Windows phone 7 . 尝试访问“valid”时出现NullReferenceException可能是因为“valid”是“license”子元素上的一个属性。其代码如下所示（实际上没有运行此代码，请自己检查）：。。。valid=bool.Parse（xe.Elemen

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. 尝试访问“valid”时出现NullReferenceException可能是因为“valid”是“license”子元素上的一个属性。其代码如下所示（实际上没有运行此代码，请自己检查）：。。。valid=bool.Parse（xe.Element（“license”）.Attribute（“valid”）…服务器生成的XML文件在第一行以开头，以

<subsonic-response status="ok" version="1.1.1">
</subsonic-response>

<subsonic-response status="ok" version="1.1.1">
<license valid="true" email="foo@bar.com" key="ABC123DEF" date="2009-09-03T14:46:43"/>
</subsonic-response>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Microsoft.Phone.Controls;
using System.Xml.Linq;

namespace RESTTest
{
public partial class MainPage : PhoneApplicationPage
{
    // Constructor
    string requestString =
    "http://WEBSITE/rest/{0}.view?u=USERNAME&p=PASSWORD&v=1.8.0&c=RestTest";
    string UriNoAppId =
    "http://WEBSITE/rest/{0}.view?u=USERNAME&p=PASSWORD&v=1.8.0&c=RestTest";
    public MainPage()
    {
        InitializeComponent();
        List<string> searchTopics = new List<string>() { "ping", "getLicense" };
        comboBox1.DataContext = searchTopics;
        comboBox1.SelectedIndex = 0;
        // Create the WebClient and associate a handler with the OpenReadCompleted event.
        wc = new WebClient();
        wc.OpenReadCompleted += new OpenReadCompletedEventHandler(wc_OpenReadCompleted);
    }
    // Call the topic service at the Bing search site.
    WebClient wc;
    private void CallToWebService()
    {
        // Call the OpenReadAsyc to make a get request, passing the url with the selected search string.
        wc.OpenReadAsync(new Uri(String.Format(requestString, comboBox1.SelectedItem.ToString())));
    }
    void wc_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
    {
        XElement resultXml;
        // You should always check to see if an error occurred. In this case, the application
        // simply returns.
        if (e.Error != null)
        {
            return;
        }
        else
        {
            XNamespace web = "http://subsonic.org/restapi";
            try
            {
                resultXml = XElement.Load(e.Result);
                // Search for the WebResult node and create a SearchResults object for each one.
                var searchResults =
                from result in resultXml.Descendants(web + "WebResult")
                select new SearchResult
                {
                    // Get the Title, Description and Url values.
                    Title = result.Element(web + "version").Value,
                    Url = result.Element(web + "status").Value
                };
                // Set the data context for the listbox to the results.
                listBox1.DataContext = searchResults;
                textBox1.DataContext = searchResults;
            }
            catch (System.Xml.XmlException ex)
            {
                textBlock2.Text = ex.Message;
            }
        }
    }
    private void button1_Click(object sender, RoutedEventArgs e)
    {
        CallToWebService();
    }
    // Update the textblock as the combo box selection changes.
    private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        uriTextBlock.DataContext = string.Format(UriNoAppId, e.AddedItems[0]);
    }

}
// Simple class to hold the search results.
public class SearchResult
{
    public string Title { get; set; }
    public string Url { get; set; }
}
}

<phone:PhoneApplicationPage 
x:Class="RESTTest.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="800"
FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="{StaticResource PhoneFontSizeNormal}"
Foreground="{StaticResource PhoneForegroundBrush}"
SupportedOrientations="Portrait" Orientation="Portrait"
shell:SystemTray.IsVisible="False">
<!--LayoutRoot is the root grid where all page content is placed-->
<Grid x:Name="LayoutRoot" Background="Transparent">
    <Grid.Resources>
        <Style x:Key="ComboBoxItemStyle" TargetType="ComboBoxItem" >
            <Setter Property="Foreground" Value="Black"/>
            <Setter Property="Background" Value="LightGray"/>
        </Style>
        <Style x:Key="ComboBoxStyle" TargetType="ComboBox" >
            <Setter Property="Foreground" Value="Black"/>
            <Setter Property="Background" Value="Gray"/>
        </Style>
    </Grid.Resources>
    <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="REST CLIENT"
                   Style="{StaticResource PhoneTextNormalStyle}"/>
        <TextBlock x:Name="PageTitle" Text="Subsonic"
                   Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"
                   Height="99" Width="453" />
    </StackPanel>
    <!--ContentPanel - place additional content here-->
    <Grid x:Name="ContentPanel" Margin="12,139,12,0" Grid.RowSpan="2">
        <Button Content="Search!" Height="89" HorizontalAlignment="Left"
                Margin="264,140,0,0" Name="button1"
                VerticalAlignment="Top" Width="189" Click="button1_Click" />
        <ComboBox Height="50" Style="{StaticResource ComboBoxStyle}"
                  HorizontalAlignment="Left" Margin="6,159" Name="comboBox1"
                  ItemContainerStyle="{StaticResource ComboBoxItemStyle}"
                  VerticalAlignment="Top" Width="235" ItemsSource="{Binding}"
                  SelectionChanged="comboBox1_SelectionChanged" />
        <TextBlock Height="36" HorizontalAlignment="Left" Margin="12,120"
                   Name="textBlock2" Text="Search Topic:" VerticalAlignment="Top" width="121" />
        <TextBlock Height="23" HorizontalAlignment="Left" Margin="12,1,0,0"
                   Name="textBlock3"
                   Text="URI:" VerticalAlignment="Top" />
        <TextBlock Height="86" HorizontalAlignment="Left" Margin="6,28"
                   Name="uriTextBlock" TextWrapping="Wrap" Text="{Binding}"
                   VerticalAlignment="Top" Width="447" />
        <TextBlock Height="23" HorizontalAlignment="Left" Margin="12,242,0,0"
                   Name="textBlock5"
                   Text="Results:" VerticalAlignment="Top" />
        <ListBox Height="169" HorizontalAlignment="Left" Margin="6,271,0,0" Name="listBox1"
                 VerticalAlignment="Top" Width="444" ItemsSource="{Binding}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <Border BorderBrush="{StaticResource PhoneForegroundBrush}" Width="418"
                            BorderThickness="2" Margin="2">
                        <StackPanel>
                            <TextBlock Text="{Binding Path=Title}" TextWrapping="Wrap" />
                            <TextBlock Text="{Binding Path=Url}" TextWrapping="Wrap"/>
                        </StackPanel>
                    </Border>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <TextBox Height="209" HorizontalAlignment="Left" Margin="6,446,0,0" Name="textBox1" Text="" VerticalAlignment="Top" Width="444" IsEnabled="True" IsReadOnly="True" />
    </Grid>
</Grid>
</phone:PhoneApplicationPage>

string fakeXML = "<subsonic-response status='ok' version='1.1.1'>" +
                 "<license valid='true' email='foo@bar.com' " +
                 "  key='ABC123DEF' date='2009-09-03T14:46:43'/>" +
                 "</subsonic-response>";
XDocument doc = XDocument.Parse(fakeXML);

var searchResults = from xe in doc.Elements("subsonic-response")
                    select new SearchResult
                    {
                      Title = xe.Attribute("version").Value,
                      Url = xe.Attribute("status").Value
                    };
listBox1.DataContext = searchResults;
textBox1.DataContext = searchResults;