C#•2y ago

Error CS353 When it doesn't make sense for this situation

A result of a stackalloc expression of type 'Span<int>' cannot be used in this context because it may be exposed outside of the containing method however it isn't exposed out of the method, I will post the code in another message as it is a lil long

15 Replies

Sherbert LemonOP•2y ago

public unsafe int[] GetConnectedPoints(int index)
    {
        const int allocationBlocks = 5;
        
        Span<int> neighboursSpan = stackalloc int[allocationBlocks]; // "allocate" allocationBlocks, if we overrun we can increase this
        int currIndex = 0; // this is pretty much a stack list, because we are not working with very large int[]s it doesn't make sense to heap allocate, also gc isn't necessary as we can clean up ourselves

        Span<int> memoryBlocks = null; // this shouldn't normally be used as the allocation block should be plenty, but to remove the chance of errors this is a backup
        int currBlock = 0;

        for (int i = 0; i < faces.Length; i++)
        {
            Face curr = faces[i];
            if (currIndex + 2 > neighboursSpan.Length)
            {
                if (currBlock == 0)
                {
                    // ReSharper disable once StackAllocInsideLoop
                    memoryBlocks = stackalloc int[allocationBlocks];
                }

                // ReSharper disable once StackAllocInsideLoop - ignore warning - it will be fine as we are only doing it in a specific circumstance in a for loop
                int* increasedAllocBlock =
                    stackalloc int[allocationBlocks]; // the pointers work without pinning because stack allocated data is always pinned
                memoryBlocks[(currIndex / allocationBlocks)] = (int)&neighboursSpan;
                memoryBlocks[(currIndex / allocationBlocks) + 1] = (int)increasedAllocBlock;

                neighboursSpan = new Span<int>(increasedAllocBlock, allocationBlocks);

                currBlock++;
            }

public unsafe int[] GetConnectedPoints(int index)
    {
        const int allocationBlocks = 5;
        
        Span<int> neighboursSpan = stackalloc int[allocationBlocks]; // "allocate" allocationBlocks, if we overrun we can increase this
        int currIndex = 0; // this is pretty much a stack list, because we are not working with very large int[]s it doesn't make sense to heap allocate, also gc isn't necessary as we can clean up ourselves

        Span<int> memoryBlocks = null; // this shouldn't normally be used as the allocation block should be plenty, but to remove the chance of errors this is a backup
        int currBlock = 0;

        for (int i = 0; i < faces.Length; i++)
        {
            Face curr = faces[i];
            if (currIndex + 2 > neighboursSpan.Length)
            {
                if (currBlock == 0)
                {
                    // ReSharper disable once StackAllocInsideLoop
                    memoryBlocks = stackalloc int[allocationBlocks];
                }

                // ReSharper disable once StackAllocInsideLoop - ignore warning - it will be fine as we are only doing it in a specific circumstance in a for loop
                int* increasedAllocBlock =
                    stackalloc int[allocationBlocks]; // the pointers work without pinning because stack allocated data is always pinned
                memoryBlocks[(currIndex / allocationBlocks)] = (int)&neighboursSpan;
                memoryBlocks[(currIndex / allocationBlocks) + 1] = (int)increasedAllocBlock;

                neighboursSpan = new Span<int>(increasedAllocBlock, allocationBlocks);

                currBlock++;
            }

the rest is irrelevant but I will add it for context

            if (curr.a == i)
            {
                neighboursSpan[currIndex] = curr.b;
                neighboursSpan[currIndex + 1] = curr.c;
                
                currIndex += 2;
            }
            else if (curr.b == i)
            {
                neighboursSpan[currIndex] = curr.a;
                neighboursSpan[currIndex + 1] = curr.c;

                currIndex += 2;
            }
            else if (curr.c == i)
            {
                neighboursSpan[currIndex] = curr.a;
                neighboursSpan[currIndex + 1] = curr.b;
                
                currIndex += 2;
            }
        }

        if (memoryBlocks != null) // null but rider didn't like null
        {
            int[] neighbours = new int[currIndex + allocationBlocks * currBlock];
            for (int i = 0; i < currBlock; i++)
            {
                neighbours.CopyTo(new Span<int>((int*)memoryBlocks[i], allocationBlocks));
            }
            
            return neighbours;
        }
        else
        {
            return neighboursSpan.ToArray();
        }
    }

            if (curr.a == i)
            {
                neighboursSpan[currIndex] = curr.b;
                neighboursSpan[currIndex + 1] = curr.c;
                
                currIndex += 2;
            }
            else if (curr.b == i)
            {
                neighboursSpan[currIndex] = curr.a;
                neighboursSpan[currIndex + 1] = curr.c;

                currIndex += 2;
            }
            else if (curr.c == i)
            {
                neighboursSpan[currIndex] = curr.a;
                neighboursSpan[currIndex + 1] = curr.b;
                
                currIndex += 2;
            }
        }

        if (memoryBlocks != null) // null but rider didn't like null
        {
            int[] neighbours = new int[currIndex + allocationBlocks * currBlock];
            for (int i = 0; i < currBlock; i++)
            {
                neighbours.CopyTo(new Span<int>((int*)memoryBlocks[i], allocationBlocks));
            }
            
            return neighbours;
        }
        else
        {
            return neighboursSpan.ToArray();
        }
    }

The error is on the line

 memoryBlocks = stackalloc int[allocationBlocks];

 memoryBlocks = stackalloc int[allocationBlocks];

I might just not use spans for this situation as I am not entirely sure but it would still be good to know what was going wrong I would check the il but it is a full on error so I can't I did the il with sharplab and that worked and it didn't seem to expose ah I made a rather silly mistake, the copyto has the destination as its param not the data nope that didn't fix it this one is really confusing me, I commented out all the return values and made it return null, the only parameter in is an integer and it is still saying that it is exposed

333fred•2y ago

Declare memoryBlocks as scoped scoped Span<int> memoryBlocks... The default safe-to-escape scope is not scoped And you're not initializing it with something that would narrow it (like assigning a stackalloc in the initializer)

Sherbert LemonOP•2y ago

I can't use scoped as my language version for this can't be high enough but I could assign it, is there any way to have it without allocating it, preferably it should be very rarely allocated, but I could allocate it if I have to also what is scoped, I haven't come across that before and I can't see it in the docs ah found it in the c# 11 page

333fred•2y ago

Do a stackalloc int[0]; I'm pretty sure that works

Sherbert LemonOP•2y ago

yeah ok will do, why is it that it has to be allocated? that does work thanks

333fred•2y ago

Sherbert LemonOP•2y ago

I don't quite understand

333fred•2y ago

By default, the local can escape the method That's it Doesn't matter whether it does. It can You can narrow that default by assigning something that not safe-to-escape Or, in C# 11, you can declare the local as scoped

Sherbert LemonOP•2y ago

ah ok that makes more sense, why isn't it implicitly not safe to escape by already being defined as a valuetype in the locals

333fred•2y ago

Being a valuetype has nothing to do with safe-to-escape

Sherbert LemonOP•2y ago

it should also be defined as a Span<int> there too also wouldn't it

333fred•2y ago

Span<char> s = " test"; is perfectly fine to escape the method

Sherbert LemonOP•2y ago

what isn't safe to escape? actually a better question would be, what does it mean for something to escape the method

333fred•2y ago

Span<char> Get() => "Test"; // escapes, it's heap data, this is fine Span<char> Get() => stackalloc char[1] { 'c' }; // escapes, oops

Sherbert LemonOP•2y ago

ahhh ok thanks that clarifies it

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Error CS353 When it doesn't make sense for this situation