Microsoft KB Archive/169571

= Microsoft Knowledge Base =

INFO: Running Visual Basic Applications as Windows NT Services
Last reviewed: June 6, 1997

Article ID: Q169571

The information in this article applies to:


 * Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 4.0, 5.0

SUMMARY
Microsoft does not currently recommend running Visual Basic applications as Microsoft Windows NT Services because they may exhibit unstable behavior when installed and run as Microsoft Windows NT Services. Microsoft Visual Basic 4.0 does not support Callbacks and neither Microsoft Visual Basic 4.0 or Microsoft Visual Basic 5.0 is thread-safe. This behavior is by design. This article includes some examples of this unstable behavior, a short explanation of the possible causes of the instability, and a possible workaround.

Examples of Unstable Behavior

 * 1) When a Microsoft Visual Basic application displays a dialog and then terminates, the name of the .EXE file will disappear from the Services applet dialog but the Visual Basic application dialog may leave behind a gray rectangle on the user interface.
 * 2) When the Microsoft Visual Basic application is launched from a service, the .EXE file name will briefly appear in the Services applet dialog but the Visual Basic application does not seem to launch.
 * 3) Microsoft Visual Basic applications which are OLE Servers will return read-only properties successfully and when the OLE Server terminates, the .EXE file name will disappear from the Services applet dialog. Yet, subsequent attempts to instantiate that OLE Server will fail with OLE automation errors or the Services process will hang.

Why This Unstable Behavior Occurs
A service in Microsoft Windows NT is a program that is written as a console application and runs whenever the operating system is running. Services are commonly employed to provide facilities like directory replication, system activity auditing, process monitoring, or communications support.

Services log on to Microsoft Windows NT under the LocalSystem account, which is a predefined local account used by system processes.

A service that runs in the context of the LocalSystem account inherits these characteristics:


 * The service can not open the registry key HKEY_CURRENT_USER.
 * The service can open the registry key HKEY_LOCAL_MACHINE\SECURITY.
 * The service has limited access to network resources such as shares and pipes because it has no credentials and must connect using a null session.
 * If the service opens a command window and runs a batch file, the user could hit CTRL+C to terminate the batch file and gain access to a command window with LocalSystem permissions.
 * The service can not share objects with other applications unless they are opened using a DACL that allows a user or group of users access or NULL DACL that allows everyone access. Specifying a NULL DACL is not the same as specifying NULL ,which means that access is only granted to applications with the same security context.
 * Service programs which require anything more than a brief moment to perform necessary initialization may be required to create another thread to do the common initialization, while the main thread continues with processing.

By convention, Microsoft Windows NT Services are designed to be non- interactive. That is, in most cases, they do their work without any user interaction. Services are usually configured and controlled with an applet installed in the Control Panel. This requires that the programmer add code to provide for these additional requirements:


 * Report warnings and errors in the system or application logs. You typically can't use output to the screen since there may be no user logged on.
 * Control the service through either a separate application or a control Panel applet. This involves implementing a communication mechanism for your service.
 * Install and remove the service from the system.

Therefore, to create an interactive Microsoft Windows NT Service requires quite a bit more effort than simply writing the program, compiling and then installing it as a service.

Microsoft Visual Basic is designed to produce application programs that expect to interact with a user instead of being run from the console. One of the most common interactions is the runtime error dialog box. Extremely careful programming can trap nearly all runtime errors, which may satisfy reliability requirements in most cases. However the possibility remains that a Microsoft Visual Basic application will eventually post an unhandled error dialog from the Services process that will hang the Microsoft Visual Basic application.

Therefore, it is not currently recommended that you install a Microsoft Visual Basic application as a Microsoft Windows NT Service.

Workaround
The Microsoft Technical Article "NT Service: An OLE Control for Creating Windows NT Services in Visual Basic" describes an OLE Control that enables you to create Visual Basic applications that function as Microsoft Windows NT services. With the NTService control, you can install a service, log events, and respond to start, stop, pause, and continue events.

Other Technologies
You can expect difficulties with efforts to employ Microsoft technologies such as MAPI, ODBC, DCOM, OLE Automation ,and DAO in a Microsoft Windows NT Service written in Microsoft Visual Basic. For instance, MAPI contains user interface elements that must be anticipated and suppressed, and may require access to parts of the registry that are unavailable to a service or require a special security context for use.

For this reason, and those already noted, Microsoft advises you to avoid using these technologies in a Microsoft Windows NT Service written in Microsoft Visual Basic.

Microsoft Visual Basic 5.0
While it is possible to write a Microsoft Windows NT service as an unattended .EXE in Microsoft Visual Basic 5.0, as the following code sample demonstrates, it is still not advisable to implement services because Microsoft Visual Basic 5.0 is not thread-safe and the possibility still remains that an unexpected dialog may post from within the Visual Basic runtime environment.

