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How To Find a Window Handle from an Instance Handle

This article was previously published under Q242308
SUMMARY
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.
MORE INFORMATION
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 = 2Public Declare Function GetParent Lib "user32" (ByVal hwnd As Long) As LongPublic Declare Function GetWindow Lib "user32" (ByVal hwnd As Long, _  ByVal wCmd As Long) As LongPublic Declare Function FindWindow Lib "user32" Alias "FindWindowA" _  (ByVal lpClassName As String, ByVal lpWindowName As String) As LongPublic Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" _  (ByVal hwnd As Long, ByVal lpString As String, ByVal cch As Long) As LongPublic Declare Function GetWindowThreadProcessId Lib "user32" _  (ByVal hwnd As Long, lpdwprocessid As Long) As LongFunction ProcIDFromWnd(ByVal hwnd As Long) As Long   Dim idProc As Long      ' Get PID for this HWnd   GetWindowThreadProcessId hwnd, idProc      ' Return PID   ProcIDFromWnd = idProcEnd 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)   LoopEnd 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 IfEnd 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."
REFERENCES
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:
183009 How To Enumerate Windows Using the WIN32 API
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:
127030 How to Find a Window Handle Based on an Instance Handle
Properties

Article ID: 242308 - Last Review: 07/16/2004 16:34:00 - Revision: 2.2

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