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Validating text box in vb

A non-trivial application may also display additional windows to do the following: Display specific information to users. Gather information from users. Both display and gather information. These types of windows are known as dialog boxes, and there are two types: A modal dialog box is displayed by a function when the function needs additional data from a user to continue. Because the function depends on the modal dialog box to gather data, the modal dialog box also prevents a user from activating other windows in the application while it remains open. In most cases, a modal dialog box allows a user to signal when they have finished with the modal dialog box by pressing either an OK or Cancel button. Pressing the OK button indicates that a user has entered data and wants the function to continue processing with that data. Pressing the Cancel button indicates that a user wants to stop the function from executing altogether. The most common examples of modal dialog boxes are shown to open, save, and print data. A modeless dialog box, on the other hand, does not prevent a user from activating other windows while it is open. For example, if a user wants to find occurrences of a particular word in a document, a main window will often open a dialog box to ask a user what word they are looking for. Since finding a word doesn't prevent a user from editing the document, however, the dialog box doesn't need to be modal. A modeless dialog box at least provides a Close button to close the dialog box, and may provide additional buttons to execute specific functions, such as a Find Next button to find the next word that matches the find criteria of a word search. Windows Presentation Foundation WPF allows you to create several types of dialog boxes, including message boxes, common dialog boxes, and custom dialog boxes. This topic discusses each, and the Dialog Box Sample provides matching examples. Message Boxes A message box is a dialog box that can be used to display textual information and to allow users to make decisions with buttons. The following figure shows a message box that displays textual information, asks a question, and provides the user with three buttons to answer the question. To create a message box, you use the MessageBox class. MessageBox lets you configure the message box text, title, icon, and buttons, using code like the following. Warning To show a message box, you call the static Show method, as demonstrated in the following code. Show messageBoxText, caption, button, icon When code that shows a message box needs to detect and process the user's decision which button was pressed , the code can inspect the message box result, as shown in the following code. Yes ' User pressed Yes button ' No ' User pressed No button ' Cancel ' User pressed Cancel button ' Although MessageBox may offer a simple dialog box user experience, the advantage of using MessageBox is that is the only type of window that can be shown by applications that run within a partial trust security sandbox see Security , such as XAML browser applications XBAPs. Most dialog boxes display and gather more complex data than the result of a message box, including text, selection check boxes , mutually exclusive selection radio buttons , and list selection list boxes, combo boxes, drop-down list boxes. For these, Windows Presentation Foundation WPF provides several common dialog boxes and allows you to create your own dialog boxes, although the use of either is limited to applications running with full trust. Common Dialog Boxes Windows implements a variety of reusable dialog boxes that are common to all applications, including dialog boxes for opening files, saving files, and printing. Since these dialog boxes are implemented by the operating system, they can be shared among all the applications that run on the operating system, which helps user experience consistency; when users are familiar with the use of an operating system-provided dialog box in one application, they don't need to learn how to use that dialog box in other applications. Because these dialog boxes are available to all applications and because they help provide a consistent user experience, they are known as common dialog boxes. Windows Presentation Foundation WPF encapsulates the open file, save file, and print common dialog boxes and exposes them as managed classes for you to use in standalone applications. This topic provides a brief overview of each. Open File Dialog The open file dialog box, shown in the following figure, is used by file opening functionality to retrieve the name of a file to open. The common open file dialog box is implemented as the OpenFileDialog class and is located in the Microsoft. The following code shows how to create, configure, and show one, and how to process the result. Note OpenFileDialog can be used to safely retrieve file names by applications running with partial trust see Security. Save File Dialog Box The save file dialog box, shown in the following figure, is used by file saving functionality to retrieve the name of a file to save. The common save file dialog box is implemented as the SaveFileDialog class, and is located in the Microsoft. Print Dialog Box The print dialog box, shown in the following figure, is used by printing functionality to choose and configure the printer that a user would like to print data to. The common print dialog box is implemented as the PrintDialog class, and is located in the System. The following code shows how to create, configure, and show one. Custom Dialog Boxes While common dialog boxes are useful, and should be used when possible, they do not support the requirements of domain-specific dialog boxes. In these cases, you need to create your own dialog boxes. As we'll see, a dialog box is a window with special behaviors. Window implements those behaviors and, consequently, you use Window to create custom modal and modeless dialog boxes. Creating a Modal Custom Dialog Box This topic shows how to use Window to create a typical modal dialog box implementation, using the Margins dialog box as an example see Dialog Box Sample. The Margins dialog box is shown in the following figure. Configuring