内容简介:Though it may sound like our occasionalApril Fools Day joke, C# 9 is looking to addIn order to make pattern matching more flexible and powerful, C#’s designers want to add the concept of conjunctive, disjunctive, and negated patterns. Superficially, they l
Though it may sound like our occasionalApril Fools Day joke, C# 9 is looking to add and
, or
, and not
to its list of keywords. Specifically, for use in pattern matching.
In order to make pattern matching more flexible and powerful, C#’s designers want to add the concept of conjunctive, disjunctive, and negated patterns. Superficially, they look just like Boolean operations where you want to match on both patterns (conjunctive), either pattern (disjunctive) or not match on a pattern (negated).
And that’s the problem. If you are working with Booleans, the && and || operators would be ambiguous; the compiler wouldn’t be able to determine if they are referring to values or patterns. To illustrate this idea, consider this disjunctive pattern:
if (myBool is true or false)
This would be interpreted as “true if myBool
equals true or if myBool
equals false”.
If we use the && and || operators for combining patterns, then we get this syntax:
if (myBool is true || false)
But this statement already means “true if myBool
equals the result of (true or false)”. Or to simplify it, “true if myBool
equals true”. Which is a completely different result than what we’d get with above disjunctive pattern.
Hence the new and, or, and not keywords are necessary to avoid ambiguity. You can learn more about them in Champion "and, or, and not patterns" . In the C# 9 feature status they are marked as being merged into the master branch.
One question frequently raised is whether or not the same variable can be declared multiple times. For example,
if ((e1, e2) is (0, int x) or (int x, 0)) { M(x); }
In this theoretical example, the variable x
is either e1
or e2
. This would be the equivalent of writing,
case (0, int x): case (int x, 0): M(x);
In fact, you would have to declare x
in both patterns for x
to be definitely assigned
. If you used the below code instead, either x
or y
wouldn’t have a value.
if ((e1, e2) is (0, int x) or (int y, 0)) { M(x); M(y); }
In order to this problem, the current proposal says,
beneath a not or or, pattern variables may not be declared.
This may be reconsidered in future versions of C#.
以上所述就是小编给大家介绍的《C# 9: New `and`, `or`, and `not` Keywords for Pattern Matching》,希望对大家有所帮助,如果大家有任何疑问请给我留言,小编会及时回复大家的。在此也非常感谢大家对 码农网 的支持!
猜你喜欢:本站部分资源来源于网络,本站转载出于传递更多信息之目的,版权归原作者或者来源机构所有,如转载稿涉及版权问题,请联系我们。
expert one-on-one J2EE Development without EJB 中文版
[美] Rod Johnson、Juergen Hoeller / JavaEye / 电子工业出版社 / 2005-9 / 59.80元
乍一看这本书的名字,Expert one on one J2EE development without EJB并没有给人带来太冲击。毕竟关于J2EE的书太多了,而without EJB看上去有点象是故意挑衅EJB的感觉。一本J2EE的书怎么可能会给人带来信念或思维的冲击呢?但是它做到了,它不仅使自己变成了不朽的经典,也使Rod Johnson成为了我最近一年的新偶像。 ......一起来看看 《expert one-on-one J2EE Development without EJB 中文版》 这本书的介绍吧!