C#：可空结构的默认文字和类型推断

Question

从 C# 7.1 开始，可以使用 default 获取默认值而不指定类型。我今天尝试了一下，发现可空结构和可空值类型的结果有些违反直觉。

[TestFixture]
public class Test
{
    private class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }

    [Test]
    public void ShouldBehaveAsExpected()
    {
        var person1 = new Person {Name = "John", Age = 58};
        var person2 = new Person {Name = "Tina", Age = 27};

        var persons = new[] {person1, person2};

        int? myAge = persons.FirstOrDefault(p => p.Name == "MyName")?.Age;
        var myDefaultAge = myAge ?? default;

        var myAgeString = myAge == null ? "null" : myAge.ToString();
        var myDefaultAgeString = myDefaultAge == null ? "null" : myDefaultAge.ToString();

        Console.WriteLine("myAge: " + myAgeString);                 // "myAge: null"
        Console.WriteLine("myDefaultAge: " + myDefaultAgeString);   // "myDefaultAge: 0"

    }
}

我本以为 myDefaultAge 是 null 而不是 0，因为 myAge 是 int? 类型而 default(int?) 是 null ].

是否在任何地方指定了此行为？ C# programming guide 仅表示“默认文字产生与等效默认值 (T) 相同的值，其中 T 是推断类型。”

Answer 1

已记录 null-coalescing operator:

The type of the expression a ?? b depends on which implicit conversions are available on the operands. In order of preference, the type of a ?? b is A0, A, or B, where A is the type of a (provided that a has a type), B is the type of b (provided that b has a type), and A0 is the underlying type of A if A is a nullable type

准确描述了我们这里的情况 - A 可以为 null，default 没有类型。

C#：可空结构的默认文字和类型推断

C#: default literal and type inference on nullable structs

c#

nullable

c#-7.1