Null coalescing operator override

Question

I know it's non sense to do something like:

xstring.ToLower()??"xx"

because i called ToLower() gets called before checking for null. is there a way around this, keeping the syntax nice and clean?

can i override the ?? operator for the string so that it only calls ToLower() when xstring is not null?

possible duplicate of [Possible to override null-coalescing operator?](http://stackoverflow.com/questions/349062/possible-to-override-null-coalescing-operator) — Lukazoid, Jan 10 '14 at 18:22

score 8 · Answer 1 · answered Jan 10 '14 at 18:23

8

No you cannot overload that operator. Just put .ToLower outside of the coalesce:

(xstring ?? "xx").ToLower();

answered Jan 10 '14 at 18:23

D Stanley

149,601
11
178
240

they don't exactly mean the same thing, what if i wanted be an * and for some reason i wanted it to be "XX"! – user1492051 Jan 10 '14 at 18:31
I didn't say they meant the same thing - but in your example they give the result you want. – D Stanley Jan 10 '14 at 18:37
i wasnt serious its a joke feel free to laugh haha – user1492051 Jan 10 '14 at 18:39

score 8 · Accepted Answer · answered Jan 10 '14 at 18:23

What you're looking for is called Monadic Null Checking. It is currently not available in C# 5, but apparently it will be available in C# 6.0.

From this post:

7. Monadic null checking

Removes the need to check for nulls before accessing properties or methods. Known as the Safe Navigation Operator in Groovy).

Before

if (points != null) {
    var next = points.FirstOrDefault();
    if (next != null && next.X != null) return next.X;
}   
return -1;

After

var bestValue = points?.FirstOrDefault()?.X ?? -1;

in the meantime, just use

(xstring ?? "xx").ToLower();

as other answers suggested.

score 4 · Answer 3 · answered Jan 10 '14 at 18:22

4

No but there are rumors of a ?. operator being added to the next version of C# for this purpose. See #7 at http://damieng.com/blog/2013/12/09/probable-c-6-0-features-illustrated

answered Jan 10 '14 at 18:22

Robert Levy

28,747
6
62
94

score 3 · Answer 4 · answered Jan 10 '14 at 18:22

3

If you want to avoid ToLower()-ing the literal value "xx", you're stuck with the ?: ternary operator.

xstring != null ? xstring.ToLower() : "xx"

Or you could write an extension method, but this looks very odd to me.

public static string ToLowerOrDefault(this string input, this string defaultValue)
{
    return (input != null ? input.ToLower() : defaultValue);
}

which you could then use like this:

xstring.ToLowerOrDefault("xx")

answered Jan 10 '14 at 18:22

Michael Gunter

12,528
1
24
58

1

There are no pretty solutions in the current versions of C# – Michael Gunter Jan 10 '14 at 18:24
2

C# is really beautiful when compared to [other languages out there](http://www.infoworld.com/d/application-development/java-faces-tough-climb-catch-net-224372). Still, we're raising the bar all the time... – Federico Berasategui Jan 10 '14 at 18:34
Agreed. It's definitely my language of choice outside of C++. Java could stand to take a few things from it. – Michael Gunter Jan 10 '14 at 19:13
even java programmers [recognize that](http://stackoverflow.com/q/21007276/643085) – Federico Berasategui Jan 10 '14 at 19:14

score 3 · Answer 5 · answered Jan 10 '14 at 18:23

3

Apply ToLower() method after checking for null:

(xstring ?? "xx").ToLower();

answered Jan 10 '14 at 18:23

Kaf

33,101
7
58
78

score 2 · Answer 6 · answered Jan 10 '14 at 18:23

2

according to ECMA-334 you can not override ?? operator

Standard ECMA-334

answered Jan 10 '14 at 18:23

santosh singh

27,666
26
83
129

score 2 · Answer 7 · answered Jan 10 '14 at 18:23

2

The only solution I would see is doing:

(xstring ?? "xx").ToLower();

However, I think it would look much nicer if you did something

xstring != null ? xstring.ToLower() : "xx"

answered Jan 10 '14 at 18:23

gleng

6,185
4
21
35

score 2 · Answer 8 · answered Jan 10 '14 at 18:24

2

You could use:

(xstring ?? "xx").ToLower()

The syntax is simple and the intent is clear. On the downside, you'll be running ToLower on "xx" and you added some parentheses.

answered Jan 10 '14 at 18:24

Tim S.

55,448
7
96
122

Null coalescing operator override

8 Answers8

7. Monadic null checking

Linked