supressing keypresses

  • Thread starter Thread starter farseer
  • Start date Start date
F

farseer

hi,
based on a few posts i have read here, i created a dll that allows me
to perform low level keyboardhooking from my C# code. this works great
and i send to a MessageWindow, certain WM_KEYDOWN messages.

What i would now like to do is if i detect certain keys (regardless of
what process they eminated from), set focus to a control on a form, and
cancel the keypress all together. Is this possible?

What i have attempted are two things:
1. don't call CallNextHookEx. when i bypass that call, the same
message is sent repeatedly to my MessageWindow (i.e. the last message
sent from the dll's HookProc). why?
2. i marhall in the KBDLLHOOKSTRUCT structure in the C# code, change
the vkcode and scancode to something like -9999, and marshall that back
to a ptr (i.e. the memory block the C code is referencing). The
marshalling works (i see the -9999 being processed by the dll), but the
keydown is still processed and does something. Why i say this is that
in by C# code, i can set focus to the control i want to when i recieve
the right arrow key press, but as soon as the HookProc call is
completed in the dll, the control looses focus (no longer has that dark
border indicating it has the focus)

anyone has any idea of how to solve this problem?
 
here's some of the code

******BEGIN C#******
public class KeyHandlerMW : MessageWindow
{
...

protected override void WndProc( ref Message msg )
{
...

switch ( msg.WParam.ToInt32( ) )
{
case ( int ) API.WM_KEYDOWN:
API.KBDLLHOOKSTRUCT kbStruct = getKeyStruct( msg.LParam );

if ( kbStruct.vkCode == 39 )
{
//*********
some code here that sets focus to form control
//*********

//try to suppress keydown event by changing vkCode and
//letting DLL know bypass CallNextHookEx
kbStruct.vkCode = -9999;
kbStruct.scanCode = -9999;
System.Runtime.InteropServices.
Marshal.StructureToPtr(
kbStruct, msg.LParam, false );
}

break;

}
}


public API.KBDLLHOOKSTRUCT getKeyStruct( IntPtr structPtr )
{
API.KBDLLHOOKSTRUCT kbStruct = ( API.KBDLLHOOKSTRUCT )
System.Runtime.InteropServices.Marshal.
PtrToStructure( structPtr, typeof( API.KBDLLHOOKSTRUCT ) );

Logger.debug("VKCode: " + kbStruct.vkCode);
Logger.debug("ScanCode: " + kbStruct.scanCode);
Logger.debug("Flags: " + kbStruct.flags);
Logger.debug("Time: " + kbStruct.time);
Logger.debug("dwExtraInfo: " + kbStruct.dwExtraInfo);

return kbStruct;
}
}

******END C#******

******BEGIN DLL******

KEYHOOK_DLL_API LRESULT CALLBACK keyboardHookProc(int ncode,WPARAM
wparam,
LPARAM lparam)
{
if ( ncode == HC_ACTION )
{
if( wparam == WM_KEYDOWN )
{
SendMessage( _messageWindow, ncode, wparam, lparam);
}
}

KBDLLHOOKSTRUCT *kbs = (KBDLLHOOKSTRUCT*) lparam;

if ( kbs->vkCode == -9999 )
{
//send dummy message back to message window to see
//if StructToPtr worked
SendMessage( _messageWindow, ncode, 9999, lparam);
return NULL;
}
else
return ( CallNextHookEx( _keyboardHook, ncode, wparam,lparam ) );

return 0;

}
******END DLL******
 
I think that, if you just bypass calling the next hook and return the right
value, the key should effectively be ignored.

Paul T.
 
What would be the "right" value to return? perhaps that is what i am
missing. right now, bypassing the CallNextHookEx and returning
something like NULL does not do it for me. I know NULL is probably not
what should be returned, but i'm not sure what should be the return
value... This would be an "app-saver" if i can get this to work...

thanks much
 
If you read the docs on LowLevelKeyboardProc function, linked from the
documentation of SetWindowsHookEx() in MSDN, it tells you what to return for
various situations, including the case where you don't want the key to be
sent to the target window procedure.

Paul T.
 
thanks...well, so you don't think i am a lazy bum, i did look, but may
have not read it carefully...i thought that not callingnexthookex would
simply not pass it on to other hookprocs (i.e. other applications that
have installed WH_KEYBOARD_LL).
i now see the last four words of the paragraph of interest..."..or the
target window procedure". i am hoping that means what i think it
does...
i'll try tonight...

thanks much
 
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