对于这个愚蠢的问题感到抱歉。我甚至不明白该问题的标题应该是什么。
我有3个按钮，当我点击它们时，应该显示列表框。 如果我在列表框中选择一个项目，它应该被导航并开始播放。
当我点击一个按钮时，列表框正在显示，当一个项目被选中时，它将导航到其他页面并播放。如果我点击任何按钮后执行选择更改，我会收到错误
tori.dll
中出现'System.NullReferenceException'类型的第一次机会异常tori.dll
中发生了未处理的“System.NullReferenceException”类型异常Archieves .xaml：
<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,-951,0">
<Button Name="btn1" Click="btn1_Click" Content="Daily" HorizontalAlignment="Left" Margin="0,88,0,0" VerticalAlignment="Top" Width="127" Height="72"/>
<Button Name="btn2" Click="btn2_Click" Content="Weekly" HorizontalAlignment="Left" Margin="132,88,0,0" VerticalAlignment="Top" Height="72" Width="140"/>
<Button Name="btn3" Click="btn3_Click" Content="CurrentMonth" HorizontalAlignment="Left" Margin="277,88,0,0" VerticalAlignment="Top" Height="72" Width="169"/>
<ListBox x:Name="itemsList" Margin="0,225,945,0"
SelectionChanged="itemsList_SelectionChanged">
<ListBox.ItemTemplate>
<DataTemplate>
<Grid Height="130">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Image delay:LowProfileImageLoader.UriSource="{Binding ThumbnailUrl}"
Grid.Column="0"
Width="500"
Height="125"
VerticalAlignment="Center"
HorizontalAlignment="Center"
Margin="6"/>
<StackPanel Margin="10,20,0,0"
Grid.Column="1"
Height="60"
Orientation="Horizontal"
VerticalAlignment="Center">
<TextBlock Text="{Binding title}"
FontSize="40" />
</StackPanel>
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
Archieves.xaml.cs：
namespace tori
{
public partial class Archieves : PhoneApplicationPage
{
public Archieves()
{
InitializeComponent();
}
private void btn1_Click(object sender, RoutedEventArgs e)
{
WebClient downloader = new WebClient();
Uri uri = new Uri(" http://www.toucheradio.com/RSSFeed/rssDaily.php ", UriKind.Absolute);
downloader.DownloadStringCompleted += new DownloadStringCompletedEventHandler(ChannelDownloaded);
downloader.DownloadStringAsync(uri);
}
void ChannelDownloaded(object sender, DownloadStringCompletedEventArgs e)
{
if (e.Result == null || e.Error != null)
{
MessageBox.Show("There was an error downloading the XML-file!");
}
else
{
// Deserialize if download succeeds
XDocument document = XDocument.Parse(e.Result);
var queue = from item in document.Descendants("item")
select new Item
{
title = item.Element("title").Value
,
link = item.Element("link").Value
,
pubDate = item.Element("pubDate").Value
,
ThumbnailUrl = item.Element(item.GetNamespaceOfPrefix("media") + "thumbnail").Attribute("url").Value
,
};
itemsList.ItemsSource = queue;
}
}
private void btn2_Click(object sender, RoutedEventArgs e)
{
WebClient downloader = new WebClient();
Uri uri = new Uri(" http://www.toucheradio.com/RSSFeed/rssWeekly.php ", UriKind.Absolute);
downloader.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Channel1Downloaded);
downloader.DownloadStringAsync(uri);
}
void Channel1Downloaded(object sender, DownloadStringCompletedEventArgs e)
{
if (e.Result == null || e.Error != null)
{
MessageBox.Show("There was an error downloading the XML-file!");
}
else
{
// Deserialize if download succeeds
XDocument document = XDocument.Parse(e.Result);
var queue = from item in document.Descendants("item")
select new Item
{
title = item.Element("title").Value
,
link = item.Element("link").Value
,
pubDate = item.Element("pubDate").Value
,
ThumbnailUrl = item.Element(item.GetNamespaceOfPrefix("media") + "thumbnail").Attribute("url").Value
,
};
itemsList.ItemsSource = queue;
}
}
private void btn3_Click(object sender, RoutedEventArgs e)
{
