C#•3y ago

✅ `??=` and `??`

This question may not fit here because it is not a question about C# but more about why it is implemented in such a way, but I'll ask anyways:

Why is

item ??= item2

item ??= item2

implemented as

            if ((object)item == null)
            {
                item = item2;
            }

            if ((object)item == null)
            {
                item = item2;
            }

instead of

            if (item == null)
            {
                item = item2;
            }

            if (item == null)
            {
                item = item2;
            }

?
The cast makes it so if the

==

==

operator is changed by the class, it will not use said operator, instead using the default

==

==

operator of

object

object

.
This makes it so that in unity for example, where the

==

==

operator is changed by all unity objects, the

??=

??=

operator cannot be used.

My other question is, does the

??

??

operator also cast to

object

object

before null checking, or does it use something else completely, like pattern matching? I've used https://sharplab.io to see the lowering of

??

??

, but it doesn't appear to be lowered.

RRuttie This question may not fit here because it is not a question about C# but more ab...

ero•12/17/22, 5:01 PM

how are you testing this?

ero•12/17/22, 5:02 PM

i don't see a cast being done

RuttieOP•12/17/22, 5:02 PM

The exact code I used to test this is

RuttieOP•12/17/22, 5:03 PM

I used to check if there is maybe a problem with my code, but the result is that it is only logged once (that once being this check).

phaseshift•12/17/22, 5:03 PM

What's the point of that if?

RuttieOP•12/17/22, 5:04 PM

as for sharplab.io, this is the result I get

Pphaseshift What's the point of that if?

RuttieOP•12/17/22, 5:04 PM

✅ `??=` and `??`

RuttieOP•12/17/22, 5:05 PM

I would expect this code to output thrice, or at least twice if

??

??

uses pattern matching, but it's only printed once

ero•12/17/22, 5:07 PM

i can't put it into words, but this is pretty much bogus

ero•12/17/22, 5:08 PM

??

??

checks

ero•12/17/22, 5:08 PM

there's nothing else to it

ero•12/17/22, 5:08 PM

all it checks is whether what's passed exists at all

ero•12/17/22, 5:08 PM

there is no "equality" being checked

RuttieOP•12/17/22, 5:08 PM

while that's the answer of my second question, it doesn't explain why

??=

??=

does a cast

ero•12/17/22, 5:10 PM

exactly so it doesn't call the equality operator of the class

ero•12/17/22, 5:11 PM

because there is no. equality. being checked.

ero•12/17/22, 5:11 PM

nothing is "equal" to null

ero•12/17/22, 5:11 PM

can't be equal to something that doesn't exist, doesn't even have a concept of a type

RuttieOP•12/17/22, 5:11 PM

unity seems to disagree then?

ero•12/17/22, 5:12 PM

because they're fucking braindead

ero•12/17/22, 5:12 PM

(no offense of course)

ero•12/17/22, 5:12 PM

unity's implementation of almost everything is a complete mistake

ero•12/17/22, 5:13 PM

they took the concept of "null equals false" from c++ (which is where their engine source code resides), and incorrectly implemented that into their c# API

ero•12/17/22, 5:13 PM

which is absolutely asinine

ero•12/17/22, 5:13 PM

a null value cannot equate to false

RuttieOP•12/17/22, 5:13 PM

Okay, so the simple answer is just, don't use

??=

??=

and

??

??

with unity.

ero•12/17/22, 5:14 PM

unfortunately

RuttieOP•12/17/22, 5:14 PM

gotcha

RuttieOP•12/17/22, 5:14 PM

thanks for your help!

ero•12/17/22, 5:15 PM

the reason "null equals false" in lower level languages like C, C++, Python (unfortunately) is because they're comparing the pointers

ero•12/17/22, 5:15 PM

and a null pointer evaluates to 0

ero•12/17/22, 5:15 PM

and 0 evaluates to false

ero•12/17/22, 5:15 PM

a non-zero value evaluates to true

RuttieOP•12/17/22, 5:15 PM

yeah

ero•12/17/22, 5:16 PM

so yes, unity completely fucked up when making an implicit bool operator for that equates to

false

false

when an object is null

Anton•12/17/22, 6:49 PM

you should be able to use the implicit bool conversions in Unity, or just check for null

Anton•12/17/22, 6:49 PM

this should work

RRuttie while that's the answer of my second question, it doesn't explain why `??=` does...

333fred•12/17/22, 7:20 PM

It doesn't

D.Mentia•12/17/22, 7:21 PM

worth adding that also does that same sort of object cast, ignoring any

==

==

overloads, because sometimes you want to make sure not to use overloaded operators for null checks

333fred•12/17/22, 7:21 PM

It does the same thing as ??

333fred•12/17/22, 7:21 PM

There is no cast involved

ero•12/17/22, 7:21 PM

there is though?

333fred•12/17/22, 7:21 PM

No there isn't

D.Mentia•12/17/22, 7:22 PM

IDK if there is one but it's as if there is one anyway, just pointing out that it's common to want to skip the overloads on null checks

333fred•12/17/22, 7:22 PM

That may be how the decompiler chooses to represent it when going back to C#

333fred•12/17/22, 7:22 PM

But it's not what happens in il

D.Mentia•12/17/22, 7:23 PM

https://ericlippert.com/2013/05/30/what-the-meaning-of-is-is/ here's a long post about why is different from which may explain why

??

??

works like the former

Fabulous adventures in codingericlippert

What the meaning of is is

Today a follow-up to my 2010 article about the meaning of the is operator. Presented as a dialog, as is my wont! I’ve noticed that the is operator is inconsistent in C#. Check this out: strin…

333fred•12/17/22, 7:23 PM

And yes, unity does bad things like overloading == so null is weird for them

333fred•12/17/22, 7:24 PM

I get why they did it, but it didn't help their users wrt this type of thing

ero•12/17/22, 7:24 PM

a bit misleading by sharplab then, or by decompilers, or whichever