什么是C＃空凝聚（？）运算符的运算符优先级？ [英] What is the operator precedence of C# null-coalescing (??) operator?

问题描述

我刚刚试过以下，想法是来连接两个字符串，代订空空字符串。

I've just tried the following, the idea being to concatenate the two strings, substituting an empty string for nulls.

string a="Hello";
string b=" World";

- 调试（？会哄即打印，不正是帮助可读性...）

-- Debug (amusing that ? is print, doesn't exactly help readability...)

 ? a ?? "" + b ?? ""

- >你好

-> "Hello"

正确的是：

? (a??"")+(b??"")
"Hello World"

我是那种期待的Hello World，或只是世界，如果为空。显然，这是待办事项与运算符优先级，可以通过支架被克服，有没有记录的优先级为这个新的运营商订单的任何地方。

I was kind of expecting "Hello World", or just "World" if a is null. Obviously this is todo with operator precedence and can be overcome by brackets, is there anywhere that documents the order of precedence for this new operator.

（共建，我也许应该是使用StringBuilder的或String.Concat）

(Realising that I should probably be using stringbuilder or String.Concat)

感谢。

推荐答案

除了从你想的如的优先级是，它是根据ECMA，它是根据MS规范，什么CSC实际上并，我有意见的一位：

Aside from what you'd like the precedence to be, what it is according to ECMA, what it is according to the MS spec and what csc actually does, I have one bit of advice:

不要这样做

我认为这是的多的更清晰的写：

I think it's much clearer to write:

string c = (a ?? "") + (b ?? "");

另外，考虑到空的字符串连接最终只是被一个空字符串无论如何，只写：

Alternatively, given that null in string concatenation ends up just being an empty string anyway, just write:

string c = a + b;

编辑：关于记录优先级，在这两个的C# 3.0规范（Word文档）和的ECMA-334 ，除了结合比??，这比结合更紧密的分配更严格。在另一个答案给出的MSDN链接是错误的和奇异的，海事组织。有一个在2008年7月做出的网页，其中移动条件运算上显示的变化 - 但显然不正确。

Regarding the documented precedence, in both the C# 3.0 spec (Word document) and ECMA-334, addition binds tighter than ??, which binds tighter than assignment. The MSDN link given in another answer is just wrong and bizarre, IMO. There's a change shown on the page made in July 2008 which moved the conditional operator - but apparently incorrectly!

这篇关于什么是C＃空凝聚（？）运算符的运算符优先级？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！