blocks|key|2637859|text|是的，它们都是只读的，但这是有区别的。在第一个函数中，有一个支持字段，它在构造函数执行之前被初始化为0。您只能在构造函数中更改该值，就像常规的只读字段一样。getter本身只返回字段的值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2637860|在第二个示例中，getter每次只返回0，不涉及任何字段。|2637861|因此，为了避免使用任何自动实现的属性或表达式主体成员，我们有：|2637862|第一版|offset|length|style|BOLD|2637863|private+readonly+int+_number+=+0;
public+int+Number+{+get+{+return+_number;+}+}|code-block|syntax|javascript|2637864|第二版|2637865|public+int+Number+{+get+{+return+0;+}+}|2637866|更清晰的差异示例可能如下所示：|2637867|public+DateTime+CreationTime+{+get;+}+=+DateTime.UtcNow;
public+DateTime+CurrentTime+=>+DateTime.UtcNow;|2637868|如果您创建单个对象，其CreationTime属性将始终提供相同的结果-因为它存储在只读字段中，在对象构造时初始化。但是，每次访问CurrentTime属性时，都会导致对DateTime.UtcNow求值，因此可能会得到不同的结果。|CODE|2637869|entityMap^0|0|0|0|0|3|0|0|0|3|0|0|0|0|B|C|1T|B|2D|F|0^^$0|@$1|2|3|4|5|6|7|13|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|14|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|15|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|16|8|@$H|17|I|18|J|K]]|9|@]|A|$]]|$1|L|3|M|5|N|7|19|8|@]|9|@]|A|$O|P]]|$1|Q|3|R|5|6|7|1A|8|@$H|1B|I|1C|J|K]]|9|@]|A|$]]|$1|S|3|T|5|N|7|1D|8|@]|9|@]|A|$O|P]]|$1|U|3|V|5|6|7|1E|8|@]|9|@]|A|$]]|$1|W|3|X|5|N|7|1F|8|@]|9|@]|A|$O|P]]|$1|Y|3|Z|5|6|7|1G|8|@$H|1H|I|1I|J|10]|$H|1J|I|1K|J|10]|$H|1L|I|1M|J|10]]|9|@]|A|$]]|$1|11|3|-4|5|6|7|1N|8|@]|9|@]|A|$]]]|12|$]]

Yes, both of them are read-only, but there is a difference. In the first one, there's a backing field which is initialized to 0 before the constructor is executed. You can change the value only in the constructor, just like a regular read-only field. The getter itself just returns the value of the field.

In the second one, the getter just returns 0 every time, with no field involved.

So to avoid using any automatically implemented properties or expression-bodied members at all, we have:

First version

<pre><code>private readonly int _number = 0;
public int Number { get { return _number; } }
</code></pre>

Second version

<pre><code>public int Number { get { return 0; } }
</code></pre>

A clearer example of the difference might be seen like this:

<pre><code>public DateTime CreationTime { get; } = DateTime.UtcNow;
public DateTime CurrentTime =&gt; DateTime.UtcNow;
</code></pre>

If you create a single object, its <code>CreationTime</code> property will always give the same result - because it's stored in a readonly field, initialized on object construction. However, every time you access the <code>CurrentTime</code> property, that will cause <code>DateTime.UtcNow</code> to be evaluated, so you'll get a potentially different result.

