.

UPDATE THE SAVE AND OPEN METHODS IN PHOTOALBUM CLASS

UPDATE THE SAVE AND OPEN METHODS IN PHOTOALBUM CLASS (continued)

This neat little use of delegates makes our code much more readable. A delegate instance is declared just like any other variable, except that in this case the type is our delegate.

Photograph.ReadDelegate ReadPhoto;

This variable is assigned by creating a new instance of the delegate object, providing a method with a matching signature. Note that in our case the method happens to be static. A delegate tracks both an object and a method, allowing both internal and static members of a class to be used. In our code, when the version number is 92, the PhotoReader variable is initialized as follows. If the method provided does not match the signature assigned to the delegate, a compiler error is generated. Such compilertime checking is a big advantage of delegates in C# over function pointers in C++.

case 92:

ReadPhoto = new Photograph.ReadDelegate(Photograph.ReadVersion92);

break;

The delegate is then used like any other function call to invoke the assigned method.

// Read each photograph in the album Photograph p = ReadPhoto(sr);

while (p != null)

{

this.Add(p);

p = ReadPhoto(sr);

}

Our PhotoAlbum class is now ready. Let’s get back to our PhotoEditDlg form and begin creating our new form.

9.2.2CREATING THE PHOTOEDITDLG PANEL AREA

Looking back at the PhotoEditDlg dialog we wish to create, let’s focus on the Panel control at the top of the form. In chapter 7 we made use of the Panel class for its drawing and scrolling capabilities. Here, we will use this class as a container for other controls to improve the overall appearance of our form. While we will not

enable scrolling in our panel, realize that controls can be placed outside of the visible portion of a panel and made accessible via the automated scroll bars discussed in chapter 7.

We have used the Label and TextBox controls previously in our program, so let’s update our panel before we talk about these classes in more detail. As you recall, the Panel object is inherited from the base form, but is modifiable by our class since it has a protected access level.

CREATE THE PANEL AREA OF THE PHOTOEDITDLG FORM

CREATE THE PANEL AREA OF THE PHOTOEDITDLG FORM

Note that the labels and text boxes here are defined within the Panel control, rather than within the form. In the InitializeComponent method, the controls are added to the Panel object much like we have seen other controls added to the Form class in previous chapters.

this.panel1.Controls.AddRange(new System.Windows.Forms.Control[] {

this.txtPhotographer,

this.txtDate,

this.txtCaption,

this.txtPhotoFile,

this.label4,

this.label3,

this.label2,

this.label1 });

The setting of the tab order in the final step, which internally assigns the TabIndex property for each control, is very important here. We could have set these values explicitly in steps 2 and 3, but the method in step 4 allowed us to set all controls at once. Since the Label and TextBox controls are located within the Panel object, the tab order defined applies only within the panel, so the TabIndex values for our labels and text boxes start at zero.

While label controls do not participate in the tab order, their TabIndex setting is still utilized. When you tab to or activate a label, the next control in the tab order will receive focus. This is the same behavior provided by the MFC CStatic class in Visual C++.

.NET Table 9.1 Label class

The Label class is a control that displays a text string or image within a form. This class is part of the of the System.Windows.Forms namespace, and inherits from the Control class. A Label object can be assigned a tab index, but when activated the next control in the tab order will always receive focus. See .NET Table 4.1 on page 104 for a list of members inherited from the Control class.

This tab order behavior is especially relevant for the assigned access key, also called a mnemonic. For example, the ampersand character “&” in the Photo File label defines the ‘F’ character as the access key. Typing the keystroke Alt+F sets the focus to this label. When you do this, the txtPhotoFile control actually receives the focus. Similarly, typing Alt+P for the Caption label will set the focus to the

tion control. Such mnemonics are very useful for users, myself included, who prefer to avoid the mouse and keep their fingers on the keyboard.

Labels include other features as well, of course. As we saw in chapter 8, they can display a border and define an alignment for displayed text. These and other features of the Label class are summarized in .NET Table 9.1. The Label class is also the parent of the LinkLabel class, which adds the ability to perform an action when the text on the label is clicked, similar to an HTML link in a web browser. We will use the LinkLabel class in chapter 18 while discussing how to include ActiveX controls on a form.

We will use Label objects again in future sections. For now, let’s move on to discuss some of the features of our TextBox controls, and the TextBox class in general.

9.2.3CREATING THE MULTILINE TEXT BOX

The TextBox controls on our form are used to display various properties of the Photograph object. We have already created the text box controls within the panel area. In this section we create the text box for displaying the Photograph.Notes property in our dialog, and implement the protected members we defined in our base class earlier in the chapter.

As we saw in the previous section, text boxes can be editable or read-only. In some ways a read-only text box is similar to a Label object with a 3D border. The major difference is that a text box supports cut and paste operations, while a label does not. The existing CaptionDlg form in our application used a 3D label to illustrate this point, so you can verify this for yourself if you prefer.

As a result, a read-only TextBox should be used when displaying text that a user may wish to copy. This, in fact, is why our PhotoEditDlg form uses a read-only TextBox to display the photograph’s file name.

The TextBox class is based on the TextBoxBase class, which provides much of the core functionality for text controls. An overview of this base class appears in .NET Table 9.2.

.NET Table 9.2 TextBoxBase class

The TextBoxBase class is a control that displays editable text and can interact with the Clipboard class to permit cut and paste operations. This class is part of the of the System.Windows.Forms namespace, and inherits from the Control class. Both the TextBox and RichTextBox classes are derived from this abstract class. See .NET Table 4.1 on page 104 for a list of members inherited from the Control class, and .NET Table 9.3 on page 288 for an overview of the TextBox class.

As shown in the table, the TextBoxBase class provides a Multiline property that indicates whether a derived control can accept multiple lines of text. The bottom part of our PhotoEditDlg form is designed to show off such a text box. This control