How To Open ADO Recordsets Asynchronously Using WithEvents

Summary

This article describes how to take advantage of asynchronous ActiveX Data Objects (ADO) functionality in Visual Basic 6.0. ADO gives developers the ability to declare ADO object variables with the WithEvents keyword, which provides enhanced control over asynchronous operations.

More Information

The following example uses the Pubs database that ships with SQL Server to demonstrate the opening of asynchronous ADO recordsets from within Visual Basic 6.0.

The example repeatedly opens and closes ADO recordsets until the user selects Cancel. The sample assumes that the Pubs sample database is used for this project and that the database contains the Publishers, Titles, and Authors tables.


To create the example, use the following steps:

Step-by-Step Example

  1. Create a new Standard.exe Visual Basic 6.0 project.
  2. From the Project menu, choose References and add the Microsoft ActiveX Data Objects Library reference to the project.
  3. Add two command buttons to Form1, the project's default form.
  4. Cut and paste the following code into the project:
       Option Explicit

    Dim WithEvents con As ADODB.Connection
    Dim rst As New ADODB.Recordset
    Dim iExecutionCount As Integer

    Private Sub Form_Load()

    Dim sConnect As String
    Dim sServer As String
    Dim sUID As String
    Dim sPWD As String

    On Error GoTo EH

    ' Specify connection parameters.

    sServer = "<your server name>"
    sUID = "<your SQL Server user ID>"
    sPWD = "<your SQL Server user ID password>"

    sConnect = "Driver={SQL Server};Server=" & sServer & _
    ";Database=Pubs;"
    sConnect = sConnect & "UID=" & sUID & ";"
    sConnect = sConnect & "PWD=" & sPWD & ";DSN='';"
    Set con = New ADODB.Connection
    con.CursorLocation = adUseClient

    ' Open the connection.

    con.Open sConnect

    command1.Caption = "Open Recordsets"
    command2.Caption = "Cancel"
    Exit Sub

    EH:
    MsgBox "Could not establish ODBC connection.", vbCritical + vbOKOnly
    Set con = Nothing
    End

    End Sub

    Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode As Integer)

    On Error Resume Next
    rst.Close
    Set rst = Nothing
    Set con = Nothing

    End Sub

    Private Sub Command1_Click()

    ' This code begins the process of repeatedly opening ADO recordsets.

    command1.Enabled = False
    command2.Enabled = True
    GetRecordsetData

    End Sub

    Private Sub Command2_Click()

    ' This code ends the process of repeatedly opening ADO recordsets.

    On Error Resume Next
    rst.Close
    MsgBox "The recordset was opened " & iExecutionCount & " time(s)."
    iExecutionCount = 0
    command1.Enabled = True
    command2.Enabled = False

    End Sub

    Private Sub GetRecordsetData()

    iExecutionCount = iExecutionCount + 1
    If rst.State <> adStateClosed Then
    rst.Close
    End If
    rst.Open _
    "Select * From Pubs..Publishers, Pubs..Titles, Pubs..Authors", _
    con, adOpenKeyset, adLockOptimistic, adAsyncExecute

    End Sub

    Private Sub con_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)

    ' When the ADO recordset has been populated with data, begin opening
    ' the next ADO recordset.
    GetRecordsetData

    End Sub
  5. Change the values of sServer, sUID, and sPWD in the Form_Load event of Form1 to valid values for your SQL Server environment.
  6. Run the project. Click Open Recordsets to begin the asynchronous opening and closing of the ADO resultsets.
  7. Select Cancel to stop opening the ADO recordsets. A message box displays telling you how many ADO recordsets were opened.
Propiedades

Id. de artículo: 190988 - Última revisión: 08/04/2008 - Revisión: 1

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