# Using EnumChildWindows API to create a hierarchical list of application windows



## John_w (Jul 10, 2020)

This code uses the EnumChildWindows API function to create a list of application windows in a hierarchical layout of parent and child windows in Excel.








						EnumChildWindows function (winuser.h) - Win32 apps
					

Enumerates the child windows that belong to the specified parent window by passing the handle to each child window, in turn, to an application-defined callback function.



					docs.microsoft.com
				



*EnumChildWindows function*

Enumerates the child windows that belong to the specified parent window by passing the handle to each child window, in turn, to an application-defined callback function. EnumChildWindows continues until the last child window is enumerated or the callback function returns FALSE.

In the code below the main procedure loops through each application window and calls EnumChildWindows, specifying a callback function, EnumChildWindowsCallback, which receives the child window handles.  The problem is that successive calls to EnumChildWindowsCallback, made by Windows, don't maintain context (like a call stack) and therefore we don't know which parent the current child belongs to.  Without this information we can't produce a hierarchical layout of parent and child windows.  

My solution is to implement a stack (using a VBA Collection and classes) which stores the parent and child windows hierarchy between calls to EnumChildWindowsCallback.  Each stack item contains the baseCell and rowOffset of a window so that the code knows where to write that window's details on the sheet.  A pointer to the stack is passed as the lParam argument to the EnumChildWindows API and therefore all variables are local.

I know a hierarchical layout could also be produced by a recursive function which calls GetWindow(hWnd, GW_CHILD) and GetWindow(hWnd, GW_NEXT), however I wanted to do it with EnumChildWindows as a learning exercise and because I've never seen it done before.

