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Ther e ar e a couple of w ays of m anaging built - in ev ent s wit h Windows Form s and t heir cont r ols fr om t he Windows For m s Designer. Double- clicking a for m s capt ion in t he Windows Form s Designer opens t he st ub for t he form s default ev ent , t he Load ev ent , in t he Code Edit or. This sam e t echnique works for t he cont r ols on a for m . For ex am ple, double- click ing a but t on on a form opens t he st ub for t he but t on s default ev ent , a Click ev ent . Aft er adding one or m or e cont r ols on a for m , y ou can select any event for any cont r ol in t he Code Edit or. Choose t he cont r ol nam e from t he Class Nam e dr op- dow n list at t he upper left of t he Code Edit or , and choose t he ev ent nam e fr om t he Met hod Nam e list at t he r ight . Aft er you click an ev ent for t he cont r ol, a st ub for t he ev ent pr ocedur e appears aut om at ically. To display a nondefault event for t he form , select ( Base Class Ev ent s) from t he Class Nam e list and t hen choose a desir ed ev ent fr om t he Met hod Nam e list . I f you search t hrough t he event s for a form or any of t he cont r ols on a form , you will quickly discover an exceedingly large array of ev ent s. Alt hough t he large num ber of ev ent s is useful for fine- grained cont rol ov er t he operat ion of an applicat ion, it m ay be difficult for som e program m ers t o discern t he order of t he ev ent s so t hey can k now which one t o use. The follow ing excerpt fr om t he Code Edit or for Form 4 in t he Ev ent sSam ples solut ion dem onst rat es a st rat er gy for t racking ev ent s. Wit hin each ev ent pr ocedur e is a MsgBox funct ion indicat ing which ev ent generat ed t he cur rent m essage box in an applicat ion. For exam ple, t he m essage box for t he form Load ev ent fir es befor e Form 4 is display ed. When you click t he form s Close but t on, y ou w ill not ice t hat t he Closing ev ent fir es pr ior t o t he Closed ev ent . See t he follow ing not e for det ailed inst ruct ions on m aking Form 4 t he st art up obj ect for t he Ev ent sSam ples solut ion.
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A Windows applicat ion st art s by default wit h For m 1, which is t he obj ect t hat Visual St udio .NET m akes aft er opening a Windows applicat ion for design. By default , t he Windows applicat ion opens t o t his obj ect when you run t he solut ion. However, you can choose anot her obj ect for a Windows applicat ion t o open when it st art s t o r un. Right - click t he solut ion s nam e in Solut ion Explor er , and choose Propert ies t o open t he Propert y Pages dialog box for t he solut ion. Use t he St art up Obj ect drop- dow n list t o select anot her obj ect . For ex am ple, select ing For m 4 will cause t his form t o open init ially when a user chooses t o run t he solut ion.
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Ev ent s som et im es fire so quick ly t hat m essage boxes can pile up and m ake discover ing t heir order confusing. I n cases lik e t his, y ou can som et im es set a propert y for an obj ect on t he form and t hus change it s appearance t o help indicat e t he order of ev ent s. The procedures for t he MouseEnt er , MouseHov er, and MouseLeav e ev ent s fr om But t on1 dem onst rat e t his appr oach. These ev ent procedur es change t he Back Color pr opert y for But t on1. I nit ially posit ioning t he m ouse over But t on1 changes t he BackColor pr opert y from it s default set t ing t o Syst em .Drawing.Color .Cyan. Because Visual St udio aut om at ically cr eat es a reference t o t he Syst em .Draw ing nam espace w hen it init ializes a Windows applicat ion, y ou can abbreviat e t he set t ing t o Color. Cyan. Leav ing t he m ouse ov er a but t on event ually invok es t he MouseHov er ev ent , which changes t he Back Color set t ing t o Sy st em .Draw ing.Color.Red. Rem ov ing t he m ouse from ov er t he but t on rest or es t he default BackColor set t ing of