This example provides for installing and uninstalling itself as a service by running the .EXE with a command line parameter as shown below:

MyService.exe install MyService.exe uninstall After installing the service, you will need to configure it using the Control Panel Services applet. From the applet you can start, stop and pause the service. You can also set it up to start automatically during boot up.

Add the following code to a .BAS module:

Option Explicit

Private Const SERVICE_WIN32_OWN_PROCESS = &H10& Private Const SERVICE_WIN32_SHARE_PROCESS = &H20& Private Const SERVICE_WIN32 = SERVICE_WIN32_OWN_PROCESS + _ SERVICE_WIN32_SHARE_PROCESS

Private Const SERVICE_ACCEPT_STOP = &H1 Private Const SERVICE_ACCEPT_PAUSE_CONTINUE = &H2 Private Const SERVICE_ACCEPT_SHUTDOWN = &H4

Private Const SC_MANAGER_CONNECT = &H1 Private Const SC_MANAGER_CREATE_SERVICE = &H2 Private Const SC_MANAGER_ENUMERATE_SERVICE = &H4 Private Const SC_MANAGER_LOCK = &H8 Private Const SC_MANAGER_QUERY_LOCK_STATUS = &H10 Private Const SC_MANAGER_MODIFY_BOOT_CONFIG = &H20

Public Const STANDARD_RIGHTS_REQUIRED = &HF0000 Private Const SERVICE_QUERY_CONFIG = &H1 Private Const SERVICE_CHANGE_CONFIG = &H2 Private Const SERVICE_QUERY_STATUS = &H4 Private Const SERVICE_ENUMERATE_DEPENDENTS = &H8 Private Const SERVICE_START = &H10 Private Const SERVICE_STOP = &H20 Private Const SERVICE_PAUSE_CONTINUE = &H40 Private Const SERVICE_INTERROGATE = &H80 Private Const SERVICE_USER_DEFINED_CONTROL = &H100 Private Const SERVICE_ALL_ACCESS = (STANDARD_RIGHTS_REQUIRED Or _                                      SERVICE_QUERY_CONFIG Or _                                       SERVICE_CHANGE_CONFIG Or _                                       SERVICE_QUERY_STATUS Or _                                       SERVICE_ENUMERATE_DEPENDENTS Or _                                       SERVICE_START Or _                                       SERVICE_STOP Or _                                       SERVICE_PAUSE_CONTINUE Or _                                       SERVICE_INTERROGATE Or _                                       SERVICE_USER_DEFINED_CONTROL)

Private Const SERVICE_DEMAND_START As Long = &H3

Private Const SERVICE_ERROR_NORMAL As Long = &H1

Private Enum SERVICE_CONTROL SERVICE_CONTROL_STOP = &H1 SERVICE_CONTROL_PAUSE = &H2 SERVICE_CONTROL_CONTINUE = &H3 SERVICE_CONTROL_INTERROGATE = &H4 SERVICE_CONTROL_SHUTDOWN = &H5 End Enum

Private Enum SERVICE_STATE SERVICE_STOPPED = &H1 SERVICE_START_PENDING = &H2 SERVICE_STOP_PENDING = &H3 SERVICE_RUNNING = &H4 SERVICE_CONTINUE_PENDING = &H5 SERVICE_PAUSE_PENDING = &H6 SERVICE_PAUSED = &H7 End Enum

Private Type SERVICE_TABLE_ENTRY lpServiceName As String lpServiceProc As Long lpServiceNameNull As Long lpServiceProcNull As Long End Type

Private Type SERVICE_STATUS dwServiceType As Long dwCurrentState As Long dwControlsAccepted As Long dwWin32ExitCode As Long dwServiceSpecificExitCode As Long dwCheckPoint As Long dwWaitHint As Long End Type

Private Declare Function StartServiceCtrlDispatcher _ Lib "advapi32.dll" Alias "StartServiceCtrlDispatcherA" _ (lpServiceStartTable As SERVICE_TABLE_ENTRY) As Long Private Declare Function RegisterServiceCtrlHandler _ Lib "advapi32.dll" Alias "RegisterServiceCtrlHandlerA" _ (ByVal lpServiceName As String, ByVal lpHandlerProc As Long) _ As Long Private Declare Function SetServiceStatus _ Lib "advapi32.dll" (ByVal hServiceStatus As Long, _     lpServiceStatus As SERVICE_STATUS) As Long Private Declare Function OpenSCManager _ Lib "advapi32.dll" Alias "OpenSCManagerA" _ (ByVal lpMachineName As String, ByVal lpDatabaseName As String, _     ByVal dwDesiredAccess As Long) As Long Private Declare Function CreateService _ Lib "advapi32.dll" Alias "CreateServiceA" _ (ByVal hSCManager As Long, ByVal lpServiceName As String, _     ByVal lpDisplayName As String, ByVal dwDesiredAccess As Long, _      ByVal dwServiceType As Long, ByVal dwStartType As Long, _      ByVal dwErrorControl As Long, ByVal lpBinaryPathName As String, _      ByVal lpLoadOrderGroup As String, ByVal lpdwTagId As String, _      ByVal lpDependencies As String, ByVal lp As String, _      ByVal lpPassword As String) As Long Private Declare Function DeleteService _ Lib "advapi32.dll" (ByVal hService As Long) As Long Declare Function CloseServiceHandle _ Lib "advapi32.dll" (ByVal hSCObject As Long) As Long Declare Function OpenService _ Lib "advapi32.dll" Alias "OpenServiceA" _ (ByVal hSCManager As Long, ByVal lpServiceName As String, _     ByVal dwDesiredAccess As Long) As Long

