在C＃中使用可空类型和空合并运算符？

Question

在下面的MSDN官方文章LINQ Outer Join中，作者正在使用?和??一起在...select new { person.FirstName, PetName = subpet?.Name ?? String.Empty };。 问题：为什么不只是使用??作为：select new { person.FirstName, PetName = subpet.Name ?? String.Empty };，我认为它可以翻译为if subpet.Name is null give me empty string。我误解了什么吗？

原因的调查：当我在我的代码使用?和??在一起（解释here）的VS2015智能感知是给我的错误在该职位还解释。

class Person 
{ 
    public string FirstName { get; set; } 
    public string LastName { get; set; } 
} 

class Pet 
{ 
    public string Name { get; set; } 
    public Person Owner { get; set; } 
} 

public static void LeftOuterJoinExample() 
{ 
    Person magnus = new Person { FirstName = "Magnus", LastName = "Hedlund" }; 
    Person terry = new Person { FirstName = "Terry", LastName = "Adams" }; 
    Person charlotte = new Person { FirstName = "Charlotte", LastName = "Weiss" }; 
    Person arlene = new Person { FirstName = "Arlene", LastName = "Huff" }; 

    Pet barley = new Pet { Name = "Barley", Owner = terry }; 
    Pet boots = new Pet { Name = "Boots", Owner = terry }; 
    Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte }; 
    Pet bluemoon = new Pet { Name = "Blue Moon", Owner = terry }; 
    Pet daisy = new Pet { Name = "Daisy", Owner = magnus }; 

    // Create two lists. 
    List<Person> people = new List<Person> { magnus, terry, charlotte, arlene }; 
    List<Pet> pets = new List<Pet> { barley, boots, whiskers, bluemoon, daisy }; 

    var query = from person in people 
       join pet in pets on person equals pet.Owner into gj 
       from subpet in gj.DefaultIfEmpty() 
       select new { person.FirstName, PetName = subpet?.Name ?? String.Empty }; 

    foreach (var v in query) 
    { 
     Console.WriteLine($"{v.FirstName+":",-15}{v.PetName}"); 
    } 
} 

// This code produces the following output: 
// 
// Magnus:  Daisy 
// Terry:   Barley 
// Terry:   Boots 
// Terry:   Blue Moon 
// Charlotte:  Whiskers 
// Arlene: 

Answer 1

正如@Lee所述，subpet可能为空。所以，在致电subpet.Name时抛出异常。因此，??检查subpet是否为空，并且?回到调用.Name属性。

Answer 2

你不是误会。程序员是为空测试subpet，由于DefaultIfEmpty()

在表达式

subpet?.Name ?? String.Empty 

• subpet可以为空，在这种情况下subpet?.Name也为空。
• String.Empty可缩短至 “”

所以要保证一个非空字符串，

subpet?.Name ?? ""