C#•2mo ago

Use of inline arrays to model arrays of structs

For a native type with the following shape:

public struct CHRDATA {
    public uint hp;
    public uint mp; // ... hundreds more fields
}
public struct SAVEDATA {
    public CHRDATA chr1;
    public CHRDATA chr2;   
    public CHRDATA chr3; // ... tens more entries
}

public struct CHRDATA {
    public uint hp;
    public uint mp; // ... hundreds more fields
}
public struct SAVEDATA {
    public CHRDATA chr1;
    public CHRDATA chr2;   
    public CHRDATA chr3; // ... tens more entries
}

it occurred to me I could represent the sequential CHRDATAs with an InlineArray as such:

[InlineArray(80)]
public struct CHRDATA_ARRAY {
    private CHRDATA _c;
}
public struct SAVEDATA {
    public CHRDATA_ARRAY chrs;
}

[InlineArray(80)]
public struct CHRDATA_ARRAY {
    private CHRDATA _c;
}
public struct SAVEDATA {
    public CHRDATA_ARRAY chrs;
}

which raises a few questions, since I'm unfamiliar with the feature: 1) is this legal? can you make InlineArrays of blittable structs, or should usage of InlineArray be restricted to primitives? (i.e. int, float) 2) does chrs[62].hp = 300; in the above case operate on a copy or actually mutate the relevant part of SAVEDATA?

12 Replies

ティナ•2mo ago

1) try compile and see 2) the indexer returns the ref to CHRDATA https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-12.0/inline-arrays#when-the-expression-type-is-int

Inline arrays, or fixed sized buffers - C# feature specifications

Inline arrays provide a general-purpose and safe mechanism for declaring inline arrays within C# classes, structs, and interfaces.

peppyOP•2mo ago

alright, that clears it up, just one follow-up: if one of these was special and I wanted to provide a named accessor, by putting a property on CHRDATA_ARRAY like:

public CHRDATA special_character_slot { get { return this[37]; } }

public CHRDATA special_character_slot { get { return this[37]; } }

I assume this will not return ref CHRDATA - is there a way to make that happen? if I try:

public ref CHRDATA special_character_slot { get { return ref this[37]; } }

public ref CHRDATA special_character_slot { get { return ref this[37]; } }

it fires CS9084: Struct member returns 'this' or other instance members by reference, but I've not been able to find documentation for this

Anton•2mo ago

you can work around this by slapping [UnscopedRef] on the getter or make an extension method that takes in a ref

peppyOP•2mo ago

:HmmNoted: Thanks, I'll write that down somewhere

cned•2mo ago

You may get in hot water with this, since you’ve got a ref to something that might be in the stack that the compiler doesn’t know is in the stack anymore… Any reason to not just add a setter and skip the dangers of ref? Heck, you might have a ref to a struct that doesn’t even really exist anymore (if it made a copy of the struct to call this method on).

peppyOP•2mo ago

in this case, the structs are just mapping/projection for something at a fixed location in memory in a subordinate native process in that sense, I don't know what would have to occur for them to move, and they're not intended to be 'used' as such in regular calls

cned•2mo ago

I guess if you have some fixed pointer to a native, returning a ref is... probably safe...? Though "safe" is certainly an interesting word when working with pointers. 🙂 I'm surpzied the compiler bothered warning you about that in an unsafe context.

Aaron•2mo ago

you do not get in hot water for this

MODiX•2mo ago

Aaron

REPL Result: Failure

struct S
{
    int a;
    [System.Diagnostics.CodeAnalysis.UnscopedRef]
    public ref int M() => ref a;
}

ref int Test()
{
    S s = default;
    // try to return data that will no longer exist, this will error because the compiler knows what UnscopedRef means and knows not to allow this
    return ref s.M();
}

struct S
{
    int a;
    [System.Diagnostics.CodeAnalysis.UnscopedRef]
    public ref int M() => ref a;
}

ref int Test()
{
    S s = default;
    // try to return data that will no longer exist, this will error because the compiler knows what UnscopedRef means and knows not to allow this
    return ref s.M();
}

Exception: CompilationErrorException

- Cannot return local 's' by reference because it is not a ref local

- Cannot return local 's' by reference because it is not a ref local

Compile: 376.485ms | Execution: 0.000ms | React with ❌ to remove this embed.

Aaron•2mo ago

the compiler keeps track of this and does not let you do invalid things with refs it does warn you about this in an unsafe context, but only because it is normally an outright error and you really shouldn't be doing it they were not in one when they tried this anyway, I don't think

MODiX•2mo ago

Aaron

REPL Result: Success

struct S
{
    int a;
    public unsafe ref int M() => ref a;
}

struct S
{
    int a;
    public unsafe ref int M() => ref a;
}

Compile: 274.488ms | Execution: 39.816ms | React with ❌ to remove this embed.

Aaron•2mo ago

(there is a warning there still, the repl just doesn't show warnings) but yeah, [UnscopedRef] isn't "shut up and do what I want, who cares about safety", it does something very specific and restricts how callers can use the ref that is returned

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Use of inline arrays to model arrays of structs

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