pinvoke.net: Edit: user32.setwindowshookex

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Summary: Installs a hook to monitor certain types of events.

!!!!C# Signature:
 [DllImport("user32.dll", SetLastError = true)]
 static extern IntPtr SetWindowsHookEx(HookType hookType, HookProc lpfn, IntPtr hMod, uint dwThreadId);

!!!!VB.NET Signature:
 <DllImport("user32.dll", SetLastError:=True)> _
 Private Shared Function SetWindowsHookEx(ByVal hookType As HookType, ByVal lpfn As HookProc, ByVal hMod As IntPtr, ByVal dwThreadId As UInteger) As IntPtr
 End Function

!!!!User-Defined Types:
A HookType constant specifying the type of hook to install. Used as the value passed into idHook.
A HookProc delegate representing the hook procedure method. Used as the value passed into lpfn (long pointer to function).

!!!!Notes:
This will enable you to install application hooks. However, you cannot implement global hooks in Microsoft .NET Framework except low level hooks. To install a global hook, a hook must have a native dynamic-link library (DLL) export to inject itself in another process that requires a valid, consistent function to call into. This requires a DLL export, which .NET Framework does not support. Managed code has no concept of a consistent value for a function pointer because these function pointers are proxies that are built dynamically.

AppDomain.GetCurrentThreadId is marked as deprecated in favour of Thread.ManagedThreadId but this will not to work with unmanaged API.
You can use the GetCurrentThreadId API as well:

To hook low level events use:
 IntPtr hHook;
 
 using (Process process = Process.GetCurrentProcess())
 using (ProcessModule module = process.MainModule)
 {
    IntPtr hModule = GetModuleHandle(module.ModuleName);
    
    hHook = SetWindowsHookEx(HookType.WH_KEYBOARD_LL, hook, hModule, 0);
 }

!!!!Tips & Tricks:
Remember to keep the HookProc delegate alive manually, such as using a class member as shown in the example below, otherwise the garbage collector will clean up your hook delegate eventually, resulting in your code throwing a System.NullReferenceException.

Remember to call CallNextHookEx in your callback delegate.

Remember to call UnhookWindowsHookEx using the handle returned by SetWindowsHookEx

Note how you can import the same function several times with different overloaded signatures to handle the common case where one parameter is an opaque pointer pointing to a struct that depends on another parameter.

!!!!Sample Code:

// this sample installs a keyboard hook

using System.Windows.Forms;
 public class MyClass
 {
     private HookProc myCallbackDelegate = null;
     
     public MyClass()
     {
         // initialize our delegate
         this.myCallbackDelegate = new HookProc(this.MyCallbackFunction);

// setup a keyboard hook
         SetWindowsHookEx(HookType.WH_KEYBOARD, this.myCallbackDelegate, IntPtr.Zero, AppDomain.GetCurrentThreadId());
     }

[DllImport("user32.dll")]
     protected static extern IntPtr SetWindowsHookEx(HookType code, HookProc func, IntPtr hInstance, int threadID);

[DllImport("user32.dll")]
     static extern int CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam);

private int MyCallbackFunction(int code, IntPtr wParam, IntPtr lParam)
     {
	if (code < 0) {
		//you need to call CallNextHookEx without further processing
		//and return the value returned by CallNextHookEx
		return CallNextHookEx(IntPtr.Zero, code, wParam, lParam);
	}
         // we can convert the 2nd parameter (the key code) to a System.Windows.Forms.Keys enum constant
         Keys keyPressed = (Keys)wParam.ToInt32();
         Console.WriteLine(keyPressed);
	//return the value returned by CallNextHookEx
	return CallNextHookEx(IntPtr.Zero, code, wParam, lParam);
     }
 }

!!!!VB.NET:
 ' My sample installs a keyboard hook
 Imports System.Windows.Forms
 Imports System.Runtime.InteropServices
 
 Public Class MyClass1
    Delegate Function HookProc(ByVal code As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As Integer
 
    Private myCallbackDelegate As HookProc = Nothing
 
    Public Sub New()
        ' initialize our delegate
        Me.myCallbackDelegate = New HookProc(AddressOf Me.MyCallbackFunction)

' setup a keyboard hook
        SetWindowsHookEx(HookType.WH_KEYBOARD, Me.myCallbackDelegate, IntPtr.Zero, AppDomain.GetCurrentThreadId())
    End Sub
 
    <DllImport("user32.dll")> _
    Friend Shared Function SetWindowsHookEx(ByVal idHook As Integer, ByVal lpfn As HookProc, ByVal hInstance As IntPtr, ByVal threadId As Integer) As Integer
    End Function
 
    <DllImport("user32.dll")> _
    Friend Shared Function CallNextHookEx(ByVal hhk As intptr, ByVal nCode As Integer, ByVal wParam As intptr, ByVal lParam As intptr) As Integer
    End Function
 
    Private Function MyCallbackFunction(ByVal code As Integer, ByVal wParam As intptr, ByVal lParam As intptr) As Integer
        If (code < 0) Then
            'you need to call CallNextHookEx without further processing
            'and return the value returned by CallNextHookEx
            Return CallNextHookEx(IntPtr.Zero, code, wParam, lParam)    'unt
        End If
        ' we can convert the 2nd parameter (the key code) to a System.Windows.Forms.Keys enum constant
        Dim keyPressed As Keys = CType(wParam.ToInt32, Keys)
        Console.WriteLine(keyPressed)
        'return the value returned by CallNextHookEx
        Return CallNextHookEx(IntPtr.Zero, code, wParam, lParam)
    End Function
 End Class

!!!!Alternative Managed API:
The ManagedWindowsApi project (http://mwinapi.sourceforge.net) provides a Hook class and subclasses for Journal hooks, Message hooks and Low-Level hooks.

Documentation: 
SetWindowsHookEx@msdn on MSDN
MSDN example of a mouse hook in C#: http://support.microsoft.com/default.aspx?scid=kb;en-us;318804#3
MSDN article on hooks in .NET: http://msdn.microsoft.com/msdnmag/issues/02/10/CuttingEdge/
MSDN article on KeyboardProc: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/winui/windowsuserinterface/windowing/hooks/hookreference/hookfunctions/keyboardproc.asp
Stephen Toub entry on low-level keyboard hook in C#: http://blogs.msdn.com/toub/archive/2006/05/03/589423.aspx
Stephen Toub entry on low-level mouse hook on C#: http://blogs.msdn.com/toub/archive/2006/05/03/589468.aspx