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Syst em .Drawing.Syst em Colors.Cont rol. Clicking But t on1 displays a m essage box and shift s t he focus fr om For m 4 t o t he m essage box. This But t on1_Click ev ent is ort hogonal t o t he MouseEnt er and MouseHover ev ent s in t hat click ing t he but t on can int errupt t he t r ansit ion fr om t he MouseEnt er event t o t he MouseHover event . Private Sub Form4_Load(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles MyBase.Load MsgBox( Just before I load. ) End Sub Private Sub Form4_Closing(ByVal sender As Object, _ ByVal e As System.ComponentModel.CancelEventArgs) _ Handles MyBase.Closing MsgBox( From Closing event. ) End Sub Private Sub Form4_Closed(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles MyBase.Closed MsgBox( From Closed event. ) End Sub Private Sub Button1_MouseEnter(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Button1.MouseEnter Me.Button1.BackColor = System.Drawing.Color.Cyan End Sub Private Sub Button1_MouseHover(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Button1.MouseHover Me.Button1.BackColor = System.Drawing.Color.Red End Sub Private Sub Button1_MouseLeave(ByVal sender As Object, _ ByVal e As System.EventArgs) Handles Button1.MouseLeave Me.Button1.BackColor = System.Drawing.SystemColors.Control End Sub Private Sub Button1_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Button1.Click MsgBox( You clicked Button1 ) End Sub Befor e proceeding t o a second sam ple, it m ay be useful t o r ev iew t he synt ax for an ev ent pr ocedure. Not ice t hat t hey ar e sub pr ocedur es m eant for operat ion in t he cur rent m odule, as specified by t he use of t he Privat e keyw ord. Pr ivat e m arks t he ev ent procedur e for exclusive use in t he cur rent m odule. The argum ent s list can offer you a way of changing t he operat ion of t he ev ent pr ocedur e. The nex t sam ple dem onst rat es t he use of an ev ent argum ent t o cont r ol t he behavior of t he Closing event . Aft er t he argum ent list , t he Handles clause specifies t he obj ect and ev ent t hat t he sub procedur e handles. You cont rol t he operat ion of t he ev ent procedur e by placing cust om code bet ween t he Sub and End Sub st at em ent s. The next select ion of ev ent pr ocedur es shows a pair of procedures for cont rolling how a user can close a form . When a user chooses t o close a form by clicking t he for m s Close but t on, t he applicat ion fir es t he Closing ev ent . This event occurs befor e t he form closes. By set t ing t he Cancel ev ent argum ent t o Tr ue in t he Closing event , you can block t he Close ev ent fr om occurr ing ( nam ely , t he form will rem ain open) . The default value for t he Cancel ev ent argum ent is False. You can use t his feat ur e t o perform ot her act ions j ust before closing a for m . For exam ple, y ou can display a m essage blocking t he operat ion of t he form s Close but t on and inst ruct ing t he user t o click a but t on t hat launches t he ot her act ions you want done befor e invok ing t he form s Close m et hod. Because t he Close
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m et hod raises t he Closing event , y ou m ust const ruct t he form s Closing ev ent procedur e t o opt ionally by pass set t ing t he Cancel argum ent t o Tr ue. The follow ing code excerpt for For m 5 dem onst r at es how t o disable a form s Close but t on and redirect t he user t o a but t on on t he for m . The solut ion uses t wo ev ent s. First t he Form 5_Closing event procedure blocks t he Close ev ent from occurr ing by set t ing t he Cancel ev ent argum ent t o bolDisableClose. The m odulelevel declarat ion for bolDisableClose set s t he variable s default value t o Tr ue. The I f Then Else st at em ent in t he pr ocedur e displays a m essage box dir ect ing t he user t o click But t on1 t o close t he form . The Click ev ent pr ocedur e for But t on1 set s bolDisableClose t o False befor e inv ok ing t he Close m et hod for t he Me keyw ord t hat r efers back t o t he curr ent form , w hich is Form 5 in t his case. The inv ocat ion of t he Close m et hod, in t ur n, launches t he Form 5_Closing event procedur e, but t his t im e t he pr ocedur e t ak es a different pat h t hr ough it s I f Then Else st at em ent because of t he new value for t he bolDisableClose variable. bolDisableClose controls Cancel argument. Dim bolDisableClose As Boolean = True Conditionally block close of form. Private Sub Form5_Closing(ByVal sender As Object, _ ByVal e As System.ComponentModel.CancelEventArgs) _ Handles MyBase.Closing If bolDisableClose Then e.Cancel = bolDisableClose MsgBox( Click Button1 to close form.", , _ After clicking Close button ) Else MsgBox( From form s Closing event.", , _ After clicking Button1 ) End If End Sub Enable form close by setting bolDisableClose to False. Private Sub Button1_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Button1.Click Perform any other necessary actions before closing Form5. bolDisableClose = False Me.Close() End Sub
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