What kind of handle does EnumWindows API returns?

[1369Times]

Hi All,
I am confused. I have tried to use the EnumWindows API to get the handles to the windows in my computer. When the code runs, I cannot find the handle of an opened window in the list of handles that EnumWindows returned. So now, I would like to know the kind of handles that EnumWindows API returns. Why is the window handle of a window that was opened via Process.Start("NotePad.exe") not listed?

Please, run the code if you can. Check the list and see if the NotePad handle is in it. Then, please, tell me what is wrong with the approach. I am looking to get the proper handle to a window so that I can position the window on a preferred parent. I do not want to use the MainWindowHandle property because some processes such as PDF do not return non-zero main window handle in my computer (Windows 7 Pro SP1, X64).

Imports System
Imports System.Runtime.InteropServices


Public Delegate Function CallBack(ByVal hwnd As Integer, ByVal lParam As Integer) As Boolean

Public Module EnumReportApp
    Dim cnt As Integer = 0
    
    Declare Function EnumWindows Lib "user32" (ByVal x As CallBack, ByVal y As Integer) As Integer

    Public Sub Main()

        EnumWindows(AddressOf EnumReportApp.Report, 0)

        Dim p As Process = Process.Start("NotePad.exe")
        p.WaitForInputIdle()
        p.Refresh()
        Console.WriteLine()
        Console.WriteLine("The current number of windows is: " & cnt)
        Console.WriteLine()
        Console.WriteLine("NotePad's current handle is: " & p.MainWindowHandle.ToString)
        Console.ReadLine()
    End Sub 'Main

    
    Public Function Report(ByVal hwnd As Integer, ByVal lParam As Integer) As Boolean
        Console.Write("Window handle is: ")
        Console.WriteLine(hwnd)
        cnt = cnt + 1
        Return True
        
    End Function 'Report
End Module 'EnumReportApp

[Joacim Andersson]

Because you launch Notepad after the call to EnumWindows. It can only list the applications that are running when you do the call, it will not list applications that are started in the future.

[1369Times]

Much respect Sir! I moved the Process.Start("NotePad.exe") to a line just before the call to EnumWindows and it works. You have just made my day.
My next issue is how to match each handle with its process. Any recommendations on this?

[Joacim Andersson]

Well, first of all I wouldn't use EnumWindows at all since you can just as well just loop through the Process.GetProcesses collection, checking if it has a MainWindowHandle and check its ProcessName.

For Each p As Process In Process.GetProcesses
  'do whatever
Next

[1369Times]

The resean I am not using Process.GetProcesses collection is that the process I am after is Adobe32.exe. This process is returning a main window handle of Zero. I cannot use this handle to set a new parent for the window. So, I am hoping that EnumWindows might just list the window handle and maybe somehow I would identify which handle belongs to Adobe.

[Joacim Andersson]

OK, well first of all you need to get the ProcessID from the hwnd, you can use GetWindowThreadProcessId for that. Next you can open that process (using OpenProcess) with the PROCESS_QUERY_INFORMATION and PROCESS_VM_READ access rights. Finally you can get the process file name by calling GetModuleFileNameEx, however I don't think it will work correctly on 64-bit applications (at least not if your application is compiled to 32-bit), but in your case I guess that doesn't matter.

[1369Times]

Thanks. I will follow your recommendations and give feedback accordingly.

[1369Times]

Hi All,
I have followed part of the steps recommended by Mr. Joacim. I did not use OpenProcess and GetModuleFileNameEx APIs. Instead I used GetWindowThreadProcessId and GetProcessById to obtain the process associated with a given window handle. The code is as shown below. I do still have issues with the process I am hunting - AcroRd32.exe. This process returns a main window handle of zero, it is not listed by the EnumWindows API and its main module filename is not listed. Please, run the code and see if you understand why AcroRd32.exe is proving elusive. I also observed that the windows listed by EnumWindows are all in "C:\Windows\System32". Adobe Reader is in "E:\Adobe files\Reader\AcroRd32.exe" in my computer. Has this got anything to do with the problem?

