How to: Determine When a Shelled Process Ends

When you run the Shell function in a Microsoft Visual Basic for Applications (VBA) procedure, it starts an executable program asynchronously and returns control to the procedure. This shelled program continues to run independently of your procedure until you close it.

If your procedure needs to wait for the shelled process to end, you can use the Windows API to poll the status of the application, but this is not very efficient. This topic explains a more efficient method.

The Windows API has integrated functionality that enables your application to wait until a shelled process has completed. To use these functions, you need to have a handle to the shelled process. To accomplish this, use the CreateProcess function instead of the Shell function to begin your shelled program.

Creating the Shelled Process

To create an addressable process, use the CreateProcess function to start your shelled application. The CreateProcess function gives your program the process handle of the shelled process via one of its passed parameters.

Waiting for the Shelled Process to End

After you use the CreateProcess function to get a process handle, you can pass that handle to the WaitForSingleObject function. This causes your VBA procedure to suspend execution until the shelled process ends.

The following steps are necessary to build a VBA procedure that uses the CreateProcess function to run the Windows Notepad application. This code shows how to use the Windows API CreateProcess and WaitForSingleObject functions to wait until a shelled process ends before resuming execution.

The syntax of the CreateProcess function is complex, so in the example code, it is encapsulated into a function called ExecCmd. ExecCmd takes one parameter, the command line of the application to execute.

Create a standard module and paste the following lines in the Declarations section:

  Option Explicit
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 Long, 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 Long, _
lpStartupInfo As STARTUPINFO, lpProcessInformation As _
PROCESS_INFORMATION) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal _
hObject As Long) As Long
Private Const NORMAL_PRIORITY_CLASS = &H20&
Private Const INFINITE = -1&

Paste the following code into the module:

  Public Sub ExecCmd(cmdline As String)
   Dim proc As PROCESS_INFORMATION
   Dim start As STARTUPINFO
   Dim ReturnValue As Integer
' Initialize the STARTUPINFO structure:
start.cb = Len(start)
' Start the shelled application:
ReturnValue = CreateProcessA(0&, cmdline$, 0&, 0&, 1&, _
NORMAL_PRIORITY_CLASS, 0&, 0&, start, proc)
' Wait for the shelled application to finish:
Do
ReturnValue = WaitForSingleObject(proc.hProcess, 0)
DoEvents
Loop Until ReturnValue <> 258
ReturnValue = CloseHandle(proc.hProcess)
End Sub
Sub Testing()
ExecCmd "NOTEPAD.EXE"
MsgBox "Process Finished"
End Sub

Type Testing in the Immediate window. Notepad will start. After you close Notepad, a message box will notify you that the process has completed.