Creating Event Handlers in VS .NET

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Creating Event Handlers
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' If the control has children, ' recursively call this function If ctrlHasChildren Then AddEventHandlers(ctrl) End If Next End Sub
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4 BUILDING THE USER INTERFACE LAYER
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The For loop processes each control that is physically on the form If the control has children, this method then calls itself passing in the control This causes the method to process any controls that are physically on the container control This repeats until all controls and all children of all controls are processed The code in this example sets only the handler for TextBox controls You can modify it to handle other controls by changing the TypeOf line as follows:
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If TypeOf ctrl Is TextBox OrElse TypeOf ctrl Is ComboBox Then
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This line of code sets the handler for TextBox and ComboBox controls You can add other control types as well by adding additional OrElse clauses
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NOTE: If you have not been using the OrElse and AndAlso operators, it is highly recommended that you use them instead of Or and And for your conditional expressions Using OrElse and AndAlso can improve your application s performance, because they allow short-circuiting Short-circuiting is the process whereby if the first expression defines the resulting condition of the statement, the second expression is not evaluated For example, in the preceding line of code, if the type of the control is a TextBox, the condition is true, and the code within the If statement is executed without evaluating the second expression
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Because these event handlers should be set up before the Enter events could occur, call the AddEventHandlers method in the Load event handler for the form, and pass in the form as the control to process
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Private Sub ProductWin_Load(ByVal sender As Object, _ ByVal e As SystemEventArgs) Handles MeLoad AddEventHandlers(Me) End Sub
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Here, Me refers to the currently running instance, which is the instance of the form that is being loaded The AddEventHandlers method starts with the controls on the form and adds event handlers for all the appropriate controls and all the children of all the controls The only thing left to do before this code is nished is documentation Add XML comments for each method (ProcessEnter, ProcessLeave, and AddEventHandlers) by inserting three comment markers (''') immediately in front of the Private keyword for the method For example, the ProcessEnter XML comments are as follows:
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''' <summary> ''' Sets the background color when the user enters the control ''' </summary> ''' <param name="sender">Control generating the event</param> ''' <param name="e">Event arguments</param> ''' <remarks></remarks> Private Sub ProcessEnter(ByVal sender As Object, _ ByVal e As SystemEventArgs) DirectCast(sender, Control)BackColor = ColorBurlyWood End Sub
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Notice how the XML comments automatically create the appropriate param tags based on the method s parameters See 5, Building the Business Logic Layer, for more information on documenting your methods using XML comments Use the AddHandler statement instead of the Handles clause on the event handler any time you want to add event handlers for a control and event at runtime The AddHandler statement is also required if you are adding the control at runtime
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For the Purchase Tracker sample application: Open the ProductWin form in the Code Editor Remove the Handles clause from the ProcessEnter and ProcessLeave event handlers Add XML comments for the methods Add the AddEventHandlers method to the form s Private Methods region using the code defined in this section Be sure to define XML comments for the method
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Create an event handler for the form s Load event, and call the AddEventHandlers method, passing Me as the parameter Run the application It displays your splash screen and then shows the ProductWin form Use the Tab key to move through the controls on the form The background color of each control changes to the defined color as you tab through it When you tab out of the control, the background color changes back If you are not seeing the background color change in the ComboBox control, check your AddEventHandlers method, and ensure that the TypeOf checks for both TextBox and ComboBox controls
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