**********************************

Imports System
Imports System.Runtime.InteropServices

Public Delegate Function CallBack(ByVal hwnd As Integer, ByVal lParam As Integer) As Boolean

Public Module EnumReportApp
    
    <DllImport("user32.dll", SetLastError:=True)> _
    Private Function GetForegroundWindow Lib "user32" () As Integer

    End Function
    
    <DllImport("user32.dll", SetLastError:=True)> _
    Private Function GetWindowThreadProcessId(ByVal hwnd As IntPtr, ByRef lpdwProcessId As Integer) As Integer

    End Function
  
      Public Sub Main()
        Dim p As Process = Process.Start("AcroRd32.exe")
        
        p.WaitForInputIdle()
        p.Refresh()

        'The last process started is presumably the topmost window, get it.
        Dim topWindow As Integer = GetForegroundWindow()

        'Walk through the windows, retrieving each window's handle
        EnumWindows(AddressOf EnumReportApp.Report, 0)
        
        'Print some relevant information
        Console.WriteLine("The current number of windows is: " & cnt)
        Console.WriteLine()
        Console.WriteLine("The process window handle is: " & p.MainWindowHandle.ToString)

        Console.WriteLine()
        Console.WriteLine("The Top Window's current handle is: " & topWindow)

        Console.WriteLine()
        Console.WriteLine("The process's main module filename is: " & p.MainModule.FileName)

        Console.ReadLine()
    End Sub 'Main
  
    Public Function Report(ByVal hwnd As Integer, ByVal lParam As Integer) As Boolean
        'We need to get the process ID.
        Dim processID As Integer
        GetWindowThreadProcessId(hwnd, processID)

        'Get the process that has this process ID
        Dim proc As Process = Process.GetProcessById(processID)

        'Get the main module filename of this process
        Dim moduleFileName As String = proc.MainModule.FileName
                  
            Console.Write("Window handle is ")
            Console.WriteLine(hwnd & "  Main module = " & moduleFileName)
            
        cnt = cnt + 1
        Return True
        
    End Function 'Report

End Module 'EnumReportApp

[1369Times]

Hi folks,
Once more, I have modified my code and I am able to get handles for AcroRd32.exe. The challenge now is that I have several handles corresponding to AcroRd32.exe and I am not able to identify which of them is the main window handle. I have tried to use the IsWindowVisible API, but even so, I still get several handles. The code is shown below. Please, help explain what is going on.

Imports System
Imports System.Runtime.InteropServices
Imports System.Text


Public Delegate Function CallBack(ByVal hwnd As Integer, ByVal lParam As Integer) As Boolean

Public Delegate Function CallBackListChild(ByVal hwnd As Intptr, ByVal lParam As Intptr) As Boolean

Public Module EnumChildWindowsApp

    Declare Function EnumWindows Lib "user32" (ByVal x As CallBack, ByVal y As Integer) As Integer

    <DllImport("user32.dll", SetLastError:=True)> _
    Private Function GetDesktopWindow() As IntPtr

    End Function

    <DllImport("user32.dll", SetLastError:=True)> _
    Private Function EnumChildWindows(<[In]()> ByVal hWndParent As IntPtr, <[In]()> ByVal z As CallBackListChild,
                                      <[In]()> ByVal lParam As IntPtr) As Boolean

    End Function

    <DllImport("user32.dll", SetLastError:=True)> _
    Private Function GetWindowThreadProcessId(ByVal hwnd As IntPtr, ByRef lpdwProcessId As Integer) As Integer

    End Function

    
    <DllImport("User32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _
    Public Function IsWindowVisible(ByVal hWnd As IntPtr) As Boolean

    End Function
    
    Dim cnt As Integer = 0
    Dim topWindow As Integer

    Public Sub Main()
        
        Dim p As Process = Process.Start("AcroRd32.exe")
        
        p.WaitForInputIdle()
        p.Refresh()

