我有一个ListBox,它显示来自web服务的注释列表。当我运行应用程序时,评论显示良好。现在,我想使用ListBox中所选项目的对象的属性,并获取ID,然后将其分配给字符串值。当我现在运行该应用程序,并单击注释选择它时,我在visualstudio中得到一个Null异常。然后,我在代码行上设置了一个断点,并试图在鼠标上获取值,它返回了所有对象,但它们都是空的。下面的代码片段:
<ListBox x:Name="lbxComments" Margin="0,0,-12,0"
ItemsSource="{Binding CommentList,Mode=TwoWay}"
SelectionChanged="lbxComments_SelectionChanged"
>
<ListBox.ItemTemplate>
<DataTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="auto"/>
<ColumnDefinition Width="480"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="auto"/>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Image Stretch="UniformToFill" Height="50" Width="50" Source="{Binding profile_pic}" Margin="8" HorizontalAlignment="Left" VerticalAlignment="Top"/>
<Border Grid.ColumnSpan="2" Grid.Row="0" HorizontalAlignment="Stretch" BorderBrush="Black" BorderThickness="0,0,0,0.5"/>
<StackPanel Grid.Column="1" >
<TextBlock VerticalAlignment="Center" HorizontalAlignment="Left" FontSize="20" Text="{Binding comment}" TextWrapping="Wrap" TextTrimming="WordEllipsis" Foreground="White" />
<StackPanel Orientation="Horizontal" Margin="0,0,0,5">
<TextBlock VerticalAlignment="Center" HorizontalAlignment="Left" FontSize="16" Text="{Binding fname}" Foreground="White"/>
<StackPanel Orientation="Horizontal">
<TextBlock VerticalAlignment="Center" HorizontalAlignment="Left" FontSize="16" Text="{Binding lname}" Margin="10 0 0 0" Foreground="White"/>
<StackPanel Orientation="Horizontal">
<TextBlock Text="Replies"/>
<TextBlock Text="("/>
<TextBlock Text="{Binding totalreplies}"/>
<TextBlock Text=")" />
</StackPanel>
</StackPanel>
</StackPanel>
</StackPanel>
<TextBlock x:Name="replyTextBox" Grid.Row="2" Margin="50,0,0,0" Visibility="Collapsed" />
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
在后面的代码中:
private void lbxComments_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
string selectedCommentId;
Comment comment = ((sender as FrameworkElement).DataContext) as Comment;
comment = new Comment();
if (comment.id != null)
{
selectedCommentId = comment.id;
repliesViewModel.SetAddress(selectedCommentId);
}
lbxComments绑定到的CommentList是一种Comment
public class Comment
{
public string fname { get; set; }
public string lname { get; set; }
public string profile_pic { get; set; }
public string id { get; set; }
public string username { get; set; }
public string comment { get; set; }
public string totalreplies { get; set; }
}
所以现在我不知道为什么当我试图获取所选项目对象时,它会返回null,但在列表框
您的代码中有两个问题:
问题1:您正在铸造错误的对象:
在这种情况下;sender
就是ListBox
,实际上是将ListBox
强制转换为Comment
对象。这就是它抛出NullReferenceException
的原因。
解决方案:
基本上,您必须强制转换SelectedItems
,这在SelectionChangedEventArgs
实例e
中可用。它具有属性AddedItems
,它包含所选项目的列表。
private void lbxComments_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
//get all selected items and cast them into 'Comment'
foreach (Comment comment in e.AddedItems)
{
string selectedCommentId;
if (comment.id != null)
{
selectedCommentId = comment.id;
repliesViewModel.SetAddress(selectedCommentId);
}
}
问题2:您正在重新初始化comment
:
您不应该重新初始化它,它会使所有内容成为null
。
Comment comment = ((sender as FrameworkElement).DataContext) as Comment;
comment = new Comment(); // <- reassignment here
您首先正确地获得了注释,但紧接着您将一个新的Comment
分配给同一个变量,因此现在它有一个空的id
。