*Standard module*:

```
'Standard module, e.g. Module1

Option Explicit

#If VBA7 Then
   
    Public Declare PtrSafe Function GetDesktopWindow Lib "user32" () As LongPtr
    Public Declare PtrSafe Function GetWindow Lib "user32" (ByVal hWnd As LongPtr, ByVal wCmd As Long) As LongPtr
    Public Declare PtrSafe Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hWnd As LongPtr, ByVal lpClassName As String, ByVal nMaxCount As Long) As Long
    Public Declare PtrSafe Function GetWindowTextLength Lib "user32" Alias "GetWindowTextLengthA" (ByVal hWnd As LongPtr) As Long
    Public Declare PtrSafe Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hWnd As LongPtr, ByVal lpString As String, ByVal cch As Long) As Long
    Public Declare PtrSafe Function EnumChildWindows Lib "user32" (ByVal hWndParent As LongPtr, ByVal lpEnumFunc As LongPtr, ByVal lParam As LongPtr) As Long
    Public Declare PtrSafe Function GetParent Lib "user32" (ByVal hWnd As LongPtr) As LongPtr
    Public Declare PtrSafe Function IsWindowVisible Lib "user32" (ByVal hWnd As LongPtr) As Long

#Else

    Public Declare Function GetDesktopWindow Lib "user32" () As Long
    Public Declare Function GetWindow Lib "user32" (ByVal hWnd As Long, ByVal wCmd As Long) As Long
    Public Declare Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hWnd As Long, ByVal lpClassName As String, ByVal nMaxCount As Long) As Long
    Public Declare Function GetWindowTextLength Lib "user32" Alias "GetWindowTextLengthA" (ByVal hWnd As Long) As Long
    Public Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hWnd As Long, ByVal lpString As String, ByVal cch As Long) As Long
    Public Declare Function EnumChildWindows Lib "user32" (ByVal hWndParent As Long, ByVal lpEnumFunc As Long, ByVal lParam As Long) As Long
    Public Declare Function GetParent Lib "user32" (ByVal hWnd As Long) As Long
    Public Declare Function IsWindowVisible Lib "user32" (ByVal hWnd As Long) As Long
   
#End If

Public Const GW_NEXT = 2
Public Const GW_CHILD = 5


Public Sub List_Desktop_Windows()
   
    #If VBA7 Then
        Dim hWndDesktop As LongPtr, hWnd As LongPtr
    #Else
        Dim hWndDesktop As Long, hWnd As Long
    #End If
   
    Dim startCell As Range
    Dim numRows As Long
    Dim lngLength As Long
    Dim strBuffer As String
    Dim lngRet As Long
    Dim thisWindowTitle As String, thisWindowClass
    Dim stack As cStack
    Dim stackItem As cStackItem
   
    hWndDesktop = GetDesktopWindow
   
    With Worksheets(1)
        .Activate
        .Cells.Clear
        .Range("A1:D1").Value = Split("Parent,Window,Title,Class", ",")
        Set startCell = .Range("A2")
        numRows = 0
    End With
   
    'Create stack to save parent and child window hierarchy between callbacks to EnumChildWindows procedure
   
    Set stack = New cStack
   
    hWnd = GetWindow(hWndDesktop, GW_CHILD)
   
    Do While hWnd <> 0
       
        lngLength = GetWindowTextLength(hWnd) + 1
        strBuffer = Space(lngLength)
        lngRet = GetWindowText(hWnd, strBuffer, lngLength)
        thisWindowTitle = Left(strBuffer, lngLength - 1)
       
        strBuffer = String$(256, Chr$(0))
        lngRet = GetClassName(hWnd, strBuffer, Len(strBuffer))
        thisWindowClass = Left$(strBuffer, lngRet)
       
        If thisWindowTitle <> "" And IsWindowVisible(hWnd) Then
           
            'Found a desktop application window so put it on the empty stack
           
            Set stackItem = New cStackItem
            With stackItem
                Set .baseCell = startCell.Offset(numRows)
                .rowOffset = 0
                .hWndParent = hWnd
            End With
            stack.Push stackItem
           
            With stackItem.baseCell
                .Offset(, 0).Value = "0x" & Hex(GetParent(hWnd))
                .Offset(, 1).Value = "0x" & Hex(hWnd)
                .Offset(, 2).Value = thisWindowTitle
                .Offset(, 3).Value = thisWindowClass
            End With
           
            'Enumerate all child windows and output to Excel cells in a hierarchical layout
           
            EnumChildWindows hWnd, AddressOf EnumChildWindowsCallback, VarPtr(stack)
           
            'Pop all stack items (emptying the stack) to calculate the total number of rows written for this application window and its child windows
           
            While stack.Count > 0
                Set stackItem = stack.Pop
                numRows = numRows + stackItem.rowOffset
            Wend
            numRows = numRows + 2 '+2 to give 1 row gap between application windows
           
            With startCell.Offset(numRows - 1).Resize(, 4).Interior
                .Pattern = xlSolid
                .PatternColorIndex = xlAutomatic
                .Color = 15773696
                .TintAndShade = 0
                .PatternTintAndShade = 0
            End With
                       
        End If
       
        hWnd = GetWindow(hWnd, GW_NEXT)
       
    Loop

End Sub


#If VBA7 Then
Public Function EnumChildWindowsCallback(ByVal hWnd As LongPtr, stack As cStack) As Long
#Else
Public Function EnumChildWindowsCallback(ByVal hWnd As Long, stack As cStack) As Long
#End If

    Dim lngLength As Long
    Dim strBuffer As String
    Dim lngRet As Long
    Dim thisWindowTitle As String, thisWindowClass As String
    Dim numChildren As Long
    Dim parentItem As cStackItem
    Dim childItem As cStackItem
       
    lngLength = GetWindowTextLength(hWnd) + 1
    strBuffer = Space(lngLength)
    lngRet = GetWindowText(hWnd, strBuffer, lngLength)
    thisWindowTitle = Left(strBuffer, lngLength - 1)
       
    strBuffer = String$(256, Chr$(0))
    lngRet = GetClassName(hWnd, strBuffer, Len(strBuffer))
    thisWindowClass = Left$(strBuffer, lngRet)
   
    'Pop items from stack until this child's parent is found, counting the total number of intervening children
   
    numChildren = 1
    Do
        Set parentItem = stack.Pop
        numChildren = numChildren + parentItem.rowOffset
    Loop Until GetParent(hWnd) = parentItem.hWndParent
   
    'Update parent row offset by number of children and put parent back on stack
   
    parentItem.rowOffset = numChildren
    stack.Push parentItem
   
    'Put this child window on stack, with its baseCell set to 'n' rows below the parent and 1 column to the right
   
    Set childItem = New cStackItem
    With childItem
        .rowOffset = 0
        Set .baseCell = parentItem.baseCell.Offset(parentItem.rowOffset, 1)
        .hWndParent = hWnd
    End With
    stack.Push childItem

    'Write child window details in columns on current row
   
    With childItem.baseCell
        .Offset(, 0).Value = "0x" & Hex(GetParent(hWnd))
        .Offset(, 1).Value = "0x" & Hex(hWnd)
        .Offset(, 2).Value = thisWindowTitle
        .Offset(, 3).Value = thisWindowClass
    End With
       
    'Continue enumerating child windows
   
    EnumChildWindowsCallback = 1
   
End Function
```

*Class module* named *cStack*:

```
'Class module cStack
'Uses a VBA Collection as the stack data structure

Option Explicit

Dim pStack As Collection

Private Sub Class_Initialize()
    Set pStack = New Collection
End Sub

Private Sub Class_Terminate()
    Set pStack = Nothing
End Sub

Public Function Push(newItem As cStackItem) As cStackItem
    With pStack
        .Add newItem
        Set Push = .Item(.Count)
    End With
End Function

Public Function Pop() As cStackItem
    With pStack
        If .Count > 0 Then
            Set Pop = .Item(.Count)
            .Remove .Count
        End If
    End With
End Function

Public Function Count() As Long
    Count = pStack.Count
End Function

Public Sub Dump()
    Dim i As Long
    Dim stackItem As cStackItem
    With pStack
        If .Count = 0 Then Debug.Print "Empty"
        For i = .Count To 1 Step -1
            Set stackItem = .Item(i)
            With stackItem
                Debug.Print i; .baseCell.Offset(.rowOffset).Address(False, False), Hex(.hWndParent), "Offset " & .rowOffset
            End With
        Next
    End With
End Sub
```
*Class module* named *cStackItem:*

```
'Class module cStackItem
'User-defined type for a stack item

Option Explicit

Public baseCell As Range
Public rowOffset As Long
#If VBA7 Then
    Public hWndParent As LongPtr
#Else
    Public hWndParent As Long
#End If
```


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## Jaafar Tribak (Jul 11, 2020)

I love this ! ... Very elegant code.
Thanks for sharing.


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