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THE FORM.CLOSE METHOD
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As you can see, this method receives a CancelEventArgs object. This class defines a Cancel property to help determine whether the application will actually exit. If this property is set to true by an override of the OnClosing method or a Closing event handler, then the close operation is cancelled and the application will continue to run. The Cancel property has a default value of false, so that the close operation is not cancelled and the application will exit. We will override the OnClosing method in our MainForm class to make sure the CloseCurrentAlbum method is called regardless of how the application exits.
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Set the version number of the MyPhotos application to 8.2. OVERRIDE THE ONCLOSING METHOD Action 1 Override the OnClosing method in the MainForm.cs source window. Within this method, call the
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CloseCurrentAlbum
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protected override void OnClosing (CancelEventArgs ce) {
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if (this.CloseCurrentAlbum() == false)
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method to see if the current album should be saved. 3 If the user clicked the Cancel button, then cancel the close operation.
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ce.Cancel = true;
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Note: This cancels the Close operation so that the application does not exit.
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else ce.Cancel = false;
Otherwise, allow the application to close.
Note: Since false is the default value, these lines are not strictly required. They are here simply to illustrate the setting when the application is permitted to exit. 5 Remember to call
OnClosing in the base
} base.OnClosing(ce);
class.
Note: This call ensures that logic internal to the Form class is performed, and ensures that any Closing event handlers for the form are called before the application exits. Of course, any registered handler can prevent the application from exiting by setting ce.Cancel to true.
Compile and run the application to see this method in action. Add a few photos and try to exit the application using the Exit menu, the Alt+F4 key, and the Close option from the system menu. In all cases, you should be queried by the CloseCurrentAlbum method with the question dialog for saving the current album. If you select the Cancel button the application will not, in fact, exit.
CHA PTE R 8
DIALOG BOXES
Before we go on, we should point out that our OnClosing override can be written more succinctly by taking advantage of the boolean value returned by our close album method.
protected override void OnClosing(CancelEventArgs ce) { ce.Cancel = (!this.CloseCurrentAlbum()); base.OnClosing(ce); }
Now that we know all about closing a dialog box, let s see how to create one of our own.
MODAL DIALOG BOXES
In earlier chapters, we added controls such as a Button, PictureBox, and StatusBar to our main form, and displayed and managed these objects within the Form class on behalf of our application. In this section we will see how a dialog box can be created and displayed to further our understanding of the Form object. As a way to introduce this concept, we will add the ability to assign a caption to an image. This caption will be a text string supplied by the user. The dialog box shown in figure 8.5 will allow the user to modify this value. The base file name of the image will be used as the default caption.
Figure 8.5 Our dialog box will contain three text labels, a text box, and two buttons.
In order to support this dialog, we will need to modify three aspects of our application:
Data layer. Our Photograph class must support a caption on an image, and our PhotoAlbum class must store and retrieve these captions when saving and opening files. Presentation layer. We need a class to display our form as a dialog box. We will call this class CaptionDlg. This class must provide the interface and a means for returning a new caption value set by the user. Application layer. Our MainForm class must provide access to the new interface, and the link between the interface layer in our CaptionDlg class and the data layer in our MyPhotoAlbum library. We will address each of these layers separately in order to create our new dialog.
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