" };

        Item item2 = new () { MyString = null };

        Item item3 = new () { MyString = "Item3" };

        List<Item>? items = new() { item1, item2, item3 };

        IEnumerable<string> strings = items.Where(x => x.MyString != null).Select(x => x.MyString!);

        Console.WriteLine(string.Join(":", strings));

    }

}