11.On the Debug menu, click Start Without Debugging to verify that the project builds and runs.

12.When the form opens, click the Tower combo box.

You will see the list of bell towers, and you can select one of them.

13.Click the drop-down arrow on the right side of the Member Since date/time picker.

You will be presented with a calendar of dates. The default value will be the current date. You can click a date and use the arrows to select a month. You can also click the month name to display the months as a drop-down list, and click the year so that you can select a year by using a numeric up-down control.

14.Click each of the radio buttons in the Experience group box.

Notice that you cannot select more than one radio button at a time.

15.In the Methods list box, click some of the methods to select the corresponding check box. If you click a method a second time, it clears the corresponding check box, just as you would expect.

16.Click the Add and Clear buttons.

Currently these buttons don’t do anything. You will add this functionality in the final set of exercises in this chapter.

17.Close the form, and return to Visual Studio 2008.

Handling Events in a WPF Form

If you are familiar with Microsoft Visual Basic, Microsoft Foundation Classes (MFC), or any of the other tools available for building GUI applications for Windows, you are aware that Windows uses an event-driven model to determine when to execute code. In Chapter 17, “Interrupting Program Flow and Handling Events,” you saw how to publish your own events and subscribe to them. WPF forms and controls have their own predefined events that you can subscribe to, and these events should be sufficient to handle the requirements of most user interfaces.

Processing Events in Windows Forms

The developer’s task is to capture the events that are relevant to the application and write the code that responds to these events. A familiar example is the Button control, which raises a

“Somebody clicked me” event when a user clicks it with the mouse or presses Enter when the button has the focus. If you want the button to do something, you write code that responds to this event. This is what you will do in the next exercise.

Handle the Click event for the Clear button

1.Display the Window1.xaml file in the Design View window. Double-click the Clear button on the form.

The Code and Text Editor window appears and creates a method called clear_Click. This is an event method that will be invoked when the user clicks the Clear button. Notice that the event method takes two parameters: the sender parameter (an object) and

an additional arguments parameter (a RoutedEventArgs object). The WPF runtime will populate these parameters with information about the source of the event and with any additional information that might be useful when handling the event. You will not use these parameters in this exercise.

WPF controls can raise a variety of events. When you double-click a control or a form in the Design View window, Visual Studio generates the stub of an event method for the default event for the control; for a button, the default event is the Click event. (If you

double-click a text box control, Visual Studio generates the stub of an event method for handling the TextChanged event.)

2.When the user clicks the Clear button, you want the form to be reset to its default values. In the body of the clear_Click method, call the Reset method, as shown here in bold type:

private void clear_Click(object sender, RoutedEventArgs e)

{

this.Reset();

}

Users will click the Add button when they have filled in all the data for a member and want to store the information. The Click event for the Add button should validate the information entered to ensure that it makes sense (for example, should you allow a tower captain to have less than one year of experience?) and, if it is okay, arrange for

the data to be sent to a database or other persistent store. You will learn more about validation and storing data in later chapters. For now, the code for the Click event of

the Add button will simply display a message box echoing the data input.

3.Return to the Design View window displaying the Window1.xaml form. In the XAML pane, locate the element that defines the Add button, and begin entering the following code shown in bold type:

<Button ... Click=”>Add</Button>

Notice that as you type the opening quotation mark after the text Click=, a shortcut menu appears, displaying two items: <New Event Handler> and clear_Click. If two but-

tons perform a common action, you can share the same event handler method between them, such as clear_Click. If you want to generate an entirely new event handling method, you can select the <New Event Handler> command instead.

4.On the shortcut menu, double-click the <New Event Handler> command. The text add_Click appears in the XAML code for the button.

Note You are not restricted to handling the Click event for a button. When you edit the

XAML code for a control, the IntelliSense list displays the properties and events for the control. To handle an event other than the Click event, simply type the name of the event,

and then select or type the name of the method that you want to handle this event. For a complete list of events supported by each control, see the Visual Studio 2008 documentation.

5.Switch to the Code and Text Editor window displaying the Window1.xaml.cs file. Notice that the add_Click method has been added to the Window1 class.

Tip You don’t have to use the default names generated by Visual Studio 2008 for the event handler methods. Rather than clicking the <New Event Handler> command on the

shortcut menu, you can just type the name of a method. However, you must then manu-

ally add the method to the window class. This method must have the correct signature; it should return a void and take two arguments—an object parameter and a RoutedEventArgs

parameter.

Important If you later decide to remove an event method such as add_Click from the Window1.xaml.cs file, you must also edit the XAML definition of the corresponding control and remove the Click=”add_Click” reference to the event; otherwise, your application will not compile.

6. Add the following code shown in bold type to the add_Click method:

private void add_Click(object sender, RoutedEventArgs e)

{

string nameAndTower = String.Format(

“Member name: {0} {1} from the tower at {2} rings the following methods:”, firstName.Text, lastName.Text, towerNames.Text);

StringBuilder details = new StringBuilder(); details.AppendLine(nameAndTower);

foreach (CheckBox cb in methods.Items)

{

if (cb.IsChecked.Value)

{

details.AppendLine(cb.Content.ToString());

}

}

MessageBox.Show(details.ToString(), “Member Information”);

}

446 Part IV Working with Windows Applications

This block of code creates a string variable called nameAndTower that it fills with the name of the member and the tower to which the member belongs.

