問題描述

我剛剛嘗試了以下方法，想法是連接兩個字符串，用空字符串替換空字符串.

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";

-- 調試(有趣的是?是打印，不完全有助于可讀性...)

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

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

->你好"

正確的是:

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

我有點期待Hello World"，或者如果 a 為空，則只是World".顯然，這與運算符優先級有關，可以通過括號來解決，是否有任何地方可以記錄此新運算符的優先級順序.

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.

(意識到我可能應該使用 stringbuilder 或 String.Concat)

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

謝謝.

推薦答案

除了您希望喜歡的優先級之外，ECMA 規定的是什么，MS 規定的是什么規范和 csc 實際做什么，我有一點建議:

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:

不要這樣做.

我認為寫起來要清楚得多:

I think it's much clearer to write:

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

或者，鑒于字符串連接中的 null 無論如何最終只是一個空字符串，只需寫:

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

string c = a + b;

關于記錄的優先級，在 C# 3.0 規范(Word 文檔)和 ECMA-334，加法綁定比 ?? 更緊，后者比賦值綁定更緊.另一個答案中給出的 MSDN 鏈接是錯誤且奇怪的，IMO.2008 年 7 月的頁面上顯示的更改移動了條件運算符 - 但顯然是錯誤的！

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!

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