为什么 C# 6.0 不允许在使用 Null 传播运算符时设置非 null 可为空结构的属性? [英] Why C# 6.0 doesn't let to set properties of a non-null nullable struct when using Null propagation operator?

问题描述

假设我们有以下代码:

struct Article
{
    public string Prop1 { get; set; }
}

Article? art = new Article();
art?.Prop1 = "Hi"; // compile-error

编译错误是

CS0131 赋值的左侧必须是变量、属性或索引器.

CS0131 The left-hand side of an assignment must be a variable, property or indexer.

实际上 art?.Prop1 是一个属性，应该被视为有效的赋值！
我不认为赋值有任何问题使此代码无效.

Actually art?.Prop1 is a property and should be considered as a valid assignment!
I don't see any problem with assignment to make this code invalid.

为什么 C# 6.0 不允许设置非空可空结构的属性?
或者，任何有关一行代码以使分配有效的建议将不胜感激.

Why C# 6.0 doesn't let to set properties of a non-null nullable struct ?
Alternately any suggestion for one line code to make assignment valid would be appreciated.

推荐答案

这段代码:

Article? art

将定义一个 Nullable

但稍后当你这样做时:

will define a Nullable<Article> but later when you do:

art?.Prop1 = "Hi";

这意味着使用 Null 传播/条件运算符.

This will mean using Null propagation/conditional operator.

空传播/条件运算符用于访问属性，而不是设置它们.因此你不能使用它.

Null propagation/conditional operator is for accessing properties, not setting them. Hence you can't use it.

正如@Servy 在评论中所说的，Null 条件运算符的结果始终是一个值，您不能为一个值赋值，因此出现错误.

As @Servy said in the comments, the result of Null conditional operator is always a value and you can't assign a value to a value, hence the error.

如果您只是想设置属性，那么您不需要 ? 和对象名称，? 和 Nullable> 声明时使用类型，语法糖为:

If you are only trying to set the property then you don't need ? with the object name, ? with Nullable<T> types is used at the time of declaration, which is syntactic sugar to:

Nullable<Article> art; //same as Article? art

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