PRB: QueryTimeout Event Not Available

• ActiveX Data Objects (ADO), versions 1.0, 1.5, 2.0

SYMPTOMS

The ActiveX Data Objects (ADO) Connection object does not expose a QueryTimeout event as does the rdoConnection object, which allows programmatic control over whether to continue waiting for query results.

CAUSE

This is a design limitation.

RESOLUTION

Execute the query asynchronously. You can use a Timer event to call code that determines whether to cancel the query.

STATUS

This behavior is a limitation of the ADO object model for the versions listed at the beginning of this article.

MORE INFORMATION

The Remote Data Objects (RDO) 2.0 rdoConnection objects expose a QueryTimeout event. For long-running queries, this event fires after QueryTimeout seconds and allows you to cancel the query or to continue for another QueryTimeout seconds, when the event fires again.

ADO does not expose a QueryTimeout event. This article lists some techniques to get similar functionality.

Detecting if a Timeout has Occurred

   If adStatus = adStatusErrorsOccurred Then

     If pError.Number = -2147217871 Then
       Debug.Print "Execute timed-out"
     End If

   End If 

Cancelling a Long-Running Query

Example

1. Create a new Visual Basic project and add a reference to the following:

   Microsoft ActiveX Data Objects Library




2. Add a new form (Form2) with a text box and two command buttons:

   Name: txtMessage Make this Textbox large enough to display a reasonable message Name: cmdRetry Caption: Retry Name: cmdCancel Caption: Cancel




3. Add the following code:

   
         Option Explicit
   
         Dim fCancel As Boolean
   
         Private Sub cmdCancel_Click ()
           fCancel = True
           Me.Visible = False
         End Sub
   
         Private Sub cmdRetry_Click ()
           fCancel = False
           Me.Visible = False
         End Sub 




4. On the default form (Form1) add two command buttons and a Timer control:

   Name: cmdDetect Caption: Detect Timeout Name: cmdChoose Caption: Time-out?




5. Add the following code:

   
         Option Explicit
   
         Dim WithEvents cn As ADODB.Connection, rs As ADODB.Recordset
   
         Private Sub cmdChoose_Click()
         Dim SQL As String
           Set cn = New ADODB.Connection
           Set rs = New ADODB.Recordset
           cn.Open "dsn=mydsn;database=pubs"  ' *** change connect string ***
           'CommandTimeout is optional; default is 30 seconds.
           cn.CommandTimeout = 15
           '
           ' This query must exceed the Timer1.Interval in order to test.
           '
           SQL = "SELECT authors.* FROM authors, titles a, titles b"
           rs.Open SQL, cn, adOpenKeyset, adLockOptimistic, adAsyncExecute
           Timer1.Interval = 2000
         End Sub
   
         Private Sub cn_ExecuteComplete(ByVal RecordsAffected As Long, _
                                        ByVal pError As ADODB.Error, _
                                        adStatus As ADODB.EventStatusEnum, _
                                        ByVal pCommand As ADODB.Command, _
                                        ByVal pRecordset As ADODB.Recordset, _
                                        ByVal pConnection As ADODB.Connection)
           If adStatus = adStatusErrorsOccurred Then
             If pError.Number = -2147217871 Then
               Debug.Print "Execute timed-out"
             End If
           End If
           Timer1.Interval = 0   ' turn off timer for async code
           if adStatus = adStatusOK Then
             If pRecordset.State = adStateOpen Then
               '
               ' Execute code now async query has completed.
               '
               Debug.Print "Query Complete."
             End If
           End If
         End Sub
   
         Private Sub cmdDetect_Click()
         Dim SQL As String
           Set cn = New ADODB.Connection
           Set rs = New ADODB.Recordset
           cn.Open "dsn=mydsn;database=pubs"  ' *** change connect string ***
           'The below is set low for demonstration purposes, it is optional.
           cn.CommandTimeout = 2
           SQL = "SELECT authors.* FROM authors, titles a, titles b"
           rs.Open SQL, cn, adOpenKeyset, adLockOptimistic, adAsyncExecute
         End Sub
   
         Private Sub Timer1_Timer()
           Select Case rs.State
   
             Case adStateConnecting, adStateExecuting, adStateFetching
               If SafeMsgBox("Query has timed-out.") = vbCancel Then
                 rs.Cancel
                 Timer1.Interval = 0
               End If
   
             Case Else
               Timer1.Interval = 0 ' catch-all
   
           End Select
         End Sub
   
         Private Function SafeMsgBox(ByVal Message As String) As Long
           Load Form2
           Form2.txtMessage = Message
           Form2.Show vbModal
           SafeMsgBox = IIf(Form2.fCancel, vbCancel, vbRetry)
           Unload Form2
         End Function 




6. Run the project and click each of the two buttons.

REFERENCES

OLE DB 2.0 SDK; search on: "CommandTimeout Property"; "ADO Events"

Additional query words: kbprb kbDatabase kbADO150 kbADO200 kbVBp kbSweepNext

Keywords          : kbADO150 kbADO200 kbDatabase kbVBp kbprb kbSweepNext 
Version           : WINDOWS:1.0,1.5,2.0
Platform          : WINDOWS 
Issue type        : kbprb 

Last Reviewed: June 30, 1999