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microsoft visual basic 2008 step by step phần 4

Những điều khiển này cho phép bạn theo dõi, quản lý, và xác thực người dùng mà không cần tạo sơ đồ của riêng bạn, dựa trên Active Directory, hoặc quản lý người sử dụng bởi các phương tiện khác. Trước khi phiên bản 2.0 của NET Framework, xác thực người dùng tùy chỉnh | Chapter 5 Visual Basic Variables and Formulas and the .NET Framework 143 The heart of the event procedure is a Select Case decision structure. In the next chapter we ll discuss how this group of program statements selects one choice from many. For now notice how each section of the Select Case block assigns a sample value to one of the fundamental data type variables and then assigns the variable to the Text property of the Label4 object on the form. I used code like this in Chapter 3 to process list box choices and you can use these techniques to work with list boxes and data types in your own programs. Note If you have more than one form in your project you need to declare variables in a slightly different way and place to give them scope throughout your program that is in each form that your project contains . The type of variable that you ll declare is a public or global variable and it s declared in a module a special file that contains declarations and procedures not associated with a particular form. For information about creating public variables in modules see Chapter 10 Creating Modules and Procedures. 12. Scroll through the ListBox1_SelectedIndexChanged event procedure and examine each of the variable assignments closely. Try changing the data in a few of the variable assignment statements and running the program again to see what the data looks like. In particular you might try assigning values to variables that are outside their accepted range as shown in the data types table presented earlier. If you make such an error Visual Basic adds a jagged line below the incorrect value in the Code Editor and the program won t run until you change it. To learn more about your mistake you can point to the jagged underlined value and read a short tooltip error message about the problem. Tip By default a green jagged line indicates a warning a red jagged line indicates a syntax error a blue jagged line indicates a compiler error and a purple jagged line indicates .