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CHA PTE R 8
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The three buttons in our dialog will correspond to the following behavior in our
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CloseCurrentAlbum method:
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Yes will save the album, then close the album and return true. No will not save the album, then close the album and return true. Cancel will not save or close the album and return false to indicate that the calling operation should be canceled. To close the album, CloseCurrentAlbum will clear the album and related settings. The following steps create this method:
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ADD A CLOSECURRENTALBUM METHOD Action 1 Add the
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CloseCurrentAlbum
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Result
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protected bool CloseCurrentAlbum() {
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method to the MainForm.cs source code window. 2 Offer to save the album if it has been modified. Define an appropriate message to display. Note: We vary the message text depending on whether the current album has a name or not. 4 Display the message box and record the result.
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if (_bAlbumChanged) { // Offer to save the current album string msg; if (_album.FileName == null) msg = "Do you want to save the " + "current album "; else msg = String.Format("Do you want to " + "save your changes to \n{0} ", _album.FileName); DialogResult result = MessageBox.Show(this, msg, "Save Current Album ", MessageBoxButtons.YesNoCancel, MessageBoxIcon.Question); if (result == DialogResult.Yes) menuSave_Click(this,EventArgs.Empty); else if (result == DialogResult.Cancel) { // Do not close the album return false; } } // Close the album and return true if (_album != null) _album.Dispose(); _album = new PhotoAlbum(); SetTitleBar(); _bAlbumChanged = false; return true; }
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Perform the action requested by the user.
Close the album and return
true.
Note: This action is only performed if the Yes or No button was selected.
We will use this new method in three different places to ensure that the user has the option of saving any changes he or she might make to the album. MESSAGE BOXES 231
In menuNew_Click to save the existing album before a new album is created. In menuOpen_Click to save the album before a new album is selected. In menuExit_Click to save the album before the application exits. We will modify the handlers for the New and Open menus here. The Exit menu presents some additional issues, which we will take up in the next section. The following table continues our previous steps.
UPDATE THE HANDLERS FOR THE NEW AND OPEN MENUS Action 7 Modify the menuNew_Click method to use the CloseCurrentAlbum method. Result
protected void menuNew_Click (object sender, System.EventArgs e) { if (this.CloseCurrentAlbum() == true) { // Make sure the window is redrawn this.Invalidate(); } } protected void menuOpen_Click (object sender, System.EventArgs e) { // Save the existing album, if necessary if (this.CloseCurrentAlbum() == false) { // Cancel this operation return; } OpenFileDialog dlg = new OpenFileDialog(); . . . }
Modify the menuOpen_Click method to use the CloseCurrentAlbum method. Note: The new code here replaces the previous code in this method to save the current album. The remainder of this method stays the same.
These changes make our application much more user-friendly by interacting with the user when they are about to discard a modified album.
TRY IT!
Before moving on, create a MessageBox dialog in the menuRemove_Click method, where the current photograph is removed without any confirmation by the user. Add a question box here to verify that the user does indeed want to remove the current photo. Another place where a message box could be used is at the beginning and end of the album. Modify the Next and Previous menus to display an information dialog whenever the user tries to move before the beginning of the album or past the end.1
For the Exit menu, life is not so easy. We will pick up this topic in the next section.
The interface designers among us will argue that the Previous and Next commands should be disabled at the beginning and end of the album, respectively. Why allow the user to invoke a menu item that does not work I would not disagree, and if you prefer this approach, please go right ahead.
CHA PTE R 8
DIALOG BOXES
THE FORM.CLOSE METHOD
In this section we pick up the thread of our previous discussion on the CloseCurrentAlbum method by discussing the Close and Dispose methods. You may think this is a little off-topic from dialog boxes, but in fact it is quite relevant. One of the key issues for C# programming in .NET is when to call the Dispose method to clean up window handlers and other nonmemory resources. This section will discuss this topic as it relates to dialog boxes, and introduce the Closing event as a way to intercept a user s request to close a form.
The relationship between Close and Dispose Before we return to the topic of calling CloseCurrentAlbum when our application exits, let s look at the relationship between Close and Dispose in .NET. It s actually quite simple: they are the same. For all classes in the .NET Framework, a call to Close is equivalent to calling the Dispose method, and a call to Dispose is equivalent to calling the Close method. The term close traditionally applies to objects like files and windows, and .NET has preserved this terminology. When you are finished with a form or a file, it seems silly to require a call to both Close and Dispose, so it makes sense to merge these two concepts together. The .NET design team could have chosen to use a common name for all classes, but programmers naturally expect to close objects such as forms and files, and closing objects like arrays or drawing objects seems a bit odd. Instead, the designers chose to use both methods and define them to be equivalent. For Form objects, the behavior of the form itself varies depending on whether the object is displayed as a modal or modeless window. For a modeless window, displayed with the Form.Show method, the nonmemory resources are automatically cleaned up when the form is closed. This makes life much easier for us programmers, since we do not have to remember anything in this case. You cannot use a modeless Form after it is closed since all of its resources are gone. The Hide method should be used if you simply want to remove a Form from the desktop and display it later via the Show method. We will see this in chapter 13 when we use a tool bar button to hide the modeless dialog created in section 8.4 of this chapter. For modal windows, displayed with the Form.ShowDialog method, there is a problem in that the dialog is typically accessed after the window disappears. As a result, a modal dialog must call Dispose explicitly to release its nonmemory resources. Typically, a modal dialog is created and destroyed in the same block of code. For example:
{ MyModalDialog dlg = new MyModalDialog(); // Initialize any dlg settings if (dlg.ShowDialog() == DialogResult.OK) { // Use dlg settings to do something
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