        'You can now iterate through the windows
        EnumWindows(AddressOf EnumChildWindowsApp.Report, 0)

        EnumChildWindows(GetDesktopWindow, AddressOf EnumChildWindowsApp.ListChildWindows, IntPtr.Zero)

        Console.WriteLine()
        Console.WriteLine("The current number of child windows is: " & cnt)
        Console.WriteLine()
        Console.WriteLine("The current child window handle is: " & p.MainWindowHandle.ToString)

        Console.WriteLine()
        Console.WriteLine("The Desktop handle is: " & GetDesktopWindow.ToString)

        Console.ReadLine()

    End Sub 'Main

    Public Function Report(ByVal hwnd As Integer, ByVal lParam As Integer) As Boolean

        Dim processID As Integer
        GetWindowThreadProcessId(hwnd, processID)

        'Get the process that has this process ID
        Dim proc As Process = Process.GetProcessById(processID)

        'Get the main module filename of this process
        Dim moduleFileName As String = proc.MainModule.FileName

        Return True

    End Function 'Report

    Public Function ListChildWindows(ByVal hwnd As IntPtr, ByVal lParam As IntPtr) As Boolean

        Dim processID As Integer
        GetWindowThreadProcessId(hwnd, processID)

        'Get the process that has this process ID
        Dim proc As Process = Process.GetProcessById(processID)

        'Get the main module filename of this process
        Dim moduleFileName As String = proc.MainModule.FileName

        'We need to walk through this process and list out each of the module in the process.
        Dim i As Integer
        Dim mods(proc.Modules.Count - 1) As String
        For i = LBound(mods) To UBound(mods) Step 1
            mods(i) = proc.Modules.Item(i).FileName
            If mods(i).EndsWith("AcroRd32.exe") AndAlso IsWindowVisible(hwnd) Then
                Console.WriteLine()
                Console.Write("Window handle is ")
                Console.WriteLine(hwnd.ToString & "  Child's Main module Filename = " & moduleFileName)
                cnt = cnt + 1
            End If
        Next  'i

        Console.WriteLine()
        
        Return True
    End Function 'ListChildWindows

End Module 'EnumChildWindowsApp

[1369Times]

Hi there,
I am still working on the code. I have somehow managed to get the handle to pdf main window. However, the procedure is circuitous and I am sure there must be a more effective way to go about it. I have tried to filter AcroRd32.exe handles from all the current window handles. I walked through the resulting list to see which handle displays a pdf window. It does seem it is the first handle in the list (in my computer). The latest version of the code is inserted below. Do note that the code may return error the first time you run it. You may have to run it a couple of times to get to work. This is something I haven't been able to understand yet. The filter does not seem to catch AcroRd32.exe until after a few minutes or perhaps AcroRd32.exe does not have handles until after a few minutes? Also, when I comment out the EnumWindows call, the programme will not return AcroRd32.exe handles. I will really be grateful if anyone can help resolve the issues.

Also, do note that when the code runs successfully and loads pdf window, you will need to manually exit the pdf process before re-running the program because the code to do that automatically has not been added. You will notice that the pdf window is not centered on the form. The code to position it properly on the form has also not been added. Thanks for your contributions.

Imports System
Imports System.Runtime.InteropServices
Imports System.Text


Public Delegate Function CallBack(ByVal hwnd As Integer, ByVal lParam As Integer) As Boolean

Public Delegate Function CallBackListChild(ByVal hwnd As Intptr, ByVal lParam As Intptr) As Boolean

Public Module EnumChildWindowsApp

    Declare Function EnumWindows Lib "user32" (ByVal x As CallBack, ByVal y As Integer) As Integer

    <DllImport("user32.dll", SetLastError:=True)> _
    Private Function GetDesktopWindow() As IntPtr

    End Function

    <DllImport("user32.dll", SetLastError:=True)> _
    Private Function EnumChildWindows(<[In]()> ByVal hWndParent As IntPtr, <[In]()> ByVal z As CallBackListChild,
                                      <[In]()> ByVal lParam As IntPtr) As Boolean

