各位大大好!小弟，最近剛學asp.net core mvc，買了一本<ASP.NET Core 7 MVC 範例實戰演練>，作者:奚江華，我目前做ch02範例!
我遇到一些問題，想請教各位大大!

問題1:請教LINQ查詢這段，這是甚麼語法呢?最基本語法是甚麼呢?

//以LINQ查詢泛集合
var friendsQuery = from f in friends where f.Country == "USA" || f.Country == "Korea" select f;

問題2:下面程式碼，我的VisualStidio 2022的Models/Frinde.cs的Name和Country出現波浪底線要 請問string後面加?嗎?，為甚麼要加[?]呢

public class Friend
{
    /*
     Models/Friends.cs為了Controllers/CSharpBasisController的IActionResult
     的函式內建立Friend物件和List集合
     */
    public string Name { get; set; }
    public string Country { get; set; }

}

問題3:

下面程式碼是正常List宣告物件語法

List<Friend> friends = new List<Friend>
{
    new Friend { Name = "Rose",Country = "USA"},
    new Friend { Name = "David", Country = "Japan" },
    new Friend { Name = "John", Country = "USA" },
    new Friend { Name = "Bob", Country = "Italy" },
    new Friend { Name = "Johnson", Country = "Thailand" },
    new Friend { Name = "Cindy", Country = "Japan" },
    new Friend { Name = "Lucy", Country = "Korea" },
    new Friend { Name = "Angel", Country = "Italy" },
    new Friend { Name = "Maya", Country = "Thailand" },
    new Friend { Name = "Max", Country = "Korea" }
};

但Visual Studio2022在上面程式碼..提示，然後我照提示，為甚麼要改成下面程式碼呢?

//集合式初始化設定式(新寫法)
List<Friend> friends = new()
{
    new Friend { Name = "Rose",Country = "USA"},
    new Friend { Name = "David", Country = "Japan" },
    new Friend { Name = "John", Country = "USA" },
    new Friend { Name = "Bob", Country = "Italy" },
    new Friend { Name = "Johnson", Country = "Thailand" },
    new Friend { Name = "Cindy", Country = "Japan" },
    new Friend { Name = "Lucy", Country = "Korea" },
    new Friend { Name = "Angel", Country = "Italy" },
    new Friend { Name = "Maya", Country = "Thailand" },
    new Friend { Name = "Max", Country = "Korea" }
};

不好意思!造成各位大大誤會!我昨天有實作出書中範例，版型畫面被我改掉!我是做完後，有不懂語法才請教各位大大，我就附上自己練習成果，照片如下!下次我在it邦幫這裡發問問題會附上練習成果!請大家多多包涵和諒解!