•Explain the difference between modal and modeless dialogs.

•Create dialog boxes using the Form class.

Before we get into generating custom dialog boxes, we will first look at how simple messages are displayed using the MessageBox class.

8.1MESSAGE BOXES

Developers, especially object-oriented developers, are always looking for shortcuts. Classes such as OpenFileDialog and SaveFileDialog not only provide a standard way to prompt a user for files, they also save programmers a lot of time and effort by encapsulating the required window display and interaction code. Another common task programmers face is the need to display a simple message to the user. Our photo album application, for example, should really display an error message when an album cannot be saved successfully, or it could pose a question by asking the user if they would like to save the album to an alternate file location.

The .NET Framework provides a MessageBox class for this purpose. This class is very similar to the MFC function of the same name. This section will show how this class is used to handle simple interactions with a user. While this class is not actually a Form object, it is the most basic type of modal dialog box.

All dialog boxes are either modal or modeless. A modal dialog box requires the user to respond before the associated program will continue. Modeless or nonmodal dialog boxes allow the application to continue while the dialog box is displayed.

All MessageBox windows are modal, while Form windows are modal if invoked via the Form.ShowDialog method and modeless if invoked via the Form.Show method.

Figure 8.1 These examples show the four types of icons available to

MessageBox dialogs.

Figure 8.1 shows some sample message boxes with various settings. Note the different button configurations, and how the Question Icon dialog has defined No as the default button. An overview of the MessageBox class is provided in .NET Table 8.1.

.NET Table 8.1 MessageBox class

The MessageBox class represents a modal dialog box that displays a message or question to the user and waits for their response. This class is part of the System.Windows.Forms namespace. A MessageBox cannot be instantiated as an object with the new keyword; instead the static Show method is used to display the dialog.

By default, a message box displays with no icon and a single OK button. The Show method is overloaded to allow these and other settings to be customized. There are four enumerations used for this purpose: MessageBoxButtons, MessageBoxIcon, MessageBoxDefaultButton, and MessageBoxOptions. In the following table, the enumeration values for some of these four types are included, since these types are only used with the MessageBox.Show method.

8.1.1THE MESSAGEBOX.SHOW METHOD

A MessageBox instance cannot be instantiated. Instead, the Show method is used to create the message dialog and return the result. There are a number of overloads available for this method, from a version that takes a single message string to one that accepts a parameter for everything from the title bar text to which button should be the default. Various forms of this method are shown in the following signatures. The comment preceding each signature refers to the characters in bold.

//The return value indicates which button was clicked by the user public static DialogResult Show( string text );

//Displays the dialog in front of the specified window object

public static DialogResult Show( IWin32Window owner, string text );

// Accepts a message string and title bar caption

public static DialogResult Show( string text, string caption );

// Displays the dialog with the specified buttons public static DialogResult Show( IWin32Window owner,

string text, string caption,

MessageBoxButtons buttons );

// The penultimate Show method: an icon, default button, and options public static DialogResult Show( IWin32Window owner,

string text, string caption,

MessageBoxButtons buttons,

MessageBoxIcon icon,

MessageBoxDefaultButton defaultButton,

MessageBoxOptions options );

Turning back to our MyPhotos application, the addition of a message box would be beneficial in some of the situations we have already encountered. These include:

•When an error occurs while trying to open an existing album.

•When an error occurs while trying to save the current album.

•When the current album has changed and is about to be discarded.

We will add a MessageBox to our program for each of these instances.

8.1.2CREATING AN OK DIALOG

When we are unable to open a selected album, there is not much to do other than inform the user that something is wrong. We will use an error dialog since a failure here is not normally expected. The resulting dialog is shown in figure 8.2.

Figure 8.2

This message box is displayed when the album contains an unrecognized version number.

Let’s add the code to create this dialog whenever an unexpected problem occurs while opening the file

Set the version number of the MyPhotos application to 8.1.

HANDLE EXCEPTION IN MENUOPEN_CLICK METHOD

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.