    End Function

    <DllImport("user32.dll", SetLastError:=True)> _
    Private Function GetWindowThreadProcessId(ByVal hwnd As IntPtr, ByRef lpdwProcessId As Integer) As Integer

    End Function

   
    <DllImport("User32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _
    Public Function IsWindowVisible(ByVal hWnd As IntPtr) As Boolean

    End Function
    <DllImport("User32.dll", CharSet:=CharSet.Auto, SetLastError:=True)> _
    Public Function SetParent(ByVal hWndChild As IntPtr, ByVal hWndNewParent As IntPtr) As IntPtr

    End Function

    Dim cnt As Integer = 0
    Dim topWindow As Integer
    Dim deskTopHandle As IntPtr
    
    Public Sub Main()
        
        Dim p As Process = Process.Start("AcroRd32.exe")
        
        p.WaitForInputIdle()
        p.Refresh()

        'You can now iterate through the windows
        EnumWindows(AddressOf EnumChildWindowsApp.Report, 0)

        deskTopHandle = GetDesktopWindow()
        EnumChildWindows(deskTopHandle, AddressOf EnumChildWindowsApp.ListChildWindows, IntPtr.Zero)

        Console.WriteLine()
        Console.WriteLine("The current number of child windows is: " & cnt)
        Console.WriteLine()
                
        Console.WriteLine()
        Console.WriteLine()
        Console.WriteLine("The Desktop handle is: " & GetDesktopWindow.ToString)

        Dim frmForm As New Form1
        frmForm.Show()
        SetParent(returnedHandles.Item(0), frmForm.Handle)

        Console.ReadLine()

    End Sub 'Main

    Public Function Report(ByVal hwnd As Integer, ByVal lParam As Integer) As Boolean

        Dim processID As Integer
        GetWindowThreadProcessId(hwnd, processID)

        'Get the process that has this process ID
        Dim proc As Process = Process.GetProcessById(processID)

        'Get the main module filename of this process
        Dim moduleFileName As String = proc.MainModule.FileName

        Return True

    End Function 'Report

    'store the returned window handles in the following list object
    Dim returnedHandles As New List(Of IntPtr)

    Public Function ListChildWindows(ByVal hwnd As IntPtr, ByVal lParam As IntPtr) As Boolean

        Dim processID As Integer
        GetWindowThreadProcessId(hwnd, processID)

        'Get the process that has this process ID
        Dim proc As Process = Process.GetProcessById(processID)

        'Get the main module filename of this process
        Dim moduleFileName As String = proc.MainModule.FileName

        If LCase(moduleFileName).EndsWith(LCase("AcroRd32.exe")) AndAlso IsWindowVisible(hwnd) Then
            Console.WriteLine("")
            Console.Write("Window handle is ")
            Console.WriteLine(hwnd.ToString & "  Child's Main module = " & moduleFileName)
            returnedHandles.Add(hwnd)
            cnt = cnt + 1
        End If

       Return True
    End Function 'ListChildWindows

End Module 'EnumChildWindowsApp

[Joacim Andersson]

Why don't you simply use FindWindow? The class name of the Adobe Reader main window is AcrobatSDIWindow.

[1369Times]

Thanks. The AcrobatSDIWindow works. However, I am still stuck with having to run the code a few times before pdf returns a window handle. I have tried increasing the WaitForInputIdle value but this doesn't seem to have any effect. I don't understand what is happening. I am just steering at the code, wondering ...


By the way, I get this message every time i try to rate your response "You must spread some Reputation around before giving it to Joacim Andersson again". Does this mean you have too many points already? I suppose I can only spread reputation to those who respond to my posts.

[Edgemeal]

Thanks. The AcrobatSDIWindow works. However, I am still stuck with having to run the code a few times before pdf returns a window handle.

If you mean getting the handle right after launching try using a loop with a short delay, always seems to works for me...

