How to wait for a shell process to finish before executing further code in VB6
The secret sauce needed to do this is the WaitForSingleObject function, which blocks execution of your application's process until the specified process completes (or times out). It's part of the Windows API, easily called from a VB 6 application after adding the appropriate declaration to your code.
That declaration would look something like this:
Private Declare Function WaitForSingleObject Lib "kernel32" (ByVal hHandle _
As Long, ByVal dwMilliseconds As Long) As Long
It takes two parameters: a handle to the process that you want to wait on, and the time-out interval (in milliseconds) that indicates the maximum amount of time that you want to wait. If you do not specify a time-out interval (a value of zero), the function does not wait and returns immediately. If you specify an infinite time-out interval, the function returns only when the process signals that it has completed.
Armed with that knowledge, the only task that remains is figuring out how to get a handle to the process that you started. That turns out to be pretty simple, and can be accomplished a number of different ways:
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One possibility (and the way I'd do it) is by using the
ShellExecuteExfunction, also from the Windows API, as a drop-in replacement for theShellfunction that is built into VB 6. This version is far more versatile and powerful, yet just as easily called using the appropriate declaration.It returns a handle to the process that it creates. All you have to do is pass that handle to the
WaitForSingleObjectfunction as thehHandleparameter, and you're in business. Execution of your application will be blocked (suspended) until the process that you've called terminates. -
Another possibility is to use the
CreateProcessfunction (once again, from the Windows API). This function creates a new process and its primary thread in the same security context as the calling process (i.e., your VB 6 application).Microsoft has published a knowledge base article detailing this approach that even provides a complete sample implementation. You can find that article here: How To Use a 32-Bit Application to Determine When a Shelled Process Ends.
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Finally, perhaps the simplest approach yet is to take advantage of the fact that the built-in
Shellfunction's return value is an application task ID. This is a unique number that identifies the program you started, and it can be passed to theOpenProcessfunction to obtain a process handle that can be passed to theWaitForSingleObjectfunction.However, the simplicity of this approach does come at a cost. A very significant disadvantage is that it will cause your VB 6 application to become completely unresponsive. Because it will not be processing Windows messages, it will not respond to user interaction or even redraw the screen.
The good folks over at VBnet have made complete sample code available in the following article: WaitForSingleObject: Determine when a Shelled App has Ended.
I'd love to be able to reproduce the code here to help stave off link rot (VB 6 is getting up there in years now; there's no guarantee that these resources will be around forever), but the distribution license in the code itself appears to explicitly forbid that.
There is no need to resort to the extra effort of calling CreateProcess(), etc. This more or less duplicates the old Randy Birch code though it wasn't based on his example. There are only so many ways to skin a cat.
Here we have a prepackaged Function for handy use, which also returns the exit code. Drop it into a static (.BAS) module or include it inline in a Form or Class.
Option Explicit
Private Const INFINITE = &HFFFFFFFF&
Private Const SYNCHRONIZE = &H100000
Private Const PROCESS_QUERY_INFORMATION = &H400&
Private Declare Function CloseHandle Lib "kernel32" ( _
ByVal hObject As Long) As Long
Private Declare Function GetExitCodeProcess Lib "kernel32" ( _
ByVal hProcess As Long, _
lpExitCode As Long) As Long
Private Declare Function OpenProcess Lib "kernel32" ( _
ByVal dwDesiredAccess As Long, _
ByVal bInheritHandle As Long, _
ByVal dwProcessId As Long) As Long
Private Declare Function WaitForSingleObject Lib "kernel32" ( _
ByVal hHandle As Long, _
ByVal dwMilliseconds As Long) As Long
Public Function ShellSync( _
ByVal PathName As String, _
ByVal WindowStyle As VbAppWinStyle) As Long
'Shell and wait. Return exit code result, raise an
'exception on any error.
