我正在尝试创建一个希望能够公开多个内容属性的UserControl。但是，我失败了！
我们的想法是创建一个优秀的用户控件（我们称之为 MultiContent ），公开两个内容属性，以便我可以执行以下操作：
<local:MultiContent>
<local:MultiContent.ListContent>
<ListBox x:Name="lstListOfStuff" Width="50" Height="50" />
</local:MultiContent.ListContent>
<local:MultiContent.ItemContent>
<TextBox x:Name="txtItemName" Width="50" />
</local:MultiContent.ItemContent>
</local:MultiContent>
这将非常有用，现在我可以根据情况更改ListContent和ItemContent，并在MultiContent用户控件中考虑常用功能。
但是，我目前实现此方法的方式，我无法访问 MultiContent 控件的这些内容属性中的UI元素。例如，当我尝试访问时， lstListOfStuff 和 txtItemName 都是 null ：
public MainPage() {
InitializeComponent();
this.txtItemName.Text = "Item 1"; // <-- txtItemName is null, so this throws an exception
}
以下是我实现MultiContent用户控件的方法：
XAML：MultiContent.xaml
<UserControl x:Class="Example.MultiContent"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Width="400" Height="300">
<Grid x:Name="LayoutRoot" Background="White">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100" />
<ColumnDefinition Width="100" />
</Grid.ColumnDefinitions>
<ContentControl x:Name="pnlList" Grid.Column="0" />
<ContentControl x:Name="pnlItem" Grid.Column="1" />
</Grid>
</UserControl>
代码背后：MultiContent.xaml.cs
// Namespaces Removed
namespace Example
{
public partial class MultiContent : UserControl
{
public UIElement ListContent
{
get { return (UIElement)GetValue(ListContentProperty); }
set
{
this.pnlList.Content = value;
SetValue(ListContentProperty, value);
}
}
public static readonly DependencyProperty ListContentProperty =
DependencyProperty.Register("ListContent", typeof(UIElement), typeof(MultiContent), new PropertyMetadata(null));
public UIElement ItemContent
{
get { return (UIElement)GetValue(ItemContentProperty); }
set
{
this.pnlItem.Content = value;
SetValue(ItemContentProperty, value);
}
}
public static readonly DependencyProperty ItemContentProperty =
DependencyProperty.Register("ItemContent", typeof(UIElement), typeof(MultiContent), new PropertyMetadata(null));
public MultiContent()
{
InitializeComponent();
}
}
}
我可能完全错了。有谁知道我怎么能让它正常工作？如何从父控件中按名称访问这些UI元素？关于如何做得更好的任何建议？谢谢！
答案 0 :(得分：3)
你绝对可以实现目标，但你需要采取不同的方法。
在您的解决方案中，您尝试为UIElement设置依赖项属性 - 并且因为它永远不会被设置并且默认值为null，这就是您获得NullReference异常的原因。您可以将默认值从null更改为新的TextBox或类似的东西，但即使它确实有效，它仍然会感觉像是黑客。
在Silverlight中，您必须自己实现Dpendency Properties。但是你没有实现它们 - 我倾向于使用这个dependency property generator来实现它们。
DP的一大优点是它们支持变更通知。因此，考虑到这一点，你需要做的就是让你的样本工作是定义为DPs：ItemContent和ListContent与Content（对象）的类型相同，当框架通知你他们中的任何一个已被更改时，只需更新你的文本框！所以这是执行此操作的代码：
MultiContent.xaml：
<Grid x:Name="LayoutRoot" Background="White">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100" />
<ColumnDefinition Width="100" />
</Grid.ColumnDefinitions>
<ContentControl x:Name="pnlList" Grid.Column="0" />
<ContentControl x:Name="pnlItem" Grid.Column="1" />
</Grid>
MultiContent.xaml.cs：
namespace MultiContent
{
public partial class MultiContent : UserControl
{
#region ListContent
/// <summary>
/// ListContent Dependency Property
/// </summary>
public object ListContent
{
get { return (object)GetValue(ListContentProperty); }
set { SetValue(ListContentProperty, value); }
}
/// <summary>
/// Identifies the ListContent Dependency Property.
/// </summary>
public static readonly DependencyProperty ListContentProperty =
DependencyProperty.Register("ListContent", typeof(object),
typeof(MultiContent), new PropertyMetadata(null, OnListContentPropertyChanged));
private static void OnListContentPropertyChanged
(object sender, DependencyPropertyChangedEventArgs e)
{
MultiContent m = sender as MultiContent;
m.OnPropertyChanged("ListContent");
}
#endregion
#region ItemContent
/// <summary>
/// ItemContent Dependency Property
/// </summary>
public object ItemContent
{
get { return (object)GetValue(ItemContentProperty); }
set { SetValue(ItemContentProperty, value); }
}
/// <summary>
/// Identifies the ItemContent Dependency Property.
/// </summary>
public static readonly DependencyProperty ItemContentProperty =
DependencyProperty.Register("ItemContent", typeof(object),
typeof(MultiContent), new PropertyMetadata(null, OnItemContentPropertyChanged));
private static void OnItemContentPropertyChanged
(object sender, DependencyPropertyChangedEventArgs e)
{
MultiContent m = sender as MultiContent;
m.OnPropertyChanged("ItemContent");
}
#endregion
/// <summary>
/// Event called when any chart property changes
/// Note that this property is not used in the example but is good to have if you plan to extend the class!
/// </summary>
public event PropertyChangedEventHandler PropertyChanged;
/// <summary>
/// Called to invoke the property changed event
/// </summary>
/// <param name="propertyName">The property that has changed</param>
protected void OnPropertyChanged(string propertyName)
{
if (propertyName == "ListContent")
{
// The ListContent property has been changed, let's update the control!
this.pnlList.Content = this.ListContent;
}
if (propertyName == "ItemContent")
{
// The ListContent property has been changed, let's update the control!
this.pnlItem.Content = this.ItemContent;
}
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
public MultiContent()
{
InitializeComponent();
}
}
}
MainPage.xaml中：
<Grid x:Name="LayoutRoot" Background="White">
<local:MultiContent>
<local:MultiContent.ListContent>
<ListBox x:Name="lstListOfStuff" Width="50" Height="50" />
</local:MultiContent.ListContent>
<local:MultiContent.ItemContent>
<TextBox x:Name="txtItemName" Width="50" />
</local:MultiContent.ItemContent>
</local:MultiContent>
</Grid>
这应该可以解决问题！