Why is Textbox Validating Event Fired Twice?

Question

I'm trying to figure out why my textbox called tbxDiscountUnit validating event fires when I programmatically set focus the textbox in the event.  I know what you're think..."why are you setting focus the textbox in the validating event, just set e.Cancel = True, and focus will be returned."  Here is the reason why I want to set focus the textbox in the validating event.  I only want to validate the textbox when the user has entered something in the textbox and leaves OR when its parent (usercontrol DiscountAdderRow) is validating.  When the DiscountAdderRow validating event is fired I call the tbxDiscountUnit validating event.  If the textbox text isn't valid I want to set focus the textbox and select all the text for the user to correct.   You can see this in my code.

For some reason, when I include "Me.tbxDiscountUnit.Focus()" in the tbxDiscountUnit_Validating event the event fires twice, but when I remove the line of code it only fires once, why?

Additionally, 

Public Class DiscountAdderRow

    Private Sub tbxDiscountUnit_Validating(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles tbxDiscountUnit.Validating

        Try
            If Me.rbnFixed.Checked Then
                Try
                    ' tbxDiscountUnit only needs to be validated when the user inputs something other than white space or DiscountAdderRow is validated
                    If (sender.Equals(Me.tbxDiscountUnit) And Not String.IsNullOrEmpty(Trim(Me.tbxDiscountUnit.Text))) Or sender.Equals(Me) Then
                        FormatCurrency(Me.tbxDiscountUnit.Text)
                    End If
                Catch ex As InvalidCastException
                    MsgBox("The Discount Amount must be proper currency in this format  $#,###.##", MsgBoxStyle.Critical)
                    e.Cancel = True
                    Me.tbxDiscountUnit.Focus()
                    Me.tbxDiscountUnit.SelectAll()
                End Try
            End If

            If Me.rbnPercent.Checked Then
                Try
                    ' tbxDiscountUnit only needs to be validated when the user inputs something other than white space or DiscountAdderRow is validated
                    If (sender.Equals(Me.tbxDiscountUnit) And Not String.IsNullOrEmpty(Trim(Me.tbxDiscountUnit.Text))) Or sender.Equals(Me) Then
                        FormatPercent(Double.Parse(Me.tbxDiscountUnit.Text.Replace("%", "")) / 100, Globalization.NumberStyles.Number)
                    End If
                Catch ex As FormatException
                    MsgBox("The Discount Amount must be proper percentage in this format  ##.#%", MsgBoxStyle.Critical)
                    e.Cancel = True
                    Me.tbxDiscountUnit.Focus()
                    Me.tbxDiscountUnit.SelectAll()
                End Try
            End If

        Catch ex As Exception
            Call PublicProcedures.AppErrorHandler(Me.GetType.Name, System.Reflection.MethodBase.GetCurrentMethod().Name, ex)
            e.Cancel = True
        End Try

    End Sub

    Private Sub DiscountAdderRow_Validating(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles Me.Validating
        If String.IsNullOrEmpty(Trim(Me.tbxDescription.Text)) Then
            MsgBox("You must enter a description.")
            e.Cancel = True
            Exit Sub
        End If
        Me.tbxDiscountUnit_Validating(sender, e)
    End Sub

    Private Sub DiscountAdderRow_Validated(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Validated
        Me.lblLastModifiedDate.Text = Now.ToString
        Me.lklLastModifiedUser.Text = My.Settings.UserFullName
    End Sub
End Class

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Ryan

Answer 1

For some reason, when I include "Me.tbxDiscountUnit.Focus()" in the tbxDiscountUnit_Validating event the event fires twice, but when I remove the line of code it only fires once, why?

Change your code so that you can distinguish between an event raised by the user changing the focus, and an event raised by your code changing the focus.

Static IsValidating As Boolean = False
If Not IsValidating Then

  Processing = True
   ...
  Processing = False

End Sub

Answer 2

Thanks Acamar for the reply.  Your suggestion seems to be a work around, but I'd like to understand why the validating event is being called twice?

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Ryan

Answer 3

I'm trying to figure out why my textbox called tbxDiscountUnit validating event fires when I programmatically set focus the textbox in the event.

That's not a good idea.

Based on this MSDN document:

"Do not attempt to set focus from within the Enter, GotFocus, Leave, LostFocus, Validating, or Validated event handlers. Doing so can cause your application or the operating system to stop responding."

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Please call me Frank :)

Answer 4

I would have given the same reply as Acamar.

The events in the windows forms control simply have an order in which they always will be called by actions. 

http://msdn.microsoft.com/en-us/library/86faxx0d(v=vs.110).aspx

so when you do focus the focus event is given followed by the others among one the validationg even. 

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Success
Cor