=> 是什么?属性签名中的 C# 赋值 [英] What is the => assignment in C# in a property signature

问题描述

我遇到了一些代码

public int MaxHealth =>内存[地址].IsValid ?内存[地址].Read(Offs.Life.MaxHp):0;

现在我对 Lambda 表达式有些熟悉了.我只是没有看到它这样使用它.

上面的语句和

有什么区别

public int MaxHealth = x ?y:z;

解决方案

你看到的是一个expression-bodied 成员 不是 lambda 表达式.

当编译器遇到表达式主体的 property 成员时，它实质上会将其转换为如下所示的 getter:

public int MaxHealth{得到{返回内存[地址].IsValid ?内存[地址].Read(Offs.Life.MaxHp) : 0;}}

(您可以通过将代码注入一个名为 TryRoslyna 的工具来验证这一点./p>

表达式主体成员 - 就​​像大多数 C# 6 功能一样 - 只是 语法糖.这意味着它们不提供通过现有功能无法实现的功能.相反，这些新功能允许使用更具表现力和简洁的语法

如您所见，表达式体成员有一些使属性成员更紧凑的快捷方式:

• 不需要使用 return 语句，因为编译器可以推断出你想要返回表达式的结果
• 不需要创建语句块，因为主体只是一个表达式
• 没有必要使用 get 关键字，因为它是通过使用表达式体成员语法来暗示的.

我将最后一点加粗，因为它与您的实际问题相关，我现在将回答.

...的区别

//表达式体成员属性公共 int MaxHealth =>X ?y:z;

还有……

//带有字段初始化器的字段公共 int MaxHealth = x ?y:z;

和...的区别是一样的

public int MaxHealth{得到{返回 x ?y:z;}}

还有……

public int MaxHealth = x ?y:z;

如果您了解属性，这应该是显而易见的.

但要明确一点:第一个清单是一个在引擎盖下有一个 getter 的属性，每次访问它时都会调用它.第二个清单是一个带有字段初始值设定项的字段，当类型被实例化时，它的表达式只计算一次.

语法上的这种差异实际上非常微妙，可能会导致 Bill Wagner 在题为 "AC# 6 gotcha:初始化与表达式主体成员".

虽然表达式体成员是 lambda 表达式-类似，但它们不是 lambda 表达式.根本区别在于 lambda 表达式生成委托实例或表达式树.表达式体成员只是编译器在幕后生成属性的指令.相似度(或多或少)以箭头 (=>) 开始和结束.

我还要补充一点，表达式体成员不限于属性成员.他们对所有这些成员都起作用:

• 属性
• 索引器
• 方法
• 运营商

添加在 C# 7.0

• 构造函数
• 终结者

但是，它们不适用于这些成员:

• 嵌套类型
• 活动
• 字段

I came across some code that said

public int MaxHealth => 
         Memory[Address].IsValid ? 
         Memory[Address].Read<int>(Offs.Life.MaxHp) : 
         0;

Now I am somewhat familiar with Lambda expressions. I just have not seen it used it this way.

What would be the difference between the above statement and

public int MaxHealth  = x ? y:z;

解决方案

What you're looking at is an expression-bodied member not a lambda expression.

When the compiler encounters an expression-bodied property member, it essentially converts it to a getter like this:

public int MaxHealth
{
    get
    {
        return Memory[Address].IsValid ? Memory[Address].Read<int>(Offs.Life.MaxHp) : 0;
    }
}

(You can verify this for yourself by pumping the code into a tool called TryRoslyn.)

Expression-bodied members - like most C# 6 features - are just syntactic sugar. This means that they don’t provide functionality that couldn't otherwise be achieved through existing features. Instead, these new features allow a more expressive and succinct syntax to be used

As you can see, expression-bodied members have a handful of shortcuts that make property members more compact:

• There is no need to use a return statement because the compiler can infer that you want to return the result of the expression
• There is no need to create a statement block because the body is only one expression
• There is no need to use the get keyword because it is implied by the use of the expression-bodied member syntax.

I have made the final point bold because it is relevant to your actual question, which I will answer now.

The difference between...

// expression-bodied member property
public int MaxHealth => x ? y:z;

And...

// field with field initializer
public int MaxHealth = x ? y:z;

Is the same as the difference between...

public int MaxHealth
{
    get
    {
        return x ? y:z;
    }
}

And...

public int MaxHealth = x ? y:z;

Which - if you understand properties - should be obvious.

Just to be clear, though: the first listing is a property with a getter under the hood that will be called each time you access it. The second listing is is a field with a field initializer, whose expression is only evaluated once, when the type is instantiated.

This difference in syntax is actually quite subtle and can lead to a "gotcha" which is described by Bill Wagner in a post entitled "A C# 6 gotcha: Initialization vs. Expression Bodied Members".

While expression-bodied members are lambda expression-like, they are not lambda expressions. The fundamental difference is that a lambda expression results in either a delegate instance or an expression tree. Expression-bodied members are just a directive to the compiler to generate a property behind the scenes. The similarity (more or less) starts and end with the arrow (=>).

I'll also add that expression-bodied members are not limited to property members. They work on all these members:

• Properties
• Indexers
• Methods
• Operators

Added in C# 7.0

• Constructors
• Finalizers

However, they do not work on these members:

• Nested Types
• Events
• Fields

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