Dropdown with combo box closes unexpectedly when managing a tab
I have a modal form with one instance of the .NET 2.0 built-in tab control. The tab control has multiple pages, and one of them has a combo box that is not populated until the user activates it the first time. When this happens, I handle the DropDown event and start a process that takes a few seconds, then I add the items returned by that process to the combo box.
It works great, except that immediately after the list portion of the combo box has dropped, it closes immediately, as if some other control was taking focus. I've narrowed it down to the fact that there is a tab control in the form, and the process that retrieves the items for the combo box takes over 4 seconds. If I create a completely empty form with just a combo box, I can't see it.
Needless to say, this is strange outside of faith. Any idea why the tab control would interfere with the control that currently has focus?
EDIT:
Here is the event handler code for this combo box. Basically I am creating a list of SQL servers on the network. The thing that takes a few seconds is the GetDataSources call.
Private Sub cmbServer_DropDown(ByVal sender As Object, ByVal e As System.EventArgs) Handles cmbServer.DropDown
Dim oTable As DataTable
Dim lstServers As List(Of String)
Dim lstAliases As List(Of String)
Try
If cmbServer.Items.Count = 0 Then
System.Windows.Forms.Cursor.Current = System.Windows.Forms.Cursors.WaitCursor
oTable = System.Data.Sql.SqlDataSourceEnumerator.Instance.GetDataSources
lstServers = New List(Of String)
For Each oRow As DataRow In oTable.Rows
If oRow("InstanceName").ToString = "" Then
lstServers.Add(oRow("ServerName").ToString)
Else
lstServers.Add(oRow("ServerName").ToString & "\" & oRow("InstanceName").ToString)
End If
Next oRow
'Retrieve any server aliases on the client and add them to the server list
lstAliases = GetSQLServerAliases()
If lstAliases IsNot Nothing Then
For Each sAlias As String In lstAliases
lstServers.Add(sAlias)
Next sAlias
End If
lstServers.Sort()
For Each sServer As String In lstServers
cmbServer.Items.Add(sServer)
Next sServer
End If
Catch ex As Exception
ErrHandler("frmRefreshDB", "cmbServer_DropDown", ex.Source, ex.Message, ex.InnerException)
Finally
System.Windows.Forms.Cursor.Current = System.Windows.Forms.Cursors.Default
If oTable IsNot Nothing Then
oTable.Dispose()
End If
End Try
End Sub
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I was able to resolve this issue.
I first added a breakpoint to the LostFocus event in the combo box and looked at the stack trace as suggested by Steve Dignan, but that didn't find anything. One feature of my form is that this combo box is usually disabled and only activated when the user checks the field on the same bookmark. The solution was to explicitly set focus on the combo box when the field was checked, like this:
Private Sub chkAltServer_CheckedChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles chkAltServer.CheckedChanged
Try
If chkAltServer.Checked Then
UnlockControl(cmbServer)
cmbServer.Focus()
Else
LockControl(cmbServer)
End If
Catch ex As Exception
ErrHandler("frmOptions", "chkAltServer_CheckedChanged", ex.Source, ex.Message, ex.InnerException)
End Try
End Sub
The combo box dropdown showed fine.
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