Is there fixed array in Struct in F# just like C#

[Xyncgas]

For example in C# there is :

internal unsafe struct Buffer
{
    public fixed char fixedBuffer[128];
}

or in C# 12 supported by the up coming .NET 8

[System.Runtime.CompilerServices.InlineArray(10)]
public struct Buffer
{
    private int _element0;
}

how do I create these struct and use it in F#

[ninjarobot]

You can use them in F#:

[<Struct>]
[<System.Runtime.CompilerServices.InlineArray(10)>]
type FixedBuffer =
    struct
        val element0:int
    end

[<Struct; StructLayout(LayoutKind.Sequential)>]
type StructWithBuffer =
    struct
        val field1:int
        val field2:int
        val otherFixedField:FixedBuffer
    end

and then use it like this:

let size = Marshal.SizeOf<StructWithBuffer>()
printfn "Size of StructWithBuffer: %i" size
let ptr = NativePtr.stackalloc<StructWithBuffer> 1
let unmarshaled : StructWithBuffer = ptr |> NativePtr.read
// The type system knows the buffer it pointing to ints, but not the size of the buffer
let fixedField = &&unmarshaled.otherFixedField.element0
fixedField |> NativePtr.read |> printfn "Fixed field first element: %i"
// You need to take care not to read past the index.
NativePtr.get fixedField 4 |> printfn "Fixed field element 4: %i"
// You need to take care not to read past the index.
NativePtr.get fixedField 11 |> printfn "Fixed field element 11 (this is something else's data): %i"

output:

Size of StructWithBuffer: 48
Fixed field: 0
Fixed field element 4: 0
Fixed field element 11 (this is something else's data): 1

You see it allocates 48 bytes:

• Field1 = int = 4 bytes
• Field2 = int = 4 bytes
• OtherFixedField = 10 ints = 40 bytes

You can read and write those elements and nativeptr<int> will give you some type safety. The one place it really can't help is ensuring you don't go out of bounds of the fixed buffer.