Can NativeWindow be used for a window whose handle isn't exposed by a Form or Control in the current WinForms app?

Robert Gustafson 606

WHAT I HAVE:

Visual Basic 2019, WinForms, .NET Framework 4 and up

MY ISSUE:

I wonder if a NativeWindow instance can be assigned the handle for a window that isn't exposed by one of the current WinForms app's forms or controls so that it can catch such a window's messages--for instance, the handle to a combo box's drop-down list (which has to be retrieved using Win32 API), or the handle to a window in a foreign process (retrieved by, say, Process.MainWindowHandle). My intuition says yes, but I want to be sure.

Please reply ASAP, and given any answers in VB.NET and as simply as possible.

2 answers

Castorix31 81,636

so that it can catch such a window's messages

or the handle to a window in a foreign process

Not for external processes, where a Global hook (DLL) is needed to intercept windows messages

Robert Gustafson 606 Reputation points

2023-01-19T11:31:38.6+00:00

What if the window is in the current process, but is not directly exposed by WinForms--i.e., the handle to the drop-down list of a ComboBox, which can be retrieved using the Win32 API GetComboBoxInfo? Can I then use that handle with NativeWindow, or should I stick to the API SetWindowSubclass, etc.? Also, what about handles to custom controls and/or constituent controls thereto?
Castorix31 81,636 Reputation points

2023-01-19T13:39:20.8233333+00:00

The handle returned from NativeWindow.FromHandle must be in the hash table

(see source : NativeWindow.cs)

Robert Gustafson 606

What does this "hash table" talk mean?

Also, I'm not talking about constructing a NativeWindow instance using NativeWindow.FromHandle. I'm talking about taking the value for a ComboBox's drop-down list's handle from the COMBOBOXINFO structure returned by the API GetComboBoxInfo, and using it for setting the AssignHandle property. Something like this:

<StructLayout(LayoutKind.Sequential)> _
Private Structure COMBOBOXINFO
   Friend cbSize As Integer
   Friend rcItem As RECT : Friend rcButton As RECT
   Friend stateButton As Integer
   Friend hwndCombo As IntPtr 'whole control
   Friend hwndItem As IntPtr  'edit portion
   Friend hwndList As IntPtr  'drop-down list
End Structure

<DllImport("User32.dll", SetLastError:=True, _
    CharSet:=CharSet.Auto)> _
Private Shared Function GetComboBoxInfo( _
      ByVal hwndCombo As IntPtr, _
   ByRef pcbi As COMBOBOXINFO) As Boolean
'   retrieve info on combo-box component handles
End Function

Private Class DropDownWindow : Inherits NativeWindow
   '   class for handling messages sent to drop-down window

   Protected Overrides Sub WndProc(ByRef msg As Message)
   '   handle messages
   End Sub
End Class

Public Class SetupDropDownWindo

   Private DropDownWindow As New DropDownWindow

   Private Sub GetDropDownWindo(ByVal cmb As ComboBox)
   '   get handle for drop-down list
   Dim cbi As COMBOBOXINFO = New COMBOBOXINFO()
   cbi.cbSize = Marshal.SizeOf(cbi) : GetComboBoxInfo(cmb.Handle, cbi) 
   DropDownWindow.AssignHandle(cbi.hWndList)
   End Sub
End Class

Will that work?

Robert Gustafson 606

I'm not talking about getting an instance of NativeWindow using NativeWindow.FromHandle. I'm talking about using the COMBOBOXINFO structure and the Win32 API GetComboBoxInfo to get the handle to a ComboBox's drop-down list, and then using that value for the AssignHandle method. Something like this:

<StructLayout(LayoutKind.Sequential)> _
Private Structure COMBOBOXINFO
   Friend cbSize As Integer
   Friend rcItem As RECT : Friend rcButton As RECT
   Friend stateButton As Integer
   Friend hwndCombo As IntPtr 'whole control
   Friend hwndItem As IntPtr  'edit window
   Friend hwndList As IntPtr  'drop-down list
End Structure

<DllImport("User32.dll", SetLastError:=True, _
   CharSet:=CharSet.Auto)> _
Private Shared Function GetComboBoxInfo( _
   ByVal hwndCombo As IntPtr, _
   ByRef pcbi As COMBOBOXINFO) As Boolean
'   retrieve info on combo-box component handles
End Function

Private Class DropDownWindow : Inherits NativeWindow
   '   class to handle drop-down list

   Protected Sub WndProc(ByRef msg As Message)
   '   handle messages here
   End Sub
End Class

Public Class SetUpDropDownWindow

   Private DropDownWindow As New DropDownWindow()

   Private Sub GetDropDownWindow(cmb As ComboBox)
   '   get handle for combo box's drop-down list
   Dim cbi As COMBOBOXINFO = New COMBOBOXINFO()
   cbi.cbSize = Marshal.SizeOf(cbi) : GetComboBoxInfo(cmb.Handle, cbi)
   DropDownWindow.AssignHandle(cbi.hWndList)
   End Sub
End Class

Will this work? Also, what's this talk about the "hash table"?

Castorix31 81,636 Reputation points

2023-01-20T07:57:33.9166667+00:00

and using it for setting the AssignHandle property

You can see in the linked source code that AssignHandle adds the window to the hash table

Robert Gustafson 606 Reputation points

2023-01-25T03:56:55.6666667+00:00

To: Castorix31

You just answered my questions. Thank you!
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RLWA32 40,286 Reputation points

2023-01-25T10:31:09.23+00:00

Since the answer and information provided by Castorix31 resolved your question I suggest you accept it as an Answer.

Jiachen Li-MSFT 26,506 Reputation points Microsoft Vendor

2023-02-02T08:43:42.82+00:00

Hi @Robert Gustafson ,

Glad to see your problem solved, you can accept the helpful answer to change its status to Answered. It will also help others to solve the similar issue.
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