Notice how the code accesses the Text property of the text box and combo box

controls to read the current values of those controls. Additionally, the code uses the static String.Format method to format the result. The String.Format method operates in a similar manner to the Console.WriteLine method, except that it returns the formatted

string as its result rather than displaying it on the screen.

The code then creates a StringBuilder object called details. The method uses this StringBuilder object to build a string representation of the information it will display. The text in the nameAndTower string is used to initially populate the details object. The code then iterates through the Items collection in the methods list box. If you recall, this

list box contains check box controls. Each check box is examined in turn, and if the user has selected it, the text in the Content property of the check box is appended to the details StringBuilder object.

Note You could use ordinary string concatenation instead of a StringBuilder object, but the StringBuilder class is far more efficient and is the recommended approach for performing the kind of tasks required in this code. In the .NET Framework and C#, the string data

type is immutable; when you modify the value in a string, the runtime actually creates a new string containing the modified value and then discards the old string. Repeatedly modifying a string can cause your code to become inefficient because a new string must

be created in memory at each change (the old strings will eventually be garbage collected). The StringBuilder class, in the System.Text namespace, is designed to avoid this inefficiency. You can add and remove characters from a StringBuilder object using the Append, Insert, and Remove methods without creating a new object each time.

Finally, the MessageBox class provides static methods for displaying dialog boxes on the screen. The Show method used here displays the contents of the details string in the

body of the message box and will put the text “Member Information” in the title bar. Show is an overloaded method, and there are other variants that you can use to specify icons and buttons to display in the message box.

7.On the Debug menu, click Start Without Debugging to build and run the application.

8.Type some sample data for the member’s first name and last name, select a tower, and pick a few methods. Click the Add button, and verify that the Member Information message box appears, displaying the details of the new member and the methods he or she can ring.

9.Click the Clear button, and verify that the controls on the form are reset to the correct default values.

10.Close the form, and return to Visual Studio 2008.

In the final exercise in this chapter, you will add an event handler to handle the Closing event

for the window so that users can confirm that they really want to quit the application. The Closing event is raised when the user attempts to close the form but before the form actually

closes. You can use this event to prompt the user to save any unsaved data or even ask the user whether he or she really wants to close the form—if not, you can cancel the event in the event handler and prevent the form from closing.

Handle the Closing event for the form

1.In the Design View window, in the XAML pane, begin entering the code shown in bold type to the XAML description of the Window1 window:

<Window x:Class=”BellRingers.Window1”

...

Title=”...” ... Closing=”>

2.When the shortcut menu appears after you type the opening quotation mark, double-click the <New Event Handler> command.

Visual Studio generates an event method called Window_Closing and associates it with the Closing event for the form, like this:

<Window x:Class=”BellRingers.Window1”

...

Title=”...” ... Closing=”Window_Closing”>

3.Switch to the Code and Text Editor window displaying the Window1.xaml.cs file.

A stub for the Window_Closing event method has been added to the Window1 class:

private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)

{

}

Observe that the second parameter for this method has the type CancelEventArgs. The CancelEventArgs class has a Boolean property called Cancel. If you set Cancel to true in the event handler, the form will not close. If you set Cancel to false (the default value),

the form will close when the event handler finishes.

4. Add the following statements shown in bold type to the memberFormClosing method:

private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)

{

MessageBoxResult key = MessageBox.Show( “Are you sure you want to quit”, “Confirm”,

MessageBoxButton.YesNo,

MessageBoxImage.Question,

MessageBoxResult.No);

e.Cancel = (key == MessageBoxResult.No);

}

These statements display a message box asking the user to confirm whether to quit

the application. The message box will contain Yes and No buttons and a question mark icon. The final parameter, MessageBoxResult.No, indicates the default button if the user

simply presses the Enter key—it is safer to assume that the user does not want to exit the application than to risk accidentally losing the details that the user has just typed.

When the user clicks either button, the message box will close and the button clicked will be returned as the value of the method (as a MessageBoxResult—an enumeration

identifying which button was clicked). If the user clicks No, the second statement will set the Cancel property of the CancelEventArgs parameter (e) to true, preventing the

form from closing.

5.On the Debug menu, click Start Without Debugging to run the application.

6.Try to close the form. In the message box that appears, click No. The form should continue running.

7.Try to close the form again. This time, in the message box, click Yes. The form closes, and the application finishes.

You have now seen how to use the essential features of WPF to build a functional user interface. WPF contains many more features than we have space to go into here, especially concerning some of its really cool capabilities for handling two-dimensional and three-

dimensional graphics and animation. If you want to learn more about WPF, you can consult a book such as Applications = Code + Markup: A Guide to the Microsoft Windows Presentation Foundation, by Charles Petzold (Microsoft Press, 2006).

If you want to continue to the next chapter

Keep Visual Studio 2008 running, and turn to Chapter 23.

If you want to exit Visual Studio 2008 now

On the File menu, click Exit. If you see a Save dialog box, click Yes (if you are using Visual Studio 2008) or Save (if you are using Visual C# 2008 Express Edition) and save the project.