a Modal Dialog Box The user interface for a typical dialog box includes the following: The various controls that are required to gather the desired data. Showing an OK button that users click to close the dialog box, return to the function, and continue processing. Showing a Cancel button that users click to close the dialog box and stop the function from further processing. Showing a Close button in the title bar. Showing Minimize, Maximize, and Restore buttons. Showing a System menu to minimize, maximize, restore, and close the dialog box. Opening above and in the center of the window that opened the dialog box. Dialog boxes should be resizable where possible so, to prevent the dialog box from being too small, and to provide the user with a useful default size, you need to set both default and a minimum dimensions respectively. Pressing the ESC key should be configured as a keyboard shortcut that causes the Cancel button to be pressed. This is achieved by setting the IsCancel property of the Cancel button to true. This is achieved by setting the IsDefault property of the OK button true. The following code demonstrates this configuration. When a menu item runs a function that requires user interaction through a dialog box before the function can continue, the menu item for the function will have an ellipsis in its header, as shown here. Opening a Modal Dialog Box A dialog box is typically shown as a result of a user selecting a menu item to perform a domain-specific function, such as setting the margins of a document in a word processor. Showing a window as a dialog box is similar to showing a normal window, although it requires additional dialog box-specific configuration. The entire process of instantiating, configuring, and opening a dialog box is shown in the following code. Margin ' Open the dialog box modally dlg. It is also setting the Window. Owner property with a reference to the window that is showing the dialog box. In general, you should always set the owner for a dialog box to provide window state-related behaviors that are common to all dialog boxes see WPF Windows Overview for more information. After the dialog box is configured, it is shown modally by calling the ShowDialog method. Validating User-Provided Data When a dialog box is opened and the user provides the required data, a dialog box is responsible for ensuring that the provided data is valid for the following reasons: From a security perspective, all input should be validated. From a domain-specific perspective, data validation prevents erroneous data from being processed by the code, which could potentially throw exceptions. From a user-experience perspective, a dialog box can help users by showing them which data they have entered is invalid. From a performance perspective, data validation in a multi-tier application can reduce the number of round trips between the client and the application tiers, particularly when the application is composed of Web services or server-based databases. To validate a bound control in WPF, you need to define a validation rule and associate it with the binding. A validation rule is a custom class that derives from ValidationRule. The following example shows a validation rule, MarginValidationRule, which checks that a bound value is a Double and is within a specified range. To associate the validation rule with the bound control, you use the following markup. When a control contains invalid data, WPF will display a red border around the invalid control, as shown in the following figure. WPF does not restrict a user to the invalid control until they have entered valid data. This is good behavior for a dialog box; a user should be able to freely navigate the controls in a dialog box whether or not data is valid. However, this means a user can enter invalid data and press the OK button. For this reason, your code also needs to validate all controls in a dialog box when the OK button is pressed by handling the Click event. GetHasError node ; if! IsValid DirectCast subnode, DependencyObject Then ' If a child dependency object is invalid, return false immediately, ' otherwise keep checking Return False End If Next ' All dependency objects are valid Return True End Function End Class End Namespace This code enumerates all dependency objects on a window and, if any are invalid as returned by GetHasError , the invalid control gets the focus, the IsValid method returns false, and the window is considered invalid. Once a dialog box is valid, it can safely close and return. As part of the return process, it needs to return a result to the calling function. When ShowDialog returns, the code that called it needs to decide whether to continue processing or stop processing, based on whether the user pressed the OK button or the Cancel button. To facilitate this decision, the dialog box needs to return the user's choice as a Boolean value that is returned from the ShowDialog method. When the OK button is clicked, ShowDialog should return true. This is achieved by setting the DialogResult property of the dialog box when the OK button is clicked. When the Cancel button is clicked, ShowDialog should return false, which also requires setting the DialogResult property. The following markup has the same effect as the preceding code, without the need to handle the Click event. Note After ShowDialog has returned, a dialog box cannot be reopened. Instead, you need to create a new instance. If the dialog result is false, the function should end processing appropriately. Creating a Modeless Custom Dialog Box A modeless dialog box, such as the Find Dialog Box shown in the following figure, has the same fundamental appearance as the modal dialog box. Validating text box in vb

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  1. From a performance perspective, data validation in a multi-tier application can reduce the number of round trips between the client and the application tiers, particularly when the application is composed of Web services or server-based databases. Does not interact with the underlying code, and can test any application including those that can cause problems for other tools such as Flash, Silverlight, etc.

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