C#•3mo ago

Explanation for array initialization peculiarities

I'd like to get an explanation for the following behavior :

        Ty[] tys = {};
        Console.WriteLine(tys.Length); // This is fine
        
        // Of is a Ty[]
        var tup = new Ty.Tuple { Of = {} };
        Console.WriteLine(tup.Of.Length); // This causes a null pointer exception

        Ty[] tys = {};
        Console.WriteLine(tys.Length); // This is fine
        
        // Of is a Ty[]
        var tup = new Ty.Tuple { Of = {} };
        Console.WriteLine(tup.Of.Length); // This causes a null pointer exception

Why does {} seem to do something different depending on where it is ?

56 Replies

Angius•3mo ago

It shouldn't behave differently

ABp16•3mo ago

And yet, it does haha

Angius•3mo ago

Just to check, though, could you try [] instead of {}?

ABp16•3mo ago

in which one ?

Angius•3mo ago

Of = []

ABp16•3mo ago

this is not proper syntax it seems

Angius•3mo ago

What's your .NET version?

ABp16•3mo ago

        // Of is a Ty[]
        var tup = new Ty.Tuple { Of = new Ty[0] };
        Console.WriteLine(tup.Of.Length); // This causes a null pointer exception

        // Of is a Ty[]
        var tup = new Ty.Tuple { Of = new Ty[0] };
        Console.WriteLine(tup.Of.Length); // This causes a null pointer exception

this works fine btw

Angius•3mo ago

This intializes a new array of 0-length explicitly, yes

ABp16•3mo ago

is this what you're looking for ?

Angius•3mo ago

Yeah I guess the [] syntax is 8.0 and up

ABp16•3mo ago

But why does {} initialize a 0 length array in some cases and not in others ?

Angius•3mo ago

¯\_(ツ)_/¯

ABp16•3mo ago

Should I upgrade to that then ? What's the reason for this syntax not working ? This I meant

Angius•3mo ago

Honestly not sure Just tried it on .NET 8 and it doesn't work either

ABp16•3mo ago

Do you think it could be a bug in the compiler ?

Angius•3mo ago

E.g.

Angius•3mo ago

Not sure

ABp16•3mo ago

I'm looking for some sort of specification that says "When classes are initialized this way {} stands for null" or something like that

Angius•3mo ago

Huh, the compiler-generated code is

[CompilerGenerated]
internal class Program
{
    private static void <Main>$(string[] args)
    {
        Ty[] array = new Ty[0];
        Ty.Tuple tuple = new Ty.Tuple();
    }
}

[CompilerGenerated]
internal class Program
{
    private static void <Main>$(string[] args)
    {
        Ty[] array = new Ty[0];
        Ty.Tuple tuple = new Ty.Tuple();
    }
}

It does not assign a new array to Of It works if I do Of = [] though

  Ty[] tys = { };
  var tup = new Ty.Tuple { Of = [] };

  Ty[] tys = { };
  var tup = new Ty.Tuple { Of = [] };

generates

[CompilerGenerated]
internal class Program
{
    private static void <Main>$(string[] args)
    {
        Ty[] array = new Ty[0];
        Ty.Tuple tuple = new Ty.Tuple();
        tuple.Of = Array.Empty<Ty>();
        Ty.Tuple tuple2 = tuple;
    }
}

[CompilerGenerated]
internal class Program
{
    private static void <Main>$(string[] args)
    {
        Ty[] array = new Ty[0];
        Ty.Tuple tuple = new Ty.Tuple();
        tuple.Of = Array.Empty<Ty>();
        Ty.Tuple tuple2 = tuple;
    }
}

So the easiest solution would be upgrade to .NET 8 and use [] Far as explanation goes, no idea

ABp16•3mo ago

Now I have to look for this little thing everywhere in my codebase I used it a bunch x) Do you know anyone who might have one ? I'm just curious

Angius•3mo ago

Huh, there are some MS people from the language and compiler team on the server

ABp16•3mo ago

I think it might be related to this syntax

Angius•3mo ago

Might be {} can, contextually, be an object initializer not an array initializer While [] is always a collection initializer

ABp16•3mo ago

Do they have a specific role / is there someone who is open to be pinged ?

