The application with the DropDownList control in C#.NET

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The application with the DropDownList control
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12: Web Forms: Client-Side Programs
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In order to populate the control's Items collection when the page is loaded you need to call this method when the form is displayed, you add a call to the method in the OnInit() method
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override protected void OnInit(EventArgs e) { InitializeComponent(); populateList(); baseOnInit(e); }
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When you run this application now, it will operate just as the previous version did, but you have now taken programmatic control of the DropDownList contents and can dynamically extend your application with little or no coding changes (see the following illustration)
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PART III
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After you have run the application a few times, you might decide that it is a waste of steps to first select the text and then have to click on the button why not eliminate the clicking of the button Indeed, you can indeed change the behavior of the application
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To make a selection in the DropDownList box trigger the update of the label, you can include the update code in the SelectedIndexChanged event handler Just add this line to handler:
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lblHelloText = strHello;
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If you run the application now, you will notice that there is no change to the label until you click the button this is the default way events are handled Remember that the event code is running on the server, based on the codebehind module In order for the event to be transmitted to the server (codebehind module) the client must initiate a postback sequence The postback sends the form data to the server, by default, the Button (and all components that inherit from the Button) is the only control that will start the postback, all other events that were created at the client are cached and transmitted the next time there is a postback It turns out to be easy to add the behavior that triggers a postback to the Drop DownList (or any other control) by adding the AutoPostBack="true" attribute to the definition of the control:
<asp:dropdownlist id=cmbHello runat="server" AutoPostBack="True"/>
The default is for the event to be handled when the server is called by a control that sends a postback, but you can control which object sends that postback message by setting the AutoPostBack attribute of a control to True When you run the application, you will now have the behavior you expected from the application We will revisit these controls later in this chapter, but first we need to have a look at the validation controls
Validation Controls
There is an old adage in computer science: garbage in = garbage out We ll look here at one thing you can do to validate the data before it is processed, to make sure garbage doesn t get in This validation can never verify that the data is absolutely correct that type of data validation must be performed closer to the data source, where the data is used The ASPNET validation controls simply validate that the data is in the correct range or style Table 12-3 lists the controls that are available
Adding Validation Controls
The best way to learn about the controls is to use them in a solution, so we will start a new project Call the project WebApplication3 and use localhost as the server
12: Web Forms: Client-Side Programs
Description
RequiredFieldValidator CompareValidator
RangeValidator RegularExpressionValidator CustomValidator
Used to verify that the user has entered data into the field Compares the value entered to a specified value This validating control uses two properties: Operator and ValueToCompare to define the validation Operator can be set to Equal, GreaterThan, GreaterThanEqual, LessThan, LessThanEqual, NotEqual, or DataTypeCheck The ValueToCompare property contains the value to compare against Validates the control based on a range of acceptable values Validates the control against the regular expression in ValidationExpression Uses the ClientValidationFunction property to set the client-side function that will be used for validation This means that you need to write the code in a scripting language and not in C#
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