WebClient downloader = new WebClient();
Uri uri = new Uri("http://www.toucheradio.com/RSSFeed/rssMonthly.php ", UriKind.Absolute);
downloader.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Channel2Downloaded);
downloader.DownloadStringAsync(uri);
}
void Channel2Downloaded(object sender, DownloadStringCompletedEventArgs e)
{
if (e.Result == null || e.Error != null)
{
MessageBox.Show("There was an error downloading the XML-file!");
}
else
{
// Deserialize if download succeeds
XDocument document = XDocument.Parse(e.Result);
var queue = from item in document.Descendants("item")
select new Item
{
title = item.Element("title").Value
,
link = item.Element("link").Value
,
pubDate = item.Element("pubDate").Value
,
ThumbnailUrl = item.Element(item.GetNamespaceOfPrefix("media") + "thumbnail").Attribute("url").Value
,
};
itemsList.ItemsSource = queue;
}
}
private void itemsList_SelectionChanged(Object sender, SelectionChangedEventArgs e)
{
var app = App.Current as App;
app.selectedItem = (Item)itemsList.SelectedItem;
this.NavigationService.Navigate(new Uri("/Details.xaml", UriKind.Relative));
}
Item.cs：
namespace tori
{
public class Item
{
public string title { get; set; }
public string ThumbnailUrl { get; set; }
public string link { get; set; }
public string pubDate { get; set; }
}
}
Details.xaml：
<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
<Grid Height="617" VerticalAlignment="Top" Margin="-27,96,0,0">
<Image x:Name="ThumbnailUrl"
Width="279"
Height="421"
VerticalAlignment="Center"
HorizontalAlignment="Center"
Margin="0,-29,204,225" />
<Image x:Name="Image1" Tap="Image1_Tap" Margin="46,430,354,92" Source="/Images/pausebutton.png"/>
<TextBlock x:Name="title" FontSize="30" TextWrapping="Wrap" Margin="284,57,-50,436" />
<TextBlock x:Name="pubDate" FontSize="25" Margin="284,186,10,363" />
<MediaElement x:Name="MediaElement1" AutoPlay="True" Margin="0,397,20,0" Height="94" VerticalAlignment="Top" />
Details.xaml.cs：
namespace tori
{
public partial class Details : PhoneApplicationPage
{
Item item;
public Details()
{
InitializeComponent();
var app = App.Current as App;
item = app.selectedItem;
title.Text = item.title;
pubDate.Text = item.pubDate;
ThumbnailUrl.Source = new BitmapImage(new Uri(item.ThumbnailUrl, UriKind.RelativeOrAbsolute));
string s = item.link;
string url = s.Replace("archivesplayer", "hostArchivesURLForMobile");
WebClient downloader = new WebClient();
Uri uri = new Uri(url,UriKind.Absolute);
downloader.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Channel3Downloaded);
downloader.DownloadStringAsync(uri);
}
void Channel3Downloaded(object sender, DownloadStringCompletedEventArgs e)
{
var textData = (string)e.Result;
Regex urlRegex = new Regex("<td height=\"25\" align=\"center\">(?<url>.*)</td>");
MatchCollection mc = urlRegex.Matches(textData);
string url = "";
if (mc.Count > 0)
{
url = mc[0].Groups["url"].Value;
MediaElement1.Source = new Uri(url, UriKind.Absolute);
MediaElement1.Play();
}
}
}
}
执行选择项目后如果单击某个按钮，我在此行中收到错误
title.Text = item.title;
任何人都可以告诉我如何克服这个空例外。当我点击一个按钮时，选择改变了事件而不是点击事件。我无法知道原因。
请有人帮帮我。
许多人提前感谢。答案 0 :(得分：1)
更改ItemsSource时，会引发SelectionChanged事件。我想每当你改变ItemsSource时，重置选择（即 - 设置为空（没有选中））。这应该是怎样的，否则ItemsSource中没有的项目可能是SelectedItem，这是错误的。
现在，要解决您的问题，只需在SelectedItem != null方法中检查是否itemsList_SelectionChanged。
关于您的代码的其他信息：方法btn1_Click，btn2_Click和btn3_Click似乎只有微小的差异，因此您可以将大部分代码放在一个方法中并将其传递给它网址。这对于ChannelDownloaded方法来说更为重要（因为它们更长）。基本上你想尽可能多地重用代码。这使得代码更易于阅读（因为它不是10页，而是一个，可以这么说），并且更容易维护（如果出现错误 - 您只需要在一个地方修复它）