Can I spawn a synchronous process in VB6 and retrieve its return value?

Question

Is it possible to spawn a synchronous process in VB6 (i.e. calling an external .exe), wait for it to terminate, and grab the return value?

We have legacy code (in VB6 obviously) that we need to call a .NET application to perform some complicated tasks, and based on the .NET app's return value, either proceed or fail. Is there a better way of doing such a thing?

Answer 1

Your first option should probably be to expose an interface to expose the .NET interfaces to COM and use that (it is much cleaner), but if for some reason you have to do it through a spawned process use this VB6 code.

 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 Declare Function GetExitCodeProcess Lib "kernel32" (ByVal hProcess As Long, lpExitCode As Long) As Long

   Private Const NORMAL_PRIORITY_CLASS = &H20&
   Private Const INFINITE = -1&
   Private Const SW_HIDE = 0
   Private Const SW_SHOWMINNOACTIVE = 7

   Public Function ExecCmd(cmdline As String, workdir As String) As Integer
      Dim proc As PROCESS_INFORMATION
      Dim start As STARTUPINFO
      Dim ret as Long

        ChDrive Left(workdir, 1) & ":"
        ChDir workdir

        start.cb = Len(start)
        start.wShowWindow = SW_SHOWMINNOACTIVE

        Call CreateProcessA(0&, cmdline, 0&, 0&, 1&, NORMAL_PRIORITY_CLASS, 0&, 0&, start, proc)
        Call WaitForSingleObject(proc.hProcess, INFINITE)
        Call GetExitCodeProcess(proc.hProcess, ret)
        Call CloseHandle(proc.hThread)
        Call CloseHandle(proc.hProcess)
        ExecCmd=ret
   End Function

Answer 2

' References: "Windows Script Host Shell Object Model" '
Function ExecuteCommand(cmd As String, ExpectedResult as Long) As Boolean
  Dim shell As New IWshRuntimeLibrary.WshShell
  Dim Result As Long

  Result = shell.Run(Command:=cmd, WindowStyle:=WshHide, WaitOnReturn:=True)

  ExecuteCommand = (Result = ExpectedResult)
End Function

Answer 3

You could use:

Option Explicit

Private Const INFINITE = &HFFFF&
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

Private Function SyncShell( _
    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
                SyncShell = lngExitCode
                CloseHandle lngHandle
            Else
                CloseHandle lngHandle
                Err.Raise &H8004AA00, "SyncShell", _
                          "Failed to retrieve exit code, error " _
                        & CStr(Err.LastDllError)
            End If
        Else
            Err.Raise &H8004AA01, "SyncShell", _
                      "Failed to open child process"
        End If
    Else
        Err.Raise &H8004AA02, "SyncShell", _
                  "Failed to Shell child process"
    End If
End Function