Managing Low-Level Keyboard Hooks in VB .NET
Trapping Key Combinations
Determining if specific key combinations are being pressed requires some tricky gyrations. (Keep in mind that we are working at a pretty low level here.) This code remains pretty much unchanged from the November article. The basic idea is to read the current key press in the KBDLLHOOKSTRUCT.vkCode. If you need to look for specific multi-key combinations, you may need to call GetAsyncKeyState to determine whether additional keys are being held. For example, we call GetAsynckeyState(VK_CONTROL) in listing 2 to see whether the Ctrl key is being held down.
Unhooking the Keyboard
The return value from SetWindowsHookEx is stored. This is the address of the hook we replaced. We don't discard this value because if we want to let some key combinations slip past our hook, we need to use the return value of SetWindwosHookEx to call the old hook. We also use this value to unhook the keyboard, returning the old hook state, when we are finished holding onto the keyboard handler. Call UnhookWindowsHookEx passing the return value from SetWindowsHookEx to restore the original keyboard hook.
The Complete Code Listing
Listing 2: The complete revised listing for implementing low-level keyboard hooks.
Imports System.Runtime.InteropServices Imports System.Reflection Imports System.Drawing Imports System.Threading Module Keyboard Public Declare Function UnhookWindowsHookEx Lib "user32" _ (ByVal hHook As Integer) As Integer Public Declare Function SetWindowsHookEx Lib "user32" _ Alias "SetWindowsHookExA" (ByVal idHook As Integer, _ ByVal lpfn As KeyboardHookDelegate, ByVal hmod As Integer, _ ByVal dwThreadId As Integer) As Integer Private Declare Function GetAsyncKeyState Lib "user32" _ (ByVal vKey As Integer) As Integer Private Declare Function CallNextHookEx Lib "user32" _ (ByVal hHook As Integer, _ ByVal nCode As Integer, _ ByVal wParam As Integer, _ ByVal lParam As KBDLLHOOKSTRUCT) As Integer Public Structure KBDLLHOOKSTRUCT Public vkCode As Integer Public scanCode As Integer Public flags As Integer Public time As Integer Public dwExtraInfo As Integer End Structure ' Low-Level Keyboard Constants Private Const HC_ACTION As Integer = 0 Private Const LLKHF_EXTENDED As Integer = &H1 Private Const LLKHF_INJECTED As Integer = &H10 Private Const LLKHF_ALTDOWN As Integer = &H20 Private Const LLKHF_UP As Integer = &H80 ' Virtual Keys Public Const VK_TAB = &H9 Public Const VK_CONTROL = &H11 Public Const VK_ESCAPE = &H1B Public Const VK_DELETE = &H2E Private Const WH_KEYBOARD_LL As Integer = 13& Public KeyboardHandle As Integer ' Implement this function to block as many ' key combinations as you'd like Public Function IsHooked( _ ByRef Hookstruct As KBDLLHOOKSTRUCT) As Boolean Debug.WriteLine("Hookstruct.vkCode: " & Hookstruct.vkCode) Debug.WriteLine(Hookstruct.vkCode = VK_ESCAPE) Debug.WriteLine(Hookstruct.vkCode = VK_TAB) If (Hookstruct.vkCode = VK_ESCAPE) And _ CBool(GetAsyncKeyState(VK_CONTROL) _ And &H8000) Then Call HookedState("Ctrl + Esc blocked") Return True End If If (Hookstruct.vkCode = VK_TAB) And _ CBool(Hookstruct.flags And _ LLKHF_ALTDOWN) Then Call HookedState("Alt + Tab blockd") Return True End If If (Hookstruct.vkCode = VK_ESCAPE) And _ CBool(Hookstruct.flags And _ LLKHF_ALTDOWN) Then Call HookedState("Alt + Escape blocked") Return True End If Return False End Function Private Sub HookedState(ByVal Text As String) Debug.WriteLine(Text) End Sub Public Function KeyboardCallback(ByVal Code As Integer, _ ByVal wParam As Integer, _ ByRef lParam As KBDLLHOOKSTRUCT) As Integer If (Code = HC_ACTION) Then Debug.WriteLine("Calling IsHooked") If (IsHooked(lParam)) Then Return 1 End If End If Return CallNextHookEx(KeyboardHandle, _ Code, wParam, lParam) End Function Public Delegate Function KeyboardHookDelegate( _ ByVal Code As Integer, _ ByVal wParam As Integer, ByRef lParam As KBDLLHOOKSTRUCT) _ As Integer <MarshalAs(UnmanagedType.FunctionPtr)> _ Private callback As KeyboardHookDelegate Public Sub HookKeyboard() callback = New KeyboardHookDelegate(AddressOf KeyboardCallback) KeyboardHandle = SetWindowsHookEx( _ WH_KEYBOARD_LL, callback, _ Marshal.GetHINSTANCE( _ [Assembly].GetExecutingAssembly.GetModules()(0)).ToInt32, 0) Call CheckHooked() End Sub Public Sub CheckHooked() If (Hooked()) Then Debug.WriteLine("Keyboard hooked") Else Debug.WriteLine("Keyboard hook failed: " & Err.LastDllError) End If End Sub Private Function Hooked() Hooked = KeyboardHandle <> 0 End Function Public Sub UnhookKeyboard() If (Hooked()) Then Call UnhookWindowsHookEx(KeyboardHandle) End If End Sub End Module
Be aware that mistakes may completely lock up your keyboard and you may need to reboot. To prevent this kind of problem, I use the ThreadPool and a separate thread to release the keyboard after 10 or 15 seconds. This strategy has been invaluable while developing low-level code. You can learn more about multithreading here in past and future articles or by picking up a copy of my book, Visual Basic .NET Unleashed, from Sams.
Run the sample code and you will see that the Windows API is alive and well in .NET. Thankfully, you will need to have very special needs indeed to resort to calling into the Windows API. This is a far cry from VB6, where almost anything useful required interaction with the Windows API.
Disclaimer: The VS IDE hooks the keyboard. You may need to run the sample code outside of the IDE for the keyboard hook API call to succeed.
One of the most important differences between VB6 and VB.NET is the notion of managed code. Code in VB.NET is managed. This means objects can be moved around in memory and garbage collected. Old Windows API methods do not represent managed code. As a result, you may get some quirky behavior when interacting between .NET and the Windows API. If you plan on writing a lot of code that interoperates with the Windows API or COM, I encourage you to pick up a good book on COM Interop and a good advanced book such as my Visual Basic .NET Power Coding from Addison-Wesley that explores these intricate nooks and crannies for you.
About the Author
Paul Kimmel is a freelance writer for Developer.com and CodeGuru.com. Look for his recent book, Visual Basic .NET Power Coding, from Addison-Wesley on Amazon.com. Paul Kimmel is available to help design and build your .NET solutions and can be contacted at firstname.lastname@example.org.
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