HOWTO: Use API to Customize DAO Registry to Close ODBC Connection

ID: Q170536

The information in this article applies to:

Microsoft Visual Basic Professional and Enterprise Editions for Windows, versions 5.0, 6.0

SUMMARY

In DAO, using the Close method on a database opened with ODBC drivers does not close the database; a cached connection remains idle before timing out. The default duration of this timeout is 600 seconds (10 minutes). If your application cannot accept the default behavior, you could control this time-out period by customizing the ConnectionTimeout value in Windows registry setting.

MORE INFORMATION

You can control the duration of ConnectionTimeout by creating a ConnectionTimeout registry entry for Jet. In Windows NT 3.51, the type is called "REG_DWORD"; in Windows 95, Windows 98, and Windows NT 4.0, the type is called "DWORD." "REG_DWORD" and "DWORD" are different terms referring to the same thing, a 32-bit number.

Although Visual Basic includes the SaveSetting and GetSetting functions to save and retrieve information from the registry, entries of types other than "REG_SZ" ("String" for Windows 95, Windows 98, and Windows NT4.0) are not recognized. This article demonstrates how to use the registry API to set the DAO ConnectionTimeout setting to 1, which closes the connection after the Close method with only a one-second delay.

Sample Program

The following example has three CommandButton controls:

Command1 creates a testing Registry entry "Software\MyApp\Jet\3.5\Engines\ODBC" under HKEY_LOCAL_MACHINE root key, and sets the ConnectionTimeOut value to 1.

Command2 establishes the connection by opening the Pubs database.

Command3 closes the database.

You can check the detailed connection information by executing SP_WHO stored procedure on the SQL Server, then open/close database with/without the registry entry created.

Start a new project in Visual Basic and choose "Standard EXE." Form1 is created by default.

Reference the Microsoft DAO 3.5 Object Library under Project, References menu items.

Add three CommandButtons, Command1, Command2, and Command3 to Form1.

Paste the following code into the General Declarations section of Form1:


    Option Explicit
    Dim ws As Workspace
    Dim db As Database
    Dim TimeOutValue As Integer

    Private Sub CreateNewKey(sNewKeyName As String, lPredefinedKey As Long)
       Dim hNewKey As Long         'handle to the new key
       Dim lRetVal As Long         'result of the RegCreateKeyEx function

         lRetVal = RegCreateKeyEx(lPredefinedKey, sNewKeyName, 0&, _
               vbNullString, REG_OPTION_NON_VOLATILE, KEY_ALL_ACCESS, _
               0&, hNewKey, lRetVal)
         RegCloseKey (hNewKey)
    End Sub

    Private Sub SetKeyValue(sKeyName As String, sValueName As String, _
         vValueSetting As Variant)
    Dim lRetVal As Long         'result of the SetValueExLong function
         Dim hKey As Long            'handle of open key

         lRetVal = RegOpenKeyEx(HKEY_LOCAL_MACHINE, sKeyName, 0, _
                            KEY_ALL_ACCESS, hKey)
         lRetVal = RegSetValueExLong(hKey, sValueName, 0&, _
                            REG_DWORD, vValueSetting, 4)
         RegCloseKey (hKey)
    End Sub

    Private Function QueryValue(sKeyName As String, sValueName As String) _
                                As Integer
         Dim lRetVal As Long        'result of the API functions
         Dim hKey As Long           'handle of opened key
         Dim vValue As Variant      'setting of queried value

         lRetVal = RegOpenKeyEx(HKEY_LOCAL_MACHINE, sKeyName, 0, _
                                KEY_ALL_ACCESS, hKey)
         lRetVal = QueryValueEx(hKey, sValueName, vValue)
         If lRetVal = ERROR_NONE Then
             QueryValue = True
             TimeOutValue = vValue
         Else
             QueryValue = False
         End If
         RegCloseKey (hKey)
    End Function

    Private Sub Command1_Click()
         If QueryValue("Software\MyApp\Jet\3.5\Engines\ODBC", _
                "ConnectionTimeout") Then
            If TimeOutValue <> 1 Then
             SetKeyValue "Software\MyApp\Jet\3.5\Engines\ODBC", _
                "ConnectionTimeout", 1
            End If
          MsgBox "Test registry key already set"
         Else
             CreateNewKey "Software\MyApp\Jet\3.5\Engines\ODBC", _
                          HKEY_LOCAL_MACHINE
            SetKeyValue "Software\MyApp\Jet\3.5\Engines\ODBC", _
                        "ConnectionTimeout", 1
            MsgBox "Test registry key created successfully"
         End If
    End Sub