Dim lngPid As Long
Dim lngHandle As Long
Dim lngExitCode As Long
lngPid = Shell(PathName, WindowStyle)
If lngPid <> 0 Then
lngHandle = OpenProcess(SYNCHRONIZE _
Or PROCESS_QUERY_INFORMATION, 0, lngPid)
If lngHandle <> 0 Then
WaitForSingleObject lngHandle, INFINITE
If GetExitCodeProcess(lngHandle, lngExitCode) <> 0 Then
ShellSync = lngExitCode
CloseHandle lngHandle
Else
CloseHandle lngHandle
Err.Raise &H8004AA00, "ShellSync", _
"Failed to retrieve exit code, error " _
& CStr(Err.LastDllError)
End If
Else
Err.Raise &H8004AA01, "ShellSync", _
"Failed to open child process"
End If
Else
Err.Raise &H8004AA02, "ShellSync", _
"Failed to Shell child process"
End If
End Function
I know it's an old thread, but...
How about using the Windows Script Host's Run method? It has a bWaitOnReturn parameter.
object.Run (strCommand, [intWindowStyle], [bWaitOnReturn])
Set oShell = CreateObject("WSCript.shell")
oShell.run "cmd /C " & App.Path & sCommand, 0, True
intWindowStyle = 0, so cmd will be hidden
Do like this :
Private Type STARTUPINFO
cb As Long
lpReserved As String
lpDesktop As String
lpTitle As String
dwX As Long
dwY As Long
dwXSize As Long
dwYSize As Long
dwXCountChars As Long
dwYCountChars As Long
dwFillAttribute As Long
dwFlags As Long
wShowWindow As Integer
cbReserved2 As Integer
lpReserved2 As Long
hStdInput As Long
hStdOutput As Long
hStdError As Long
End Type
Private Type PROCESS_INFORMATION
hProcess As Long
hThread As Long
dwProcessID As Long
dwThreadID As Long
End Type
Private Declare Function WaitForSingleObject Lib "kernel32" (ByVal _
hHandle As Long, ByVal dwMilliseconds As Long) As Long
Private Declare Function CreateProcessA Lib "kernel32" (ByVal _
lpApplicationName As String, ByVal lpCommandLine As String, ByVal _
lpProcessAttributes As Long, ByVal lpThreadAttributes As Long, _
ByVal bInheritHandles As Long, ByVal dwCreationFlags As Long, _
ByVal lpEnvironment As Long, ByVal lpCurrentDirectory As String, _
lpStartupInfo As STARTUPINFO, lpProcessInformation As _
PROCESS_INFORMATION) As Long
Private Declare Function CloseHandle Lib "kernel32" _
(ByVal hObject As Long) As Long
Private Declare Function GetExitCodeProcess Lib "kernel32" _
(ByVal hProcess As Long, lpExitCode As Long) As Long
Private Const NORMAL_PRIORITY_CLASS = &H20&
Private Const INFINITE = -1&
Public Function ExecCmd(cmdline$)
Dim proc As PROCESS_INFORMATION
Dim start As STARTUPINFO
' Initialize the STARTUPINFO structure:
start.cb = Len(start)
' Start the shelled application:
ret& = CreateProcessA(vbNullString, cmdline$, 0&, 0&, 1&, _
NORMAL_PRIORITY_CLASS, 0&, vbNullString, start, proc)
' Wait for the shelled application to finish:
ret& = WaitForSingleObject(proc.hProcess, INFINITE)
Call GetExitCodeProcess(proc.hProcess, ret&)
Call CloseHandle(proc.hThread)
Call CloseHandle(proc.hProcess)
ExecCmd = ret&
End Function
Sub Form_Click()
Dim retval As Long
retval = ExecCmd("notepad.exe")
MsgBox "Process Finished, Exit Code " & retval
End Sub
Reference : http://support.microsoft.com/kb/129796
Great code. Just one tiny little problem: you must declare in the ExecCmd (after Dim start As STARTUPINFO):
Dim ret as Long
You will get an error when trying to compile in VB6 if you don't. But it works great :)
Kind regards