I'm attempting to set up a low-level keyboard hook using P/Invoke in an F# application. The Win32 function SetWindowsHookEx
takes a HOOKPROC
for its second argument, which I've represented as a delegate of (int * IntPtr * IntPtr) -> IntPtr
, similar to how this would be handled in C#. When calling the method, I get a MarshalDirectiveException
stating that the delegate parameter cannot be marshaled because
Generic types cannot be marshaled
I'm not sure how generics are involved, as all types are concretely specified. Can anyone shed some light on this? Code follows.
EDIT
This may have to do with the way the F# compiler deals with type signatures - Reflector indicates that the delegate LowLevelKeyboardProc
is implemented as a method which accepts one argument of type Tuple<int, IntPtr, IntPtr>
- and there would be the un-marshalable generic type. Is there a way around this somehow, or are F# functions simply not capable of being marshaled to native function pointers?
let WH_KEYBOARD_LL = 13
type LowLevelKeyboardProc = delegate of (int * IntPtr * IntPtr) -> IntPtr
[<DllImport("user32.dll")>]
extern IntPtr SetWindowsHookEx(int idhook, LowLevelKeyboardProc proc, IntPtr hMod, UInt32 threadId)
[<DllImport("kernel32.dll")>]
extern IntPtr GetModuleHandle(string lpModuleName)
let SetHook (proc: LowLevelKeyboardProc) =
use curProc = Process.GetCurrentProcess ()
use curMod = curProc.MainModule
SetWindowsHookEx(WH_KEYBOARD_LL, proc, GetModuleHandle(curMod.ModuleName), 0u)
C - this is a grade that rests right in the middle. C is anywhere between 70% and 79% D - this is still a passing grade, and it's between 59% and 69% F - this is a failing grade.
If you get an F, you still have to pay for the class without receiving any credit toward your degree. What's more, failed classes don't count toward graduation requirements either. In some majors, even a passing grade like a D might not meet those requirements.
An F letter grade is equivalent to a 0.0 GPA, or Grade Point Average, on a 4.0 GPA scale, and a percentage grade of 65 or below.
Your LowLevelKeyboardProc
definition is wrong. Change from
type LowLevelKeyboardProc = delegate of (int * IntPtr * IntPtr) -> IntPtr
to
type LowLevelKeyboardProc = delegate of int * IntPtr * IntPtr -> IntPtr
or better yet
type LowLevelKeyboardProc = delegate of int * nativeint * nativeint -> nativeint
or better yet
[<StructLayout(LayoutKind.Sequential)>]
type KBDLLHOOKSTRUCT =
val vkCode : uint32
val scanCode : uint32
val flags : uint32
val time : uint32
val dwExtraInfo : nativeint
type LowLevelKeyboardProc =
delegate of int * nativeint * KBDLLHOOKSTRUCT -> nativeint
In all of the above cases, proc
will need to use curried form rather than tupled form.
Also note that you should add SetLastError = true
to all of the extern
ed functions whose documentation says to call GetLastError
upon failure (which is the case for GetModuleHandle
, SetWindowsHookEx
, and UnhookWindowsHookEx
). That way if any fail (and you should be checking the return values...), you can simply raise a Win32Exception
or call Marshal.GetLastWin32Error
to get proper diagnostics.
EDIT: Just for the sake of clarity, here are all the P/Invoke signatures I successfully tested locally:
[<Literal>]
let WH_KEYBOARD_LL = 13
[<StructLayout(LayoutKind.Sequential)>]
type KBDLLHOOKSTRUCT =
val vkCode : uint32
val scanCode : uint32
val flags : uint32
val time : uint32
val dwExtraInfo : nativeint
type LowLevelKeyboardProc = delegate of int * nativeint * KBDLLHOOKSTRUCT -> nativeint
[<DllImport("kernel32.dll")>]
extern uint32 GetCurrentThreadId()
[<DllImport("kernel32.dll", SetLastError = true)>]
extern nativeint GetModuleHandle(string lpModuleName)
[<DllImport("user32.dll", SetLastError = true)>]
extern bool UnhookWindowsHookEx(nativeint hhk)
[<DllImport("user32.dll", SetLastError = true)>]
extern nativeint SetWindowsHookEx(int idhook, LowLevelKeyboardProc proc, nativeint hMod, uint32 threadId)
Also note that this would work equally as well, if you prefer value semantics for KBDLLHOOKSTRUCT
:
[<Struct; StructLayout(LayoutKind.Sequential)>]
type KBDLLHOOKSTRUCT =
val vkCode : uint32
val scanCode : uint32
val flags : uint32
val time : uint32
val dwExtraInfo : nativeint
type LowLevelKeyboardProc = delegate of int * nativeint * byref<KBDLLHOOKSTRUCT> -> nativeint
Have you tried to use managed C++ with this. It can make a lot of the translation pretty seamless. You won't need P/Invoke then.
EDIT: I'd like to note one fairly important thing: compiler will do more type-checking for you. I am sure you love your type-checking, since you use F# for the rest of the app (hopefully).
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