Whenever an application throws an exception, Visual Studio will stop execution at the line where it's thrown. To observe this behavior, let's run the application and take a closer look using the Visual Studio IDE.

Fire up Visual Studio, select File

{unzippeddirectory}ch04ErrorHandlingDemoStartErrorHandlingDemo.sln.

Once the solution file is loaded, press F5 to run it. Notice the raised exception message in Figure 4-2, which points to the code that has caused ArgumentOutOfRangeException to be thrown. From DeviceExtendedProperties you can obtain Windows Phone 7 system information, and only the following keys—DeviceManufacturer, DeviceName, DeviceUniqueId, DeviceTotalMemory, ApplicationCurrentMemoryUsage, and ApplicationPeakMemoryUsage—will be available. And when you try to query the system information DName that does not exist, the application throws ArgumentOutOfException.

Figure 4.2. A raised exception in Visual Studio

Whenever an application running in Visual Studio pauses at a line where an exception has been thrown, you always have an opportunity to observe its variables in Visual Studio's Immediate Window.

Immediate Window is a most useful debugging feature because it allows you to evaluate any statement when the code execution pauses at the breakpoint. If you do not see the immediate window when the breakpoint is hit, you can go to Debug

Unhandled exceptions in a Windows Phone application will be caught by one of two main methods: RootFrame_NavigationFailed and Application_UnhandledException. RootFrame_NavigationFailed catches unhandled exceptions thrown while a page is being loaded; Application_UnhandledException catches exceptions thrown in all other cases.

When an exception is thrown in the MainPage of an application, the exception will be caught by the RootFrame_NavigationFailed method, and this is where you want to handle it in order to stop the exception from bubbling up to the Application_UnhandledException method.

In ErrorHandlingDemo, replace the RootFrame_NavigationFailed method with following code. Notice the use of MessageBox in the code to display the proper error with stack trace and set e.Handled to true, which will stop the breakpoint to move to the Application_UnhandledException method.

// Code to execute if a navigation fails
        private void RootFrame_NavigationFailed(object sender, NavigationFailedEventArgs e)
        {
            if (System.Diagnostics.Debugger.IsAttached)
            {
                // A navigation has failed; break into the debugger
                System.Diagnostics.Debugger.Break();
            }

            MessageBox.Show(
string.Format("Page {0} failed to load because of with error: {1}",
e.Uri.ToString(), e.Exception.StackTrace));
            e.Handled = true;
        }

In the previous section, you added a MessageBox display in case any other page fails to load. In the following steps, you will be fixing the actual cause of the exception in MainPage. But first, let's fix the error in MainPage.xaml.cs.

Fix the error in MainPage.xaml.cs by replacing

txtDeviceName.Text = DeviceExtendedProperties.GetValue("DName").ToString()

with

txtDeviceName.Text = DeviceExtendedProperties.GetValue("DeviceName").ToString().

You will be stepping through the breakpoints in order to understand the behavior of the thrown exception. Before you begin, make sure that both the Windows Phone project and the web service project start simultaneously when you Press F5.

After stepping through the breakpoints, you now have enough information to fix the exception.

First, let's check the values received from the caller in CalculatorService. Replace Service1.svc.cs codes with the following snippet. The CheckValue method will make sure that the received value is not null and convert the value to the integer.

public int Add(object x, object y)
        {
            int xValue = CheckValue(x);
            int yValue = CheckValue(y);

            return xValue + yValue;

}

        private int CheckValue(object value)
        {
            int convertedValue = −1;
            if (value == null)
            {
                throw new ArgumentNullException("value");
            }
            else if (!int.TryParse(value.ToString(), out convertedValue))
            {
                throw new ArgumentException(
string.Format("The value '{0}' is not an integer.", value));
            }

            return convertedValue;
        }

In MainPage.xaml.cs, replace the AddCompleted event delegate with following code. You will be checking to make sure e.Error is empty before retrieving e.Result; if e.Error is not empty, you will be throwing the proper error message.

_svc.AddCompleted += (s, e) =>
            {
                if (e.Error == null)
                {
                    txtAnswer.Text = e.Result.ToString();
                }
                else
                {
                    MessageBox.Show(
string.Format("CalculatorService return an error {0}",
e.Error.Message));
                }
            };

The namespaces you see in the following code snippet are typically declared by default when you first create a Windows Phone project. In particular, namespaces xmlns:phone="clr-namespace:Microsoft.Phone.Controls; assembly=Microsoft.Phone" allow you to add common Windows Phone controls to the application main page.

<phone:PhoneApplicationPage
    x:Class="CatchingDeviceExceptionsDemo.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"

xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" d:DesignWidth="480" d:DesignHeight="768"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    shell:SystemTray.IsVisible="True">

Create two buttons to start and stop the accelerometer.

<!--LayoutRoot is the root grid where all page content is placed-->
    <Grid x:Name="LayoutRoot" Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <!--TitlePanel contains the name of the application and page title-->
        <StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="12,17,0,28">
            <TextBlock x:Name="ApplicationTitle" Text="CatchingDeviceExceptionsDemo"
Style="{StaticResource PhoneTextNormalStyle}"/>
        </StackPanel>

        <!--ContentPanel - place additional content here-->
        <Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
            <Button Content="Start Accelerometer" Height="72" HorizontalAlignment="Left"
Margin="84,45,0,0" Name="btnStartAcc" VerticalAlignment="Top"
Width="284" Click="btnStartAcc_Click" />
            <Button Content="Stop Accelerometer" Height="72" HorizontalAlignment="Left"
Margin="84,123,0,0" Name="btnStopAcc" VerticalAlignment="Top"
Width="284" Click="btnStopAcc_Click" />
        </Grid>
    </Grid>

</phone:PhoneApplicationPage>

Once you have loaded the XAML code, you should see the layout shown in Figure 4-19. In the next section, you will be coding the application.

Figure 4.19. CatchDeviceExceptionDemo design view

Begin by listing the namespaces the application will use. Note that the inclusion of Microsoft.Devices.Sensors will allow you to start and stop Windows Phone's accelerometer.

using System.Windows;
using Microsoft.Phone.Controls;
using Microsoft.Devices.Sensors;

The variable _acc, Accelerometer object, will be used to start and stop.

Accelerometer _acc;

        public MainPage()
        {
            InitializeComponent();
            _acc = new Accelerometer();
        }

Implement a button event for stopping and starting the accelerometer. Note that you are catching AccelerometerFailedException, which can be raised during the start and stop of the accelerometer. In the exception property, you will find ErrorId and Message that contains specific error code and a description that could explain why the error was raised

private void btnStartAcc_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                _acc.Start();

                _acc.ReadingChanged += (s1, e1) =>
                    {
                        // Do something with captured accelerometer data
                    };
            }
            catch (AccelerometerFailedException ex)
            {
                string errorMessage = string.Format(@"
                            Accelerometer threw an error with ErrorId {0}
    during the start operation
                            with error message {1}
                            ", ex.ErrorId, ex.Message);
                MessageBox.Show(errorMessage);
            }
        }

        private void btnStopAcc_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                 _acc.Stop();
            }
            catch (AccelerometerFailedException ex)
            {
                string errorMessage = string.Format(@"

Accelerometer threw an error with ErrorId {0}
    during the stop operation
                            with error message {1}

                            ", ex.ErrorId, ex.Message);
                MessageBox.Show(errorMessage);
            }
        }