【正文】 s mercial implementation of the mon language infrastructure (CLI), an international standard that is the basis for creating execution and development environments in which languages and libraries work together seamlessly. Source code written in C is piled into an intermediate language (IL) that conforms to the CLI specification. The IL code, along with resources such as bitmaps and strings, is stored on disk in an executable file called an assembly, typically with an extension of .exe or .dll. An assembly contains a manifest that provides information on the assembly39。s entry point, are encapsulated within class definitions. A class may inherit directly from one parent class, but it may implement any number of interfaces. Methods that override virtual methods in a parent class require the override keyword as a way to avoid accidental redefinition. In C, a struct is like a lightweight class。 you don39。s Visible property has the value False, the control is not displayed on the page。t piled into native machine code until it is actually requested by a browser. At that point, the class file contained in the Temporary Files directory is piled with the .NET framework Just in Time (JIT) piler and executed. The magical aspect of this whole process is that it happens automatically in the background. All you have to do is create a text file with the source code for your page. Forms with Web Server Controls Useingseveral of the basic Web controls to represent standard HTML form elements such as radio buttons, text boxes, and list boxes. You can use these controls in your pages to create the user interface for your Web application.. Form Validation with Validation Controls Traditionally, Web developers have faced a tough choice when adding form validation logic to their pages. You can add form validation routines to your serverside code, or you can add the validation routines to your clientside code. The advantage of writing validation logic in clientside code is that you can provide instant feedback to your users. For example, if a user neglects to enter a value in a required form field, you can instantly display an error message without requiring a roundtrip back to the really like clientside validation. It looks great and creates a better overall user experience. The problem, however, is that it does not work with all browsers. Not all browsers support JavaScript, and different versions of browsers support different versions of JavaScript, so clientside validation is never guaranteed to this reason, in the past, many developers decided to add all their form validation logic exclusively to serverside code. Because serverside code functions correctly with any browser, this course of action was safer. At the same time, The Validation controls automatically generate both clientside and serverside code. If a browser is capable of supporting JavaScript, clientside validation scripts are automatically sent to the browser. If a browser is incapable of supporting JavaScript, the validation routines are automatically implemented in serverside code. 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 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 parisons 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. Control Programming Working with View State By default, almost all controls retain the values of their properties between form posts. For example, if you assign text to a Label control and submit