How To Find a Window Handle from an Instance Handle

The Shell function in Microsoft Visual Basic is used to execute an application. Often, it is useful to get a Window handle (hWnd) to the application so you can manipulate it using the Windows APIs. However, the Shell function returns an Instance handle (hInstance), which is different from a Window handle. This article shows how to create and use a GetWinHandle() function to return a Window handle based on an Instance handle.

By using the FindWindow() and GetWindow() APIs (or using EnumWindows in Visual Basic 5.0 or later), you can loop through the Window handle list. For each window handle, you can check to see if it has a parent window with GetParent(). If the Window handle does not have a parent handle, you have reached the main window for an application. You can call GetWindowThreadProcessID(), which yields the instance handle of the process for the given window handle, to check the instance handle of the application against the instance handle received from the Shell() function.

Step-by-Step Example

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.
2. Add a CommandButton (Command1) to the form.
3. From the Project menu, add a new module to the project, and enter the following declarations and functions:

   Public Const GW_HWNDNEXT = 2
   
   Public Declare Function GetParent Lib "user32" (ByVal hwnd As Long) As Long
   Public Declare Function GetWindow Lib "user32" (ByVal hwnd As Long, _
     ByVal wCmd As Long) As Long
   Public Declare Function FindWindow Lib "user32" Alias "FindWindowA" _
     (ByVal lpClassName As String, ByVal lpWindowName As String) 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 GetWindowThreadProcessId Lib "user32" _
     (ByVal hwnd As Long, lpdwprocessid As Long) As Long
   
   Function ProcIDFromWnd(ByVal hwnd As Long) As Long
      Dim idProc As Long
      
      ' Get PID for this HWnd
      GetWindowThreadProcessId hwnd, idProc
      
      ' Return PID
      ProcIDFromWnd = idProc
   End Function
         
   Function GetWinHandle(hInstance As Long) As Long
      Dim tempHwnd As Long
      
      ' Grab the first window handle that Windows finds:
      tempHwnd = FindWindow(vbNullString, vbNullString)
      
      ' Loop until you find a match or there are no more window handles:
      Do Until tempHwnd = 0
         ' Check if no parent for this window
         If GetParent(tempHwnd) = 0 Then
            ' Check for PID match
            If hInstance = ProcIDFromWnd(tempHwnd) Then
               ' Return found handle
               GetWinHandle = tempHwnd
               ' Exit search loop
               Exit Do
            End If
         End If
      
         ' Get the next window handle
         tempHwnd = GetWindow(tempHwnd, GW_HWNDNEXT)
      Loop
   End Function
   					

4. Add the following code to the form:

   Sub Command1_Click()
      Dim hInst As Long             ' Instance handle from Shell function.
      Dim hWndApp As Long           ' Window handle from GetWinHandle.
      Dim buffer As String          ' Holds caption of Window.
      Dim numChars As Integer       ' Count of bytes returned.
      
      ' Shell to an application
      hInst = Shell("calc.exe")
      
      ' Begin search for handle
      hWndApp = GetWinHandle(hInst)
      
      If hWndApp <> 0 Then
         ' Init buffer
         buffer = Space$(128)
         
         ' Get caption of window
         numChars = GetWindowText(hWndApp, buffer, Len(buffer))
      
         ' Display window's caption
         MsgBox "You shelled to the Application: " & Left$(buffer, numChars)
      End If
   End Sub
   					

5. Press the F5 key to run the application. Click Command1 to see that the Calculator has been shelled and to see a message box displaying "You shelled to the Application: Calculator."

Topic in MSDN : "More about handles and process IDs"

For additional information on using the EnumWindows functions to find a specified window, click the article number below to view the article in the Microsoft Knowledge Base: For additional information on performing this task in 16-bit versions of VB, click the article number below to view the article in the Microsoft Knowledge Base: