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SelectionChanging for a combobox in vb.net

Posted on 2012-03-14
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Last Modified: 2012-06-27
Hi

Is it possible to have a "SelectionChanging" event (rather than the SelecionChanged)?

The reason I ask is that I would find it extremely useful to prevent the selection change if certain criteria in the form weren't satisfied.

Note that I don't want to disable the combo to get round the problem.

Thanks in advance

John
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Question by:IssacJones
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4 Comments
 
LVL 40

Expert Comment

by:Kyle Abrahams
ID: 37721940
On your selection changed,

use the following pseudo_code:


if CriteriaMet then
   cmb.Tag = cmb.SelectedValue
else
   WarnUser()
   cmb.SelectedValue = cmb.Tag
end if
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Author Comment

by:IssacJones
ID: 37722059
Interesting. But won't result in recursive calls to selectionchanged?
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LVL 17

Expert Comment

by:nepaluz
ID: 37722089
I suppose the only way to achieve that is by having a custom method added to an inherited combobox. I have not tested this but the logic seems good.
Public Class xCombo
    Inherits ComboBox
    Private _ValidatingEventRaised As Boolean = False
    Protected Overrides Sub OnValidating(e As System.ComponentModel.CancelEventArgs)
        If Me.SelectedValue = "YourTest" Then
            _ValidatingEventRaised = True
            MyBase.OnValidating(e)
        End If
    End Sub
    Protected Overrides Sub OnValidated(e As System.EventArgs)
        If Not _ValidatingEventRaised Then Return
        MyBase.OnValidated(e)
    End Sub
End Class

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Accepted Solution

by:
Kyle Abrahams earned 250 total points
ID: 37722134
But won't result in recursive calls to selectionchanged?  

you can get around that by removing the handler before setting the value back:

RemoveHandler cmb.SelectedIndexChanged, AddressOf cmb_SelectedIndexChanged
cmb.SelectedValue = cmb.Tag
AddHandler cmb.SelectedIndexChanged, AddressOf cmb_SelectedIndexChanged


But the call is negligble as it's only going to set it back to what it was already.
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