Working with SafeHandles in F#

Question

I'm working with some F# code that uses platform invoke. One of the APIs I'm using returns a handle. Instead of using a nativeint, I've implemented my own SafeHandle (specifically SafeHandleMinusOneIsInvalid.) This makes working with the module containing the pinvoke signature a little clunky. Here is an example:

type MySafeHandle() = 
    inherit SafeHandleZeroOrMinusOneIsInvalid(true)

    override this.ReleaseHandle() =
        NativeMethods.FreeHandle(base.handle)            
        true

module NativeMethods = 
    [<DllImport("mylibrary.dll")>]
    extern void GetHandle([<Out>]MySafeHandle& handle)
    [<DllImport("mylibrary.dll")>]
    extern void FreeHandle(nativeint handle)

This won't compile because the module and the class recursively reference each other, which doesn't work. If I move the module above MySafeHandle, then GetHandle won't see the SafeHandle.

I can't move the platform invoke methods inside of MySafeHandle since it appears that extern methods in F# must be in modules (even though the compiler won't stop you from trying to put them in a class).

It also appears that F#'s recursive types don't work between a module and a class, just classes.

Is there a solution to this problem that does not require declaring two different modules? Ideally I'd like to keep all of my platform invoke code organized into one module.

Two modules really is the cleanest solution. – Daniel Jul 28 '14 at 21:51 — Daniel, Jul 28 '14 at 21:51

score 1 · Answer 1 · answered Jul 28 '14 at 21:18

Well I know of one, because I had the same problem myself.

The thing is, it's kinda ugly I guess:

It involves a static reference that you set to the imported function later in the module.

type MySafeHandle() = 
    inherit SafeHandleZeroOrMinusOneIsInvalid(true)
    static let freeHandle = ref Unchecked.defaultof<_>
    static member internal SetFreeHandleRef value = freeHandle := value

    override this.ReleaseHandle() =
        !freeHandle base.handle            
        true

module NativeMethods = 
    [<DllImport("mylibrary.dll")>]
    extern void GetHandle([<Out>]MySafeHandle& handle)
    [<DllImport("mylibrary.dll")>]
    extern void FreeHandle(nativeint handle)

    MySafeHandle.SetFreeHandleRef FreeHandle

Working with SafeHandles in F#

1 Answers1