Is there an “opposite” to the null coalescing oper

2019-01-13 09:24发布

null coalescing translates roughly to return x, unless it is null, in which case return y

I often need return null if x is null, otherwise return x.y

I can use return x == null ? null : x.y;

Not bad, but that null in the middle always bothers me -- it seems superfluous. I'd prefer something like return x :: x.y;, where what follows the :: is evaluated only if what precedes it is not null.

I see this as almost an opposite to null coalescence, kind of mixed in with a terse, inline null-check, but I'm [almost] certain that there is no such operator in C#.

Are there other languages that have such an operator? If so, what is it called?

(I know that I can write a method for it in C#; I use return NullOrValue.of(x, () => x.y);, but if you have anything better, I'd like to see that too.)

12条回答
Animai°情兽
2楼-- · 2019-01-13 09:44

It just felt right to add this as an answer.

I guess the reason why there is no such thing in C# is because, unlike the coalescing operator (which is only valid for reference types), the reverse operation could yield either a reference or value type (i.e. class x with member int y - therefore it would unfortunately be unusable in many situations.

I'm not saying, however, that I wouldn't like to see it!

A potential solution to that problem would for the operator to automatically lift a value type expression on the right-hand-side to a nullable. But then you have the issue that x.y where y is an int will actually return an int? which would be a pain.

Another, probably better, solution would be for the operator to return the default value (i.e. null or zero) for the type on the right hand side if the expression on the left is null. But then you have issues distinguishing scenarios where a zero/null was actually read from x.y or whether it was supplied by the safe-access operator.

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老娘就宠你
3楼-- · 2019-01-13 09:44

This is being added in C# vNext (Roslyn powered C#, releases with Visual Studio 2014).

It is called Null propagation and is listed here as complete. https://roslyn.codeplex.com/wikipage?title=Language%20Feature%20Status

It is also listed here as complete: https://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/3990187-add-operator-to-c

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冷血范
4楼-- · 2019-01-13 09:46

Create a static instance of your class somewhere with all the right default values for the members.

For example:

z = new Thingy { y=null };

then instead of your

return x != null ? x.y : null;

you can write

return (x ?? z).y;
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来,给爷笑一个
5楼-- · 2019-01-13 09:47

We considered adding ?. to C# 4. It didn't make the cut; it's a "nice to have" feature, not a "gotta have" feature. We'll consider it again for hypothetical future versions of the language, but I wouldn't hold my breath waiting if I were you. It's not likely to get any more crucial as time goes on. :-)

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我想做一个坏孩纸
6楼-- · 2019-01-13 09:48

There's the null-safe dereferencing operator (?.) in Groovy... I think that's what you're after.

(It's also called the safe navigation operator.)

For example:

homePostcode = person?.homeAddress?.postcode

This will give null if person, person.homeAddress or person.homeAddress.postcode is null.

(This is now available in C# 6.0 but not in earlier versions)

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Lonely孤独者°
7楼-- · 2019-01-13 09:54

Haskell has fmap, which in this case I think is equivalent toData.Maybe.map. Haskell is purely functional, so what you are looking for would be

fmap select_y x

If x is Nothing, this returns Nothing. If x is Just object, this returns Just (select_y object). Not as pretty as dot notation, but given that it's a functional language, styles are different.

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