Angius•3mo ago

No such role, alas

ABp16•3mo ago

I thought it should be preceded by "new" if it's an object initializer

Angius•3mo ago

I'll post it in the associates channel, though

ABp16•3mo ago

and it's weird that it just gives null Alright thank you! Thank you for your time as well there seems to be a verified microsoft employee role but I think it might be best not to ping them directly

Angius•3mo ago

@mtreit? :when:

ABp16•3mo ago

Oh well x)

Angius•3mo ago

Care to weigh in?

mtreit•3mo ago

What are we discussing? Haven't scrolled up.

ABp16•3mo ago

reason those two

        Ty[] tys = {};
        Console.WriteLine(tys.Length); // This is fine
        
        // Of is a Ty[]
        var tup = new Ty.Tuple { Of = {} };
        Console.WriteLine(tup.Of.Length); // This causes a null pointer exception

        Ty[] tys = {};
        Console.WriteLine(tys.Length); // This is fine
        
        // Of is a Ty[]
        var tup = new Ty.Tuple { Of = {} };
        Console.WriteLine(tup.Of.Length); // This causes a null pointer exception

are different

mtreit•3mo ago

Good question, I don't think I've ever used that syntax.

Aaron•3mo ago

Thing = {} in an object initializer is the syntax for adding things, not creating a collection for example

MODiX•3mo ago

Aaron

REPL Result: Failure

class C
{
    public List<int> List { get; set; }
}
new C() {
    List = { 1, 2, 3, 4 }
}

class C
{
    public List<int> List { get; set; }
}
new C() {
    List = { 1, 2, 3, 4 }
}

Exception: NullReferenceException

- Object reference not set to an instance of an object.

- Object reference not set to an instance of an object.

Quoted by

<@270457259065081856> from #taxed-void (click here)

Compile: 283.852ms | Execution: 60.462ms | React with ❌ to remove this embed.

Aaron•3mo ago

the list is null, it tries to do List.Add, and throws

ABp16•3mo ago

Ty[] tys = {};
Console.WriteLine(tys.Length); // This is fine

Ty[] tys = {};
Console.WriteLine(tys.Length); // This is fine

It works on arrays though

Aaron•3mo ago

thats not in an object initializer though thats where the difference exists the syntax means something different in a difference place

ABp16•3mo ago

Ah, I figured it was something like that

Aaron•3mo ago

if you want to create the collection, use [] instead of {}

ABp16•3mo ago

Well, apparently that's in dotnet 8.0, I haven't upgraded to that yet I'm using Array.Empty now

Angius•3mo ago

Just switch the number Upgrade to 8.0 should literally be just it

Aaron•3mo ago

that is also fine

Angius•3mo ago

No other changes required

ABp16•3mo ago

don't I need to install it though

Angius•3mo ago

And that is not a problem either

Aaron•3mo ago

besides, yknow, the list of changes required https://learn.microsoft.com/en-us/dotnet/core/compatibility/8.0

Breaking changes in .NET 8

Navigate to the breaking changes in .NET 8.

Angius•3mo ago

Those are few and mostly to obscure APIS, IME winget install --id Microsoft.DotNet.SDK.8

ABp16•3mo ago

Alright alright fine I'll do it now x)

Angius•3mo ago

Alternatively, if you use VS, chances are the SDK should be installed with it dotnet --list-sdks will answer that question

ABp16•3mo ago

I use Rider It didn't seem to be installed yet

Angius•3mo ago

Ah Then yeah, install it with Winget

ABp16•3mo ago

Well, thank you all for your help Much appreciated !

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Explanation for array initialization peculiarities