    Private Sub Command2_Click()
         Dim strConnect As String

         DBEngine.IniPath = "HKEY_LOCAL_MACHINE\Software\MyApp\Jet\3.5"
         Set ws = DBEngine.Workspaces(0)
         strConnect = "ODBC;SERVER=MyServer;" & _
               "DRIVER={SQL  SERVER};DATABASE=pubs;UID=sa;PWD=;"
         Set db = ws.OpenDatabase("", False, False, strConnect)
         MsgBox "Connection established"
    End Sub

    Private Sub Command3_Click()
         If db Is Nothing Then
            MsgBox "Already Disconnected"
            Exit Sub
         End If

         db.Close
         ws.Close

         Dim Start As Long
         Start = Timer
         Do
           DBEngine.Idle dbFreeLocks
         DoEvents
         Loop While Timer <= Start + 1
         MsgBox "Disconnected"
    End Sub

    Private Sub Form_Load()
         Command1.Caption = "Create/Set test registry entry"
         Command2.Caption = "Open database"
         Command3.Caption = "Close database"
    End Sub

From the Project menu, select Add Module, then click Module. Copy and paste the following code into the General Declarations section of Module1:


    Option Explicit

    Global Const REG_DWORD As Long = 4
    Global Const HKEY_LOCAL_MACHINE = &H80000002
    Global Const ERROR_NONE = 0
    Global Const KEY_ALL_ACCESS = &H3F
    Global Const REG_OPTION_NON_VOLATILE = 0
    Global TimeOut As Integer

    Declare Function RegCloseKey Lib "advapi32.dll" _
                       (ByVal hKey As Long) As Long
    Declare Function RegCreateKeyEx Lib "advapi32.dll" Alias _
      "RegCreateKeyExA" (ByVal hKey As Long, ByVal lpSubKey As String, _
      ByVal Reserved As Long, ByVal lpClass As String, ByVal dwOptions _
      As Long, ByVal samDesired As Long, ByVal lpSecurityAttributes _
      As Long, phkResult As Long, lpdwDisposition As Long) As Long
    Declare Function RegOpenKeyEx Lib "advapi32.dll" Alias _
      "RegOpenKeyExA" (ByVal hKey As Long, ByVal lpSubKey As String, _
      ByVal ulOptions As Long, ByVal samDesired As Long, phkResult As _
      Long) As Long
    Declare Function RegQueryValueExLong Lib "advapi32.dll" Alias _
      "RegQueryValueExA" (ByVal hKey As Long, ByVal lpValueName As _
      String, ByVal lpReserved As Long, lpType As Long, lpData As _
      Long, lpcbData As Long) As Long
    Declare Function RegQueryValueExNULL Lib "advapi32.dll" Alias _
      "RegQueryValueExA" (ByVal hKey As Long, ByVal lpValueName As _
      String, ByVal lpReserved As Long, lpType As Long, ByVal lpData _
      As Long, lpcbData As Long) As Long

    Declare Function RegSetValueExLong Lib "advapi32.dll" Alias _
      "RegSetValueExA" (ByVal hKey As Long, ByVal lpValueName As String, _
      ByVal Reserved As Long, ByVal dwType As Long, lpValue As Long, _
      ByVal cbData As Long) As Long

    Function QueryValueEx(ByVal lhKey As Long, ByVal szValueName As _
             String, vValue As Variant) As Long
      Dim cch As Long
      Dim lrc As Long
      Dim lType As Long
      Dim lValue As Long
      Dim sValue As String

      On Error GoTo QueryValueExError

      lrc = RegQueryValueExNULL(lhKey, szValueName, 0&, lType, 0&, cch)
      If lrc <> ERROR_NONE Then Error 5

      lrc = RegQueryValueExLong(lhKey, szValueName, 0&, lType, _
            lValue, cch)
      If lrc = ERROR_NONE Then vValue = lValue

    QueryValueExExit:
      QueryValueEx = lrc
      Exit Function
    QueryValueExError:
      Resume QueryValueExExit
    End Function

Make sure to change your SERVER, UID, and PWD parameters in the connection string under Command2_Click.

Start the program or press the F5 key.

You can observe the connection information by executing the SP_WHO stored procedure at SQL Server. Compare the result before and after opening a database and before and after closing the database.

REFERENCES

For additional information, please see the following articles in the Microsoft Knowledge Base:

Q145679 HOWTO: Use the Registry API to Save and Retrieve Setting

Q110227 PRB: ODBC Database Remains Open After a Close Method Used

Additional query words:


Keywords          : kbADO kbVBp kbVBp500 kbVBp600 kbWinOS98 kbGrpVBDB kbDSupport 
Version           : WINDOWS:5.0,6.0
Platform          : WINDOWS 
Issue type        :

Last Reviewed: July 15, 1999