blocks|key|1133224|text|你还没有提供答案吗？它正是为了支持你所提到的那些类型的结构。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1133225|使用赋值操作符的此属性的常见情况是从文件中读取行...|1133226|string+line;
while+((line+=+streamReader.ReadLine())+!=+null)
++++//+...|code-block|syntax|javascript|1133227|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Haven’t you provided the answer? It’s to enable exactly the kinds of constructs you have mentioned.

A common case where this property of the assignment operator is used is reading lines from a file...

<pre><code>string line;
while ((line = streamReader.ReadLine()) != null)
 // ...
</code></pre>

blocks|key|918845|text|我最喜欢的赋值表达式是用于延迟初始化的属性。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|918846|private+string+_name;
public+string+Name
{
++++get+{+return+_name+??+(_name+=+ExpensiveNameGeneratorMethod());+}
}|code-block|syntax|javascript|918847|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

My favorite use of assignment expressions is for lazily initialized properties.

<pre><code>private string _name;
public string Name
{
 get { return _name ?? (_name = ExpensiveNameGeneratorMethod()); }
}
</code></pre>

blocks|key|918501|text|首先，它允许你链接你的赋值，如你的例子所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|918502|a+=+b+=+c+=+16;|code-block|syntax|javascript|918503|另一方面，它允许您在单个表达式中分配和检查结果：|918504|while+((s+=+foo.getSomeString())+!=+null)+{+/*+...+*/+}|918505|这两个都可能是可疑的原因，但肯定有人喜欢这些构造。|918506|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

For one, it allows you to chain your assignments, as in your example:

<pre><code>a = b = c = 16;
</code></pre>

For another, it allows you to assign and check a result in a single expression:

<pre><code>while ((s = foo.getSomeString()) != null) { /* ... */ }
</code></pre>

Both are possibly dubious reasons, but there are definitely people who like these constructs.

blocks|key|1133382|text|除了前面提到的原因(赋值链接、while循环内的设置和测试等)之外，要使用using语句，您需要以下功能：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1133383|using+(Font+font3+=+new+Font("Arial",+10.0f))
{
++++//+Use+font3.
}|code-block|syntax|javascript|1133384|MSDN不鼓励在using语句之外声明disposable对象，因为即使在它被释放之后，它仍将保留在作用域中(请参阅我链接的MSDN文章)。|1133385|entityMap^0|11|5|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

Aside from the reasons already mentioned (assignment chaining, set-and-test within while loops, etc), to <a href="http://msdn.microsoft.com/en-us/library/yh598w02.aspx" rel="noreferrer">properly</a> use the <code>using</code> statement you need this feature:

<pre><code>using (Font font3 = new Font("Arial", 10.0f))
{
 // Use font3.
}
</code></pre>

MSDN discourages declaring the disposable object outside of the using statement, as it will then remain in scope even after it has been disposed (see the MSDN article I linked).

blocks|key|918582|text|如果赋值没有返回值，那么a+=+b+=+c+=+16行也不会工作。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|918583|此外，能够编写像while+((s+=+readLine())+!=+null)这样的东西有时也会很有用。|918584|所以让赋值返回赋值的原因，就是为了让你做这些事情。|918585|entityMap^0|C|E|0|8|W|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|R|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|K|$]]

If assignment didn't return a value, the line <code>a = b = c = 16</code> wouldn't work either.

Also being able to write things like <code>while ((s = readLine()) != null)</code> can be useful sometimes.

So the reason behind letting assignment return the assigned value, is to let you do those things.

blocks|key|918655|text|我认为主要原因是(有意地)与C%2B%2B和C相似。让赋值运算符(和许多其他语言构造)的行为像它们的C%2B%2B对应运算符一样只遵循最少惊讶的原则，任何来自另一种花括号语言的程序员都可以使用它们，而不需要花费太多的精力。让C%2B%2B程序员容易上手是C#的主要设计目标之一。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|918656|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I think the main reason is the (intentional) similarity with C++ and C. Making the assigment operator (and a lot of other language constructs) behave like their C++ counterparts just follows the principle of least surprise, and any programmer coming from another curly-bracket language can use them without spending much thought. Being easy to pick up for C++ programmers was one of the main design goals for C#.

blocks|key|1133288|text|因为你在帖子中提到的两个原因|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1133289|1)所以你可以做a+=+b+=+c+=+16|offset|length|style|CODE|1133290|2)这样你就可以测试赋值是否成功if+((s+=+openSomeHandle())+!=+null)|1133291|entityMap^0|0|8|E|0|G|Z|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@$D|Q|E|R|F|G]]|9|@]|A|$]]|$1|J|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|K|$]]

