qr code with c# Figure 8.2 This message box is displayed when the album contains an unrecognized version number. in Visual C#.NET

Painting ECC200 in Visual C#.NET Figure 8.2 This message box is displayed when the album contains an unrecognized version number.

Figure 8.2 This message box is displayed when the album contains an unrecognized version number.
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MESSAGE BOXES
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Let s add the code to create this dialog whenever an unexpected problem occurs while opening the file
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Set the version number of the MyPhotos application to 8.1. HANDLE EXCEPTION IN MENUOPEN_CLICK METHOD Action 1 Locate the
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menuOpen_Click method
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Result
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private void menuOpen_Click (object sender, System.EventArgs e) { . . . if (dlg.ShowDialog() == DialogResult.OK) { try { // Open the new album. _album.Open(dlg.FileName); _album.FileName = dlg.FileName; _bAlbumChanged = false; this.Invalidate(); }
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in the MainForm.cs source file. 2 Enclose the code to open the album in a try block.
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Catch any Exception that occurs. Display the dialog in the
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catch block.
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catch (Exception ex)
{ MessageBox.Show(this, "Unable to open file " + dlg.FileName + "\n (" + ex.Message + ")", "Open Album Error", MessageBoxButtons.OK, MessageBoxIcon.Error); } } . . . }
Note: The text string is constructed using the + (plus sign) notation for strings. Also note that a new line is inserted in the dialog with the \n character.
In this code, we cheated a little by catching any and all Exception objects in the catch block. It is normally safer to catch specific exceptions that may occur so you can provide feedback or take actions based on the specific error. In this code, an IOException will occur if an unexpected error occurs during a file I/O operation. If you recall, the PhotoAlbum.Open method throws an IOException explicitly if the version number in the file is not recognized. It is also worth noting that we ignore the result returned by the Show method, since there is only a single OK button in the dialog.
CHA PTE R 8
DIALOG BOXES
CREATING A YESNO DIALOG As an alternate example, what happens when an error occurs while saving an album We could simply display an OK dialog as we did while opening an album. This would just duplicate the previous code, so we will do something different. Instead, we will allow the user to save the album under an alternate file name. This permits the user to save the album to an alternate location that is less likely to fail, or retry the save to the same location. The new message box is shown in figure 8.3.
Figure 8.3 This message box is displayed when an exception occurs in the menuSave_Click method.
The steps required to generate this message dialog are shown in the following table:
HANDLE EXCEPTION IN MENUSAVE_CLICK METHOD Action 1 Locate the menuSave_Click method in the MainForm.cs file. Result
private void menuSave_Click (object sender, System.EventArgs e) { . . . else { try { // Save album in current file _album.Save(); _bAlbumChanged = false; } catch (Exception ex) { string msg = "Unable to save file {0}" + " - {1}\nWould you like to save" + " the album in an alternate file "; DialogResult result = MessageBox.Show(this, String.Format(msg, _album.FileName, ex.Message), "Save Album Error", MessageBoxButtons.YesNo, MessageBoxIcon.Error, MessageBoxDefaultButton.Button2); if (result == DialogResult.Yes) { menuSaveAs_Click(sender, e); } } . . . }
Enclose the code to save the album in a try block.
Catch any exception that occurs. Within the catch block, display the dialog and record the selected button.
If the user wishes to save under an alternate name, prompt the user for the new file name.
How-to
Use the Save As menu handler.
MESSAGE BOXES
Unlike our message for the Open handler, this code makes use of the result returned by the Show method. This result is a DialogResult enumeration that indicates the button pressed. The values in this enumeration are shown in .NET Table 8.2, and correspond to the kinds of buttons typically found in Windows dialogs.
.NET Table 8.2 DialogResult enumeration
The DialogResult enumeration represents a value returned by a dialog box. This class is part of the System.Windows.Forms namespace, and is used with all dialog boxes in Windows Forms. In particular, a DialogResult is returned by the MessageBox.Show method as well as the ShowDialog method in both the Form class and common dialogs derived from the CommonDialog class. This enumeration is also used by the Button class to indicate the result to automatically return from a modal dialog when the button is clicked. Abort Cancel Ignore Enumeration Values No None OK Retry Yes The dialog return value is Abort. Typically, this means the user clicked an Abort button. The dialog returns Cancel, typically from a Cancel button. The dialog returns Ignore, typically from an Ignore button. The dialog returns No, typically from a No button. The dialog returns nothing, indicating that the dialog box is still running. The dialog returns OK, typically from an OK button. The dialog returns Retry, typically from a Retry button. The dialog returns Yes, typically from a Yes button.
You can compile and run this code if you would like to see the message boxes we created. You can generate an open error easily enough by selecting a file that is not, in fact, an album file. A save error can be generated by attempting to save to a read-only CD, or by filling up a floppy disk and then saving a file to it. Our last example will generate a message box for closing an existing album. 8.1.4 Creating A YesNoCancel dialog Our final example is the case where an album has changed but is about to be discarded. This can occur when the application is about to exit, when loading a new album with the Open menu item, and when creating a new album with the New menu item. To handle these situations in a consistent way, we will create a protected method to gracefully close the current album for all three cases using the dialog in figure 8.4. We will call this method CloseCurrentAlbum and have it return a boolean value indicating whether the album was closed or the user Figure 8.4 This dialog is displayed when an album is about to be discarded. clicked the Cancel button. DIALOG BOXES
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