blocks|key|2849524|text|这些是C#+6的语言特性。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2849525|第一个示例|offset|length|style|BOLD|2849526|public+int+Number+{+get;+}+=+0|code-block|syntax|javascript|2849527|第一个示例是getter-only+auto+property。仅限getter的auto-property的支持字段被隐式声明为readonly。|2849528|第二个示例|2849529|public+int+Number+=>+0;|2849530|第二个例子是expression+bodies+on+property-like+function+members。请注意，没有任何get关键字:它是通过使用表达式主体语法隐含的。|CODE|2849531|两者都是只读的。|2849532|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/dotnet/roslyn/wiki/New-Language-Features-in-C%2523-6#getter-only-auto-properties|1|https://github.com/dotnet/roslyn/wiki/New-Language-Features-in-C%2523-6#expression-bodies-on-property-like-function-members^0|0|0|5|0|0|6|P|0|0|0|5|0|0|1U|3|6|1F|1|0|0^^$0|@$1|2|3|4|5|6|7|17|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|18|8|@$D|19|E|1A|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|1B|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|1C|8|@]|9|@$D|1D|E|1E|1|1F]]|A|$]]|$1|O|3|P|5|6|7|1G|8|@$D|1H|E|1I|F|G]]|9|@]|A|$]]|$1|Q|3|R|5|J|7|1J|8|@]|9|@]|A|$K|L]]|$1|S|3|T|5|6|7|1K|8|@$D|1L|E|1M|F|U]]|9|@$D|1N|E|1O|1|1P]]|A|$]]|$1|V|3|W|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|1R|8|@]|9|@]|A|$]]]|Y|$Z|$5|10|11|12|A|$13|14]]|15|$5|10|11|12|A|$13|16]]]]

These are C# 6 language features.

First example

<pre><code>public int Number { get; } = 0
</code></pre>

The first example is a <a href="https://github.com/dotnet/roslyn/wiki/New-Language-Features-in-C%23-6#getter-only-auto-properties" rel="noreferrer">getter-only auto property</a>. The backing field of a getter-only auto-property is implicitly declared as readonly.

Second example

<pre><code>public int Number =&gt; 0;
</code></pre>

And the second example is <a href="https://github.com/dotnet/roslyn/wiki/New-Language-Features-in-C%23-6#expression-bodies-on-property-like-function-members" rel="noreferrer">expression bodies on property-like function members</a>. Note that there isn't any <code>get</code> keyword: It is implied by the use of the expression body syntax.

Both are readonly.

blocks|key|100845|text|一个不同之处在于何时计算该0：在创建对象时还是在使用属性时。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|100846|使用DateTime属性可以更好地看到这一点：|100847|class+SomeTestClass
{
++++public+DateTime+Start+{+get;+}+=+DateTime.Now;

++++public+DateTime+Now+=>+DateTime.Now;
}|code-block|syntax|javascript|100848|Start属性始终返回相同的时间(创建实例时的时间)，而Now会更改以反映当前时间。|100849|说明|BOLD|100850|第一个版本("Start")提供了一个初始值，该初始值甚至可能被构造函数覆盖。所以这只需要计算一次。|100851|第二个版本("Now")提供了将成为该属性的"getter“的表达式。因此，每次读取该属性时都会对其进行评估。甚至没有构造函数可以覆盖的后备字段。|100852|entityMap^0|D|1|0|0|0|0|5|S|3|0|0|2|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|10|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|11|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|17|8|@$9|18|A|19|B|Q]]|D|@]|E|$]]|$1|R|3|S|5|6|7|1A|8|@]|D|@]|E|$]]|$1|T|3|U|5|6|7|1B|8|@]|D|@]|E|$]]|$1|V|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|W|$]]

One difference is when that <code>0</code> is evaluated: at object creation or when the property is used. 

You can see this better with DateTime properties:

<pre><code>class SomeTestClass
{
 public DateTime Start { get; } = DateTime.Now;

 public DateTime Now =&gt; DateTime.Now;
}
</code></pre>

The <code>Start</code> property keeps returning the same time (of when the instance was created), while <code>Now</code> changes to reflect the current time.

Explanation:

The first version ("Start") supplies an initial value that may even be overwritten by the constructor. So this is evaluated just once. 
The second version ("Now") provides the expression that will be the "getter" of this property. So this is evaluated every time the property is read. There isn't even a backing field that the constructor can overwrite.

I do sometimes see abbreviations in properties for the getter. E.g. those two types:

<pre><code>public int Number { get; } = 0

public int Number =&gt; 0;
</code></pre>

Can someone please tell me if there are any differences between those two. How do they behave? Are both of them read-only?

Difference in C# between different getter styles

我有时确实会在getter的属性中看到缩写。例如，这两种类型：public int Number { get; } = 0public int Number => 0;有人能告诉我这两者之间有什么不同吗？他们的行为是怎样的？它们都是只读的吗？

问不同吸气剂风格的C#差异
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