Dim p As Process = Process.Start("AcroRd32.exe")
p.WaitForInputIdle()
Dim hWnd As IntPtr = FindWindow("AcrobatSDIWindow", Nothing)
Do While hWnd.Equals(IntPtr.Zero)
    Threading.Thread.Sleep(33) ' short delay
    hWnd = FindWindow("AcrobatSDIWindow", Nothing)
Loop
' SetParent(hWnd, Me.Handle) ' < do something with the handle >

[Joacim Andersson]

By the way, I get this message every time i try to rate your response "You must spread some Reputation around before giving it to Joacim Andersson again". Does this mean you have too many points already? I suppose I can only spread reputation to those who respond to my posts.

No, it doesn't mean that I have to many rep points. However you're not allowed to rate the same person over and over again because that could lead up to that someone creates a second account just to rate their own posts over and over. Before you can give rep points to the same person a second time you must give it to a number of others first (I think it's 10, but I'm unsure about the number). However you are allowed to rate any answer in any thread, you don't have to have created the thread yourself, since you might find an answer in someone elses thread just as valuable as an answer in your own thread.

[1369Times]

Wow! Apology for the very long delay in responding. I have tried looping method. It works when I specify the process class name. However, this approach is not very suitable for me as I cannot tell what application a user might open. I cannot use FindWindow() because I need a class name for this and I need to know the window's handle to dynamically get the class name. What I am trying out at the moment is to remove the FindWindow() and the Thread.sleep() from the Do . . . Loop and allow the cycle to repeat until there is a match between the file name (without extension) derived from the process and the file name derived from the window' s caption. This way, I don't need to know the class name of the process. It does seem to work well for Microsoft Windows applications, but it is uncomfortably slow for PDF documents, especially when the file is large.

Any suggestions of a more elegant way to optimize this code will be appreciated.

Dim relativeName2 As String
Dim nameLength As Integer
Private Const MAX_LEN = 1024
Private Const GW_CHILD = 5
Private Const GW_HWNDNEXT = 2
Private Const WS_VISIBLE = &H10000000

Private Function EnumerateOpenedWindows() As Boolean
        Dim desktopHandle, windowHandle As IntPtr
        Dim caption As String = vbNullString

        Try
            Do
                desktopHandle = GetDesktopWindow()

                'Get the first (top most) child window on the Desktop
                windowHandle = GetWindow(desktopHandle, GW_CHILD)
                newHandle = windowHandle
                Do
                    If IsWindowVisible(windowHandle) Then
                        'Get the file name (without extension) from the window's caption and compare it with
                        'the file name (without extension) from the application that has justed been started.
                        caption = GetWindowCaption(windowHandle)
                        If LSet(caption, nameLength) = relativeName2 Then
                            SetParent((windowHandle), Me.Handle)
                            

                            'There is no need to continue.
                            Return True
                            Exit Function

                        End If
                    End If
                    
                    windowHandle = GetWindow(windowHandle, GW_HWNDNEXT)
                    newHandle = windowHandle
                Loop Until windowHandle = 0    'reached the end of the child windows collection

            Loop Until LSet(caption, nameLength) = relativeName2

        Catch ex As Exception
            MsgBox(ex.Message, vbApplicationModal + vbAbort, "Error!")

            'The function has failed
            Return False
            Exit Function
        End Try

        'If the execution got to this point, then it was successful.
        Return True

    End Function



'*************************************************************************

Private Function GetWindowCaption(ByVal winHandle As IntPtr) As String
        Dim caption As String = vbNullString
        Dim counter As Integer = 0

        Try
            
            caption = StrDup(MAX_LEN - 1, "*")

            counter = GetWindowText(winHandle, caption, MAX_LEN)
            If counter <= 0 Then
                GetWindowCaption = vbNullString
                Exit Function
            End If
            GetWindowCaption = Trim(caption)
        Catch ex As Exception
            MsgBox(ex.Message, vbApplicationModal + vbAbort, "Error!")
            Return vbNullString
            Exit Function
        End Try

    End Function