Determine when a shelled process ends

When you run the Shell function in a 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.

Create 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.

Wait 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.

1. 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& 
   
   

2. 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
   

3. To test the function, paste the following code in the Immediate window and press Enter. Notepad starts. After a moment, close Notepad. The message box appears when Notepad closes.

   ExecCmd "NOTEPAD.EXE": MsgBox "Process Finished" 
   

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