Multi threading and garbage collected problem using RegisterShellHook and SetWindowLong API.

slam Iqbal

Multi threading and garbage collected problem using RegisterShellHook and SetWindowLong API.
The purpose of this to notify when a Removable Drive is Inserted AND Removed.
Most of the time it works properly but when I shake the form it causes an error. This is the code of my class:

Public Class Shell_Hook_Pointer
Public Shadows Delegate Function WndProc(ByVal hWnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer

Private Declare Function RegisterShellHook Lib "Shell32" Alias "#181" (ByVal hwnd As Integer, ByVal nAction As Integer) As Integer
Private Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Integer, ByVal nIndex As Integer, ByVal dwNewLong As Integer) As Integer
Private Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Integer, ByVal nIndex As Integer, ByVal dwNewLong As WndProc) As Integer

Private Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" (ByVal hwnd As Integer, ByVal nIndex As Integer) As Integer

Private Const GWL\_WNDPROC As Short = (-4)
Private Const RSH\_DEREGISTER As Short = 0

Public Enum SheelHookTypes
    RSH\_REGISTER = 1
    RSH\_REGISTER\_PROGMAN = 2
    RSH\_REGISTER\_TASKMAN = 3
End Enum
Private Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" (ByVal lpPrevWndFunc As Integer, ByVal hwnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer
Private OldProc As Integer
Private uRegMsg As Integer
Private hWnd As Integer, ParentWindowHadler As Form
Public Const wMsg\_REMOVABLE\_DEVICE = 537
Public Const wParam\_REMOVABLE\_DEVICE\_INSERTED = 32768
Public Const wParam\_REMOVABLE\_DEVICE\_REMOVED = 32772
''' <summary>
''' Envent argumetns
''' </summary>
''' <remarks></remarks>
Class ShellHookEventArgs
    Inherits System.EventArgs
    Public Enum REMOVABLE\_DEVICE
        NONE = 0
        INSERTED = 1
        REMOVED = 2
    End Enum
    Public hWnd, wMsg, wParam, lParam As Integer
    Public REMOVABLE\_DEVICE\_INFO As REMOVABLE\_DEVICE
    Public TempMsg As String
    Public SheelHookTypes = SheelHookTypes
End Class

Public Function WndProcedure(ByVal hWnd As Integer, ByVal wMsg As Integer, \_
                             ByVal wParam As Integer, ByVal lParam As Integer) As Integer

Luc Pattyn

Hi, I don't think multi-threading is involved; however shaking your Form will create new objects, possibly causing the GC to run and remove a delegate that is still in use as you have stored it in unmanaged memory (SetWindowLong) so the CG cannot work out it is still in use. This is not the right way to handle this. I do have similar functionality implemented in C#, without using any shell hooks: Windows will always broadcast Windows messages whenever a device gets added or removed, there is no need to subscribe to anything; all it takes is overriding WndProc and checking for such messages. This is the heart of the matter (the constant values are 0x0219, 0x8000, 0x8004):

protected override void WndProc(ref Message m) {
if (m.Msg==LPWIN_Constants.WM_DEVICECHANGE) {
int wParam=(int)m.WParam;
int lParam=(int)m.LParam;
if (xpanel!=null) {
if (wParam==LPWIN_Constants.DBT_DEVICEARRIVAL) xpanel.DeviceAdded();
else if (wParam==LPWIN_Constants.DBT_DEVICEREMOVECOMPLETE) xpanel.DeviceRemoved();
}
}
}

:)

Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles] Nil Volentibus Arduum

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slam Iqbal

"I do have similar functionality implemented in C#, without using any shell hooks"
Yeah you are right. I used shell hooks to monitor processes or forms.
Here is my simplyfied code:

Public Class HOOK_RMV_DEVICE
Private Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" (ByVal lpPrevWndFunc As Integer, _
ByVal hwnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer

Private Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Integer, ByVal nIndex As Integer, ByVal dwNewLong As Integer) As Integer
Private Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Integer, ByVal nIndex As Integer, ByVal dwNewLong As WndProc) As Integer

Private Shadows Delegate Function WndProc(ByVal hWnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer

Private Const GWL\_WNDPROC = -4
Private IsHooked As Boolean, lpPrevWndProc As Integer, WindowHandler As Integer
'
Private Const wMsg\_REMOVABLE\_DEVICE = 537
Private Const wParam\_REMOVABLE\_DEVICE\_INSERTED = 32768
Private Const wParam\_REMOVABLE\_DEVICE\_REMOVED = 32772

Public Function Hook() As Boolean
    If IsHooked Then
        Return False
    Else
        lpPrevWndProc = SetWindowLong(WindowHandler, GWL\_WNDPROC, AddressOf WindowProc)
        IsHooked = True
        Return True
    End If
End Function
Public Sub Unhook()
    Dim temp As Long
    temp = SetWindowLong(WindowHandler, GWL\_WNDPROC, lpPrevWndProc)
    IsHooked = False
End Sub

Function WindowProc(ByVal hWnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer
    Dim e As New ShellHookEventArgs
    e.hWnd = hWnd
    e.wMsg = wMsg
    e.wParam = wParam
    e.lParam = lParam
    e.REMOVABLE\_DEVICE\_INFO = ShellHookEventArgs.REMOVABLE\_DEVICE.NONE

    If wMsg = wMsg\_REMOVABLE\_DEVICE Then
        If wParam = wParam\_REMOVABLE\_DEVICE\_INSERTED Then
            e.REMOVABLE\_DEVICE\_INFO = ShellHookEventArgs.REMOVABLE\_DEVICE.INSERTED
            RaiseEvent Removable\_Device(Me, e)
        ElseIf wParam = wParam\_REMOVABLE\_DEVICE\_REMOVED Then
            e.REMOVABLE\_DEVICE\_INFO = ShellHookEventArgs.REMOVABLE\_DEVICE.REMOVED
            RaiseEvent Removable\_Device(Me, e)
        End If
    Else
        '

Luc Pattyn

The code I gave earlier is all there is to it. You should remove your P/Invoke stuff, the SetWindowLong calls, etc. However, you are missing the Overrides keyword that makes your windowproc replace the default one for your Form. Look at the documentation for Overrides, and search some examples. I don't have any VB code around right now. :)

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slam Iqbal

Thanks I got it :-D
I forget add this line:

MyBase.WndProc(m)

It gives all my solution.

Luc Pattyn

you're welcome. :)

Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles] Nil Volentibus Arduum

Please use < PRE > tags for code snippets, it preserves indentation, and improves readability.