'** Change SERVICE_NAME as needed Private Const SERVICE_NAME As String = "MyService"

Private hServiceStatus As Long Private ServiceStatus As SERVICE_STATUS

Sub Main Dim hSCManager As Long Dim hService As Long Dim ServiceTableEntry As SERVICE_TABLE_ENTRY Dim b As Boolean Dim cmd As String

cmd = Trim(LCase(Command)) Select Case cmd Case "install"                     'Install service on machine hSCManager = OpenSCManager(vbNullString, vbNullString, _                        SC_MANAGER_CREATE_SERVICE) hService = CreateService(hSCManager, SERVICE_NAME, _                      SERVICE_NAME, SERVICE_ALL_ACCESS, _                       SERVICE_WIN32_OWN_PROCESS, _                       SERVICE_DEMAND_START, SERVICE_ERROR_NORMAL, _                       App.Path & "\" & App.EXEName, vbNullString, _                       vbNullString, vbNullString, vbNullString, _                       vbNullString) CloseServiceHandle hService CloseServiceHandle hSCManager Case "uninstall"                  'Remove service from machine hSCManager = OpenSCManager(vbNullString, vbNullString, _                        SC_MANAGER_CREATE_SERVICE) hService = OpenService(hSCManager, SERVICE_NAME, _                      SERVICE_ALL_ACCESS) DeleteService hService CloseServiceHandle hService CloseServiceHandle hSCManager Case Else                                   'Start the service ServiceTableEntry.lpServiceName = SERVICE_NAME ServiceTableEntry.lpServiceProc = _ FncPtr(AddressOf ServiceMain) b = StartServiceCtrlDispatcher(ServiceTableEntry) End Select End Sub

Sub ServiceMain(ByVal dwArgc As Long, ByVal lpszArgv As Long) Dim b As Boolean

'Set initial state ServiceStatus.dwServiceType = SERVICE_WIN32_OWN_PROCESS ServiceStatus.dwCurrentState = SERVICE_START_PENDING ServiceStatus.dwControlsAccepted = SERVICE_ACCEPT_STOP _ Or SERVICE_ACCEPT_PAUSE_CONTINUE _ Or SERVICE_ACCEPT_SHUTDOWN ServiceStatus.dwWin32ExitCode = 0 ServiceStatus.dwServiceSpecificExitCode = 0 ServiceStatus.dwCheckPoint = 0 ServiceStatus.dwWaitHint = 0

hServiceStatus = RegisterServiceCtrlHandler(SERVICE_NAME, _                      AddressOf Handler) ServiceStatus.dwCurrentState = SERVICE_START_PENDING b = SetServiceStatus(hServiceStatus, ServiceStatus)

'** Do Initialization Here

ServiceStatus.dwCurrentState = SERVICE_RUNNING b = SetServiceStatus(hServiceStatus, ServiceStatus)

'** Perform tasks -- if none exit

''** If an error occurs, the following should be used for shutting ''** down: ''  SetServerStatus SERVICE_STOP_PENDING   Clean up         SetServerStatus SERVICE_STOPPED End Sub

Sub Handler(ByVal fdwControl As Long) Dim b As Boolean

Select Case fdwControl Case SERVICE_CONTROL_PAUSE '** Do whatever it takes to pause here. ServiceStatus.dwCurrentState = SERVICE_PAUSED Case SERVICE_CONTROL_CONTINUE '** Do whatever it takes to continue here. ServiceStatus.dwCurrentState = SERVICE_RUNNING Case SERVICE_CONTROL_STOP ServiceStatus.dwWin32ExitCode = 0 ServiceStatus.dwCurrentState = SERVICE_STOP_PENDING ServiceStatus.dwCheckPoint = 0 ServiceStatus.dwWaitHint = 0    'Might want a time estimate b = SetServiceStatus(hServiceStatus, ServiceStatus) '** Do whatever it takes to stop here. ServiceStatus.dwCurrentState = SERVICE_STOPPED Case SERVICE_CONTROL_INTERROGATE 'Fall through to send current status. Case Else End Select 'Send current status. b = SetServiceStatus(hServiceStatus, ServiceStatus) End Sub

Function FncPtr(ByVal fnp As Long) As Long FncPtr = fnp End Function