C#•2y 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 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. 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 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.

37 Replies

ero•2y ago

how are you testing this? i don't see a cast being done

RuttieOP•2y ago

The exact code I used to test this is

    public static partial class Program
    {
        private static ClassThingie? item;
        public static unsafe void Main(string[] args)
        {
            ClassThingie inst = null;
            if (inst == null) ;
            inst ??= item ?? GetThingie();
        }

        public static ClassThingie GetThingie() =>
            new();
    }

    public class ClassThingie
    {
        public static bool operator ==(ClassThingie left, ClassThingie right)
        {
            Console.WriteLine($"{nameof(ClassThingie)} equality operator called.");
            return true;
        }

        public static bool operator !=(ClassThingie left, ClassThingie right)
        {
            Console.WriteLine($"{nameof(ClassThingie)} inequality operator called.");
            return true;
        }

        public override bool Equals(object? obj)
        {
            return this == (ClassThingie)obj!;
        }
    }

    public static partial class Program
    {
        private static ClassThingie? item;
        public static unsafe void Main(string[] args)
        {
            ClassThingie inst = null;
            if (inst == null) ;
            inst ??= item ?? GetThingie();
        }

        public static ClassThingie GetThingie() =>
            new();
    }

    public class ClassThingie
    {
        public static bool operator ==(ClassThingie left, ClassThingie right)
        {
            Console.WriteLine($"{nameof(ClassThingie)} equality operator called.");
            return true;
        }

        public static bool operator !=(ClassThingie left, ClassThingie right)
        {
            Console.WriteLine($"{nameof(ClassThingie)} inequality operator called.");
            return true;
        }

        public override bool Equals(object? obj)
        {
            return this == (ClassThingie)obj!;
        }
    }

I used if (inst == null) ; 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•2y ago

What's the point of that if?

RuttieOP•2y ago

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

RuttieOP•2y ago

https://discord.com/channels/143867839282020352/1053717640025210892/1053718999537557574 I would expect this code to output ClassThingie equality operator called. thrice, or at least twice if ?? uses pattern matching, but it's only printed once

ero•2y ago

i can't put it into words, but this is pretty much bogus ?? checks if (thing is null) { } there's nothing else to it all it checks is whether what's passed exists at all there is no "equality" being checked

RuttieOP•2y ago

while that's the answer of my second question, it doesn't explain why ??= does a cast

ero•2y ago

exactly so it doesn't call the equality operator of the class because there is no. equality. being checked. nothing is "equal" to null can't be equal to something that doesn't exist, doesn't even have a concept of a type

RuttieOP•2y ago

unity seems to disagree then?

ero•2y ago

because they're fucking braindead (no offense of course) unity's implementation of almost everything is a complete mistake 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 which is absolutely asinine a null value cannot equate to false

RuttieOP•2y ago

Okay, so the simple answer is just, don't use ??= and ?? with unity.

ero•2y ago

unfortunately

RuttieOP•2y ago

gotcha thanks for your help!

ero•2y ago

the reason "null equals false" in lower level languages like C, C++, Python (unfortunately) is because they're comparing the pointers and a null pointer evaluates to 0 and 0 evaluates to false a non-zero value evaluates to true

RuttieOP•2y ago

yeah

ero•2y ago

so yes, unity completely fucked up when making an implicit bool operator for UnityObject that equates to false when an object is null

Anton•2y ago

you should be able to use the implicit bool conversions in Unity, or just check for null obj ? whenNotNull : whenNull this should work

333fred•2y ago

It doesn't

D.Mentia•2y ago

worth adding that item is null 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•2y ago

It does the same thing as ?? There is no cast involved

ero•2y ago

there is though?

333fred•2y ago

No there isn't

D.Mentia•2y ago

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•2y ago

That may be how the decompiler chooses to represent it when going back to C# But it's not what happens in il

D.Mentia•2y ago

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

ericlippert

Fabulous adventures in coding

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•2y ago

And yes, unity does bad things like overloading == so null is weird for them I get why they did it, but it didn't help their users wrt this type of thing

ero•2y ago

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

MODiX•2y ago

Ero#1111

sharplab.io (click here)

Foo foo = null;
Foo bar = new();
foo ??= bar ?? new();
record Foo();

Foo foo = null;
Foo bar = new();
foo ??= bar ?? new();
record Foo();

React with ❌ to remove this embed.

ero•2y ago

Foo foo = new Foo();
if ((object)null == null && (object)foo == null)
{
    new Foo();
}

Foo foo = new Foo();
if ((object)null == null && (object)foo == null)
{
    new Foo();
}

333fred•2y ago

It's a reasonable way to represent in C# But when you look at the il, you'll see no casts Think about it this way: reference conversions (such upcasting a class) mean nothing. They tell the compiler things, but absolutely nothing needs to happen at runtime The only time they matter is when they can't be proved at compile time, such as downcasts

ero•2y ago

so then, what is the reason for the operator not to be executed? actually, i guess a null check is indeed entirely different

333fred•2y ago

The == operator? That's not how any of the null-conditional operators are defined

ero•2y ago

isn't it just like a cmp against 0 or something

333fred•2y ago

They check for null, and only null

ero•2y ago

mh, yeah this, basically

333fred•2y ago

Correct

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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✅ `??=` and `??`