How to set a hook in Visual Basic .NET

This article describes how to set a hook that is specific to a thread and to a hook procedure. This article uses the mouse hook as an example.

You can use hooks to monitor certain types of events. You can associate these events with a specific thread or with all threads that are in the same desktop as a calling thread.

Set a mouse hook

To set a hook, call the SetWindowsHookEx function from the User32.dll file. This function installs an application-defined hook procedure in the hook chain that is associated with the hook.

To set a mouse hook and to monitor the mouse events, follow these steps:

1. Start Microsoft Visual Studio .NET.
2. On the File menu, point to New, and then click Project.
3. In the New Project dialog box, click Visual Basic Projects under Project Types. Then, click Windows Application under Templates.
4. In the Name box, type ThreadSpecificMouseHook. By default, a form that is named Form1 is created.
5. At the beginning of the Form1.vb file, paste the following code.

   Imports System.Runtime.InteropServices
   Public Delegate Function CallBack( _
       ByVal nCode As Integer, _
       ByVal wParam As IntPtr, _
       ByVal lParam As IntPtr) As Integer

6. In the Form1 class, paste the following code.

   'Declare the mouse hook constant.
   'For other hook types, obtain these values from Winuser.h in Microsoft SDK.
       Dim WH_MOUSE As Integer = 7
       Shared hHook As Integer = 0
   
       'Keep the reference so that the delegate is not garbage collected.
       Private hookproc As CallBack
   
       'Import for the SetWindowsHookEx function.
       <DllImport("User32.dll", CharSet:=CharSet.Auto, CallingConvention:=CallingConvention.StdCall)> _
        Public Overloads Shared Function SetWindowsHookEx _
             (ByVal idHook As Integer, ByVal HookProc As CallBack, _
              ByVal hInstance As IntPtr, ByVal wParam As Integer) As Integer
       End Function
   
       'Import for the CallNextHookEx function.
       <DllImport("User32.dll", CharSet:=CharSet.Auto, CallingConvention:=CallingConvention.StdCall)> _
        Public Overloads Shared Function CallNextHookEx _
             (ByVal idHook As Integer, ByVal nCode As Integer, _
              ByVal wParam As IntPtr, ByVal lParam As IntPtr) As Integer
       End Function
       'Import for the UnhookWindowsHookEx function.
       <DllImport("User32.dll", CharSet:=CharSet.Auto, CallingConvention:=CallingConvention.StdCall)> _
            Public Overloads Shared Function UnhookWindowsHookEx _
                 (ByVal idHook As Integer) As Boolean
       End Function
   
       'Point structure declaration.
       <StructLayout(LayoutKind.Sequential)> Public Structure Point
           Public x As Integer
           Public y As Integer
       End Structure
   
       'MouseHookStruct structure declaration.
       <StructLayout(LayoutKind.Sequential)> Public Structure MouseHookStruct
           Public pt As Point
           Public hwnd As Integer
           Public wHitTestCode As Integer
           Public dwExtraInfo As Integer
       End Structure

7. Add a button to the form. Then, paste the following code in the button1_click procedure.

       Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
   
           If hHook.Equals(0) Then
               hookproc = AddressOf MouseHookProc
               hHook = SetWindowsHookEx(WH_MOUSE, _
                                        hookproc, _
                                        IntPtr.Zero, _
   AppDomain.CurrentDomain.GetCurrentThreadId())
               If hHook.Equals(0) Then
                   MsgBox("SetWindowsHookEx Failed")
                   Return
               Else
                   Button1.Text = "UnHook Windows Hook"
               End If
           Else
               Dim ret As Boolean = UnhookWindowsHookEx(hHook)
   
               If ret.Equals(False) Then
                   MsgBox("UnhookWindowsHookEx Failed")
                   Return
               Else
                   hHook = 0
                   Button1.Text = "Set Windows Hook"
                   Me.Text = "Mouse Hook"
               End If
           End If
   
       End Sub

8. In the Form1 class, paste the following code for the MouseHookProc function.

       Public Shared Function MouseHookProc( _
       ByVal nCode As Integer, _
       ByVal wParam As IntPtr, _
       ByVal lParam As IntPtr) As Integer
   Dim MyMouseHookStruct As New MouseHookStruct()
   
           Dim ret As Integer
   
           If (nCode < 0) Then
               Return CallNextHookEx(hHook, nCode, wParam, lParam)
           End If
   
           MyMouseHookStruct = CType(Marshal.PtrToStructure(lParam, MyMouseHookStruct.GetType()), MouseHookStruct)
   
           Dim tempForm As Form
           tempForm = Form.ActiveForm()
   
           Dim strCaption As String
           strCaption = "x = " & MyMouseHookStruct.pt.x & " y = " & MyMouseHookStruct.pt.y
   
           tempForm.Text = strCaption
           Return CallNextHookEx(hHook, nCode, wParam, lParam)
   
       End Function

9. Press F5 to run the project. Click the button on the form to set the hook. The pointer coordinates appear on the form caption bar when the pointer moves on the form.
10. Click the button again to remove the hook.

Global hooks are not supported in the .NET Framework

You cannot implement global hooks in Microsoft .NET Framework. To install a global hook, a hook must have a native DLL export to insert itself in another process that requires a valid, consistent function to call into. This behavior requires a DLL export. The .NET Framework does not support DLL exports. Managed code has no concept of a consistent value for a function pointer because these function pointers are proxies that are built dynamically.

For more information about hooks, visit the following Microsoft Developer Network (MSDN) Web site: