For this reason, in the past, many developers decided to add all their form validation logic exclusively to server-side code. Because server-side code functions correctly with any browser, this course of action was safer.本文来自辣.文,论'文·网原文请找腾讯752018766
Fortunately, the Validation controls discussed in this chapter do not force you to make this difficult choice. The Validation controls automatically generate both client-side and server-side code. If a browser is capable of supporting JavaScript, client-side validation scripts are automatically sent to the browser. If a browser is incapable of supporting JavaScript, the validation routines are automatically implemented in server-side code.
You should be warned, however, that client-side validation works only with Microsoft Internet Explorer version 4.0 and higher. In particular, the client-side scripts discussed in this chapter do not work with any version of Netscape Navigator.
Requiring Fields: The RequiredFieldValidator Control
You use RequiredFieldValidator in a Web form to check whether a control has a value. Typically, you use this control with a TextBox control. However, nothing is wrong with using RequiredFieldValidator with other input controls such as RadioButtonList.
Validating Expressions: The RegularExpressionValidator Control
You can use RegularExpressionValidator to match the value entered into a form field to a regular expression. You can use this control to check whether a user has entered, for example, a valid e-mail address, telephone number, or username or password. Samples of how to use a regular expression to perform all these validation tasks are provided in the following sections.
Comparing Values: The CompareValidator Control
The CompareValidator control performs comparisons between the data entered into a form field and another value. The other value can be a fixed value, such as a particular number, or a value entered into another control.
Summarizing Errors: The ValidationSummary Control
Imagine that you have a form with 50 form fields. If you use only the Validation controls discussed in the previous sections of this chapter to display errors, seeing an error message on the page might be difficult. For example, you might have to scroll down to the 48th form field to find the error message.
Fortunately, Microsoft includes a ValidationSummary control with the Validation controls. You can use this control to summarize all the errors at the top of a page, or wherever else you want.
4. Advanced Control Programming
Working with View State
By default, almost all ASP.NET controls retain the values of their properties between form posts. For example, if you assign text to a Label control and submit the form, when the page is rendered again, the contents of the Label control are preserved.
The magic of view state is that it does not depend on any special server or browser properties. In particular, it does not depend on cookies, session variables, or application variables. View state is implemented with a hidden form field called VIEWSTATE that is automatically created in every Web Forms Page.
When used wisely, view state can have a dramatic and positive effect on the performance of your Web site. For example, if you display database data in a control that has view state enabled, you do not have to return to the database each time the page is posted back to the server. You can automatically preserve the data within the page's view state between form posts.
Displaying and Hiding Content
Imagine that you are creating a form with an optional section. For example, imagine that you are creating an online tax form, and you want to display or hide a section that contains questions that apply only to married tax filers.
Or, imagine that you want to add an additional help button to the tax form. You might want to hide or display detailed instructions for completing form questions depending on a user's preferences.
Finally, imagine that you want to break the tax form into multiple pages so that a person views only one part of the tax form at a time.
In the following sections, you learn about the properties that you can use to hide and display controls in a form. You learn how to use the Visible and Enabled properties with individual controls and groups of controls to hide and display page content.
Using the Visible and Enabled Properties
Every control, including both HTML and Web controls, has a Visible property that determines whether the control is rendered. When a control's Visible property has the
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