How to Add Code and Validation Controls to a Form
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Welcome to the fourth part of a five-part article series on building web applications with ASP.NET. In this part our form really starts taking shape, as we add validation controls -- something which belongs on every web-facing form. This article is excerpted from chapter two of
Murach's ASP.NET 3.5 Web Programming with VB 2008, written by Anne Boehm (Murach, 2008; ISBN: 1890774472).
How to add validation controls to a form
A validation control is a type of ASP.NET control that’s used to validate input data. The topics that follow introduce you to the validation controls and show you how to use two of the commonly used controls. Then, in chapter 7, you can learn all the skills that you need to master the use of these controls.
An introduction to the validation controls
Figure 2-13 shows the Validation group in the Toolbox. It offers five controls that can be called validators. These are the controls that you use to check that the user has entered valid data. You can use the last control in this group, the validation summary control, to display all the errors that have been detected by the validators on the form.
The easiest way to add a validation control to a web form is to drag it from the Toolbox. In this example, four validators have been added to the form: two required field validators and two range validators. In this case, the controls have been added below the table so ASP.NET will use flow layout to position the controls. However, these controls could have been added to a third column of the table. Although these controls aren’t displayed when the form is displayed, the messages in their ErrorMessage properties are displayed if errors are detected.
In this case, the first required field validator checks to make sure that a value has been entered in the text box for the interest rate, and the first range validator checks to make sure that this value ranges from 1 to 20. Similarly, the second required field validator checks to make sure that a value has been entered in the text box for the number of years, and the second range validator checks to make sure that this value ranges from 1 to 45.
Validation tests are typically done on the client before the page is posted to the server. That way, a round trip to the server isn’t required to display error messages if any invalid data is detected.
In most cases, client-side validation is done when the focus leaves an input control that has validators associated with it. That can happen when the user presses the Tab key to move to the next control or clicks another control to move the focus to that control. Validation is also done when the user clicks on a button that has its CausesValidation property set to True.
To perform client-side validation, a browser must support Dynamic HTML, or DHTML. Because most browsers in use today support DHTML, validation can usually be done on the client. However, validation is always done on the server too when a page is submitted. ASP.NET does this validation after it initializes the page.
When ASP.NET performs the validation tests on the server, it sets the IsValid property of each validator to indicate if the test was successful. In addition, after all the validators are tested, it sets the IsValid property of the page to indicate if all the input data is valid. You can test this property in the event handler for the event that causes the page to be posted to the server. You’ll see how this works when you review the code-behind file for this form.
The validation controls on the Future Value form
Description
You can use validation controls to test user input and produce error messages. The validation is performed when the focus leaves the control that’s being validated and also when the user clicks on a button control whose CausesValidation property is set to True.
Each validation control is associated with a specific server control, but you can associate one or more validation controls with a single server control.
The validation controls work by running client-side script. Then, if the validation fails, the page isn’t posted back to the server. However, the validation is also performed on the server in case the client doesn’t support scripts.
If the client doesn’t support scripts, you can test whether validation has been successful on the server by testing whether the IsValid property of the page is True.
--------------------------------------------Figure 2-13 An introduction to the validation controls
Next: How to use the required field validator >>
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 This article is an excerpt from chapter two of Murach's ASP.NET 3.5 Web Programming with VB 2008, written by Anne Boehm (Murach, 2008; ISBN: 1890774472). Check it out today at your favorite bookstore. Buy this book now.
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