For the two reasons you include in your post 
1) so you can do <code>a = b = c = 16</code> 
2) so you can test if an assignment succeeded
<code>if ((s = openSomeHandle()) != null)</code>

blocks|key|918788|text|'a%2B%2B‘或'printf("foo")’作为独立语句或更大表达式的一部分可能是有用的，这意味着C必须考虑到表达式结果可能被使用，也可能不被使用。鉴于此，有一个通用的概念，即可能有效地“返回”值的表达式也可以这样做。赋值链接在C中可能有点“有趣”，在C%2B%2B中更有趣，如果所有有问题的变量都不是完全相同的类型。最好避免这样的用法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|918789|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The fact that 'a++' or 'printf("foo")' may be useful either as a self-contained statement or as a part of a larger expression means that C has to allow for the possibility that expression results may or may not be used. Given that, there's a general notion that expressions which might usefully 'return' a value may as well do so. Assignment chaining can be slightly "interesting" in C, and even more interesting in C++, if all the variables in question do not have precisely the same type. Such usages are probably best avoided.

blocks|key|918918|text|另一个很好的示例用例，我一直在用这个：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|918919|var+x+=+_myVariable+??+(_myVariable+=+GetVariable());
//for+example:+when+used+inside+a+loop,+"GetVariable"+will+be+called+only+once|code-block|syntax|javascript|918920|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Another great example use-case, I use this all the time:

<pre><code>var x = _myVariable ?? (_myVariable = GetVariable());
//for example: when used inside a loop, "GetVariable" will be called only once
</code></pre>

blocks|key|1133578|text|当我需要更新一堆东西并返回是否有任何更改时，我喜欢使用赋值返回值：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1133579|bool+hasChanged+=+false;

hasChanged+%7C=+thing.Property+!=+(thing.Property+=+"Value");
hasChanged+%7C=+thing.AnotherProperty+!=+(thing.AnotherProperty+=+42);
hasChanged+%7C=+thing.OneMore+!=+(thing.OneMore+=+"They+get+it");

return+hasChanged;|code-block|syntax|javascript|1133580|不过要小心。你可能会认为你可以把它缩短为：|1133581|return+thing.Property+!=+(thing.Property+=+"Value")+%7C%7C
++++thing.AnotherProperty+!=+(thing.AnotherProperty+=+42)+%7C%7C
++++thing.OneMore+!=+(thing.OneMore+=+"They+get+it");|1133582|但这实际上会在找到第一个true之后停止计算或语句。在这种情况下，这意味着一旦它分配了第一个不同的值，它就会停止分配后续的值。|1133583|请看https://dotnetfiddle.net/e05Rh8来解决这个问题|offset|length|1133584|entityMap|0|LINK|mutability|MUTABLE|url|https://dotnetfiddle.net/e05Rh8^0|0|0|0|0|0|2|V|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|12|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@]|9|@$O|14|P|15|1|16]]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

I like using the assignment return value when I need to update a bunch of stuff and return whether or not there were any changes:

<pre><code>bool hasChanged = false;

hasChanged |= thing.Property != (thing.Property = "Value");
hasChanged |= thing.AnotherProperty != (thing.AnotherProperty = 42);
hasChanged |= thing.OneMore != (thing.OneMore = "They get it");

return hasChanged;
</code></pre>

Be careful though. You might think you can shorten it to this:

<pre><code>return thing.Property != (thing.Property = "Value") ||
 thing.AnotherProperty != (thing.AnotherProperty = 42) ||
 thing.OneMore != (thing.OneMore = "They get it");
</code></pre>

But this will actually stop evaluating the or statements after it finds the first true. In this case that means it stops assigning subsequent values once it assigns the first value that is different. 

See <a href="https://dotnetfiddle.net/e05Rh8" rel="nofollow noreferrer">https://dotnetfiddle.net/e05Rh8</a> to play around with this

This is allowed:

<pre><code>int a, b, c;
a = b = c = 16;

string s = null;
while ((s = "Hello") != null) ;
</code></pre>

To my understanding, assignment <code>s = ”Hello”;</code> should only cause <code>“Hello”</code> to be assigned to <code>s</code>, but the operation shouldn’t return any value. If that was true, then <code>((s = "Hello") != null)</code> would produce an error, since <code>null</code> would be compared to nothing. 

What is the reasoning behind allowing assignment statements to return a value?

Why do assignment statements return a value?

这是允许的：int a, b, c;a = b = c = 16;string s = null;while ((s = "Hello") != null) ;据我所知，赋值s = ”Hello”;应该只会将“Hello”赋值给s，但该操作不应该返回任何值。如果这是真的，那么((s = "Hello") != null)将产生一个错误，因为null将被比作nothing。允许赋值语句返回值的原因是

问为什么赋值语句返回值？
EN

回答 10

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么赋值语句返回值？EN

回答 10

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么赋值语句返回值？
EN