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

• Microsoft Visual Basic Professional and Enterprise Editions for Windows, version 5.0

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 and Windows NT4.0, 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 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.

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

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

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

4. 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()
            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
   
   

5. 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
   
   

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

7. Start the program or press the F5 key.

8. 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:

   ARTICLE-ID: Q145679
   TITLE:      HOWTO: Use the Registry API to Save and Retrieve Setting

   ARTICLE-ID: Q110227
   TITLE:      PRB: ODBC Database Remains Open After a Close Method Used