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.