PRB: rdoConnection_Connect Event Does Not Always Fire (191788)


The information in this article applies to:

  • Microsoft Visual Basic Enterprise Edition for Windows 4.0
  • Microsoft Visual Basic Enterprise Edition for Windows 5.0
  • Microsoft Visual Basic Enterprise Edition for Windows 6.0
This article was previously published under Q191788

SYMPTOMS

When using the OpenConnection method of a rdoEnvironment object to establish a connection to a local database, the Connect event of the rdoConnection object will not fire.

The Connect event will fire, however, when using the EstablishConnection method of the rdoConnection object.

CAUSE

This is primarily caused by the speed of the connection. When accessing a database server that resides locally, the connection is made and the Connect event is fired before the object can be returned to your program. Because the object has not yet been returned, custom event handlers are not called.

Basically, when using the rdoEnvironment.OpenConnection on a synchronous call, a connection is established first and then the object is returned, whereas using rdoConnection.EstablishConnection, an object has already been created before the connection is established.

Specifying rdAsyncEnable (an asynchronous connection) in the OpenConnection method will not solve the problem on machines where the database server is local. Because the database server is local, the connection is created before RDO has an object to return to the calling program. When the database is remote, RDO has enough time to return the rdoConnection object to the calling program before the connection is established. When the connection is finally established, the Connect event will fire normally.

NOTE: You must specify rdAsyncEnable to make OpenConnection fire the Connect event or the Connect event will never fire under any circumstances. If rdAsyncEnable is not specified, the connection is always made to the database before the object is returned.

RESOLUTION

To work around this problem, try using the EstablishConnection method of a valid connection object. Because the object has already been created and your program has the event handler established before the connect call has been made, the Connect event will always fire correctly.

STATUS

Microsoft is researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

If you are running SQL Server or other similar database system locally, you can reproduce this behavior by following the steps below.

Steps to Reproduce Behavior

  1. Open a new Standard EXE project in Visual Basic. Form1 is created by default. Add a reference to the RDO Object Library.
  2. Add the following controls with the following properties to Form1: 
        Control         Property         Value
    -------         --------         -----

    Label           Name             lblDSN
                    Caption          DSN
    TextBox         Name             txtDSN
    CommandButton   Name             cmdUseEstablishConnection
                    Caption          Use EstablishConnection
    CommandButton   Name             cmdUseOpenConnection
                    Caption          Use OpenConnection
  3. Paste the following code into the Code Window of Form1:

    Note You must change UID=<username> and PWD=<strong password> to the correct values before you run this code. Make sure that UID has the appropriate permissions to perform this operation on the database. the database.
         Option Explicit
     Dim WithEvents ConnectionObj As RDO.rdoConnection
     Dim Env As RDO.rdoEnvironment

     Private Sub cmdUseEstablishConnection_Click()

         'Here, we are creating the Object First
         Set ConnectionObj = New RDO.rdoConnection

         ConnectionObj.Connect = txtDSN

         'and then establishing the connection
         ConnectionObj.EstablishConnection

     End Sub

     Private Sub cmdUseOpenConnection_Click()

         'Here, we are establishing the connection first
         'and then creating the object
         Set ConnectionObj = Env.OpenConnection(dsname:="", _
         prompt:=rdDriverNoPrompt, Connect:=txtDSN.Text, _
         Options:=rdAsyncEnable)

     End Sub

     Private Sub ConnectionObj_Connect(ByVal ErrorOccurred As Boolean)
         MsgBox "We are connected!", vbInformation
     End Sub

     Private Sub Form_Load()

         Set Env = rdoEngine.rdoEnvironments(0)

         'This sample is designed to work with the pubs database
         'on a local SQL Server.  You may change your connect string
         'here.
         txtDSN = "Driver={SQL Server};Server=(local);" & _
                  "Database=Pubs;UID=<username>;PWD=<strong password>;"

     End Sub
  4. Run the form. Click on each of the command buttons and note that the Use EstablishConnection will always display a Message Box whereas OpenConnection will not.
(c) Microsoft Corporation 1998. All Rights Reserved.
Contributions by Jonathan Johnson, Microsoft Corporation
Modification Type:MinorLast Reviewed:3/2/2005
Keywords:kbprb kbRDO KB191788
©2004 Microsoft Corporation. All rights reserved.