WPF錯誤：無法在創建自定義控件時創建類型實例

Question

我正嘗試創建自定義標籤雲控件。它應該工作的方式是用戶可以在itemsSource中給它一個字符串集合，並且轉換後的字符串將顯示在UI中。問題是，當我嘗試將控件放到應用程序中時出現以下錯誤：「無法創建類型爲TagCloudControl的實例」。誰能幫忙？

代碼背後

public class TagCloudControl : ListBox 
{ 
    static TagCloudControl() 
    { 
     DefaultStyleKeyProperty.OverrideMetadata(typeof(TagCloudControl), new FrameworkPropertyMetadata 
      (typeof(TagCloudControl))); 
    } 
    //tags dependency property 
    public static DependencyProperty TagsProperty = DependencyProperty.Register("Tags", 
     typeof(IEnumerable<Tag>), 
     typeof(TagCloudControl)); 

    public CollectionView GroupedTagsView { get; set; } 

    public TagCloudControl() 
    { 
     ItemsSource = Tags; 

     //group my tags by "name" property 
     GroupedTagsView = (ListCollectionView)CollectionViewSource.GetDefaultView(Tags); 
     GroupedTagsView.GroupDescriptions.Add(new PropertyGroupDescription("Name")); 

    } 

    public IEnumerable<Tag> Tags 
    { 
     get { return (IEnumerable<Tag>)GetValue(TagsProperty); } 
     set { SetValue(TagsProperty, value); } 
    } 
} 

XAML

<Window x:Class="TagCloudDemo.Window1" 
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
xmlns:src="clr-namespace:TagCloudControlLibrary;assembly=TagCloudControlLibrary" 
Title="Window1" Height="300" Width="300"> 
<Grid> 

    <src:TagCloudControl HorizontalAlignment="Center" VerticalAlignment="Center" Name="firstTag"/> 

</Grid> 
</Window> 

TagControl XAML

<ResourceDictionary 
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
xmlns:local="clr-namespace:TagCloudControlLibrary"> 

<local:CountToBrushConverter x:Key="CountToBrushConverter"/> 
<local:CountToFontSizeConverter x:Key="CountToFontSizeConverter"/> 

<Style TargetType="{x:Type local:TagCloudControl}"> 
    <Setter Property="Template"> 
     <Setter.Value> 
      <ControlTemplate TargetType="{x:Type local:TagCloudControl}"> 
       <Grid> 
        <WrapPanel Orientation="Horizontal" 
           Margin="2" 
           IsItemsHost="True"> 
        </WrapPanel> 

       </Grid> 
      </ControlTemplate> 
     </Setter.Value> 
    </Setter> 
</Style> 

<DataTemplate x:Key="TagsTemplate"> 
    <WrapPanel> 
     <TextBlock Text="{Binding Name}" 
        FontSize="{Binding ItemCount, Converter={StaticResource CountToBrushConverter}}" 
        Foreground="{Binding ItemCount, Converter={StaticResource CountToFontSizeConverter}}"/> 
    </WrapPanel> 
</DataTemplate> 
</ResourceDictionary> 

標記類

public class Tag 
{ 
    public Tag(string name) 
    { 
     Name = name;   
    } 

    public string Name { get; set;} 

} 

Answer 1

你需要實例化的標籤？

我猜這是一個空引用異常，你的項目源設置爲構造函數中的標籤，但是你沒有實例化標籤。

public TagCloudControl() 
{ 
    Tags = new ObservableCollection<Tags>(); 

    ItemsSource = Tags; 
    ... 
} 

編輯：

與代碼玩弄後......我在想，標籤可能並不需要一個DependencyProperty。看來要綁定的ItemsSource到標籤...但你可以只讓標籤一個簡單的屬性，並在其中，設置的ItemsSource爲傳入的值：

public TagCloudControl() 
{ 
    //this is now empty. 
} 

public IEnumerable<Tag> Tags 
{ 
    get { return (this.ItemsSource as IEnumerable<Tag>); } 
    set { this.ItemsSource = value; } 
} 

而且我覺得你的風格可能要更像是這樣的：

<Style TargetType="{x:Type local:TagCloudControl}"> 
    <Setter Property="ItemTemplate"> 
     <Setter.Value> 
      <DataTemplate DataType="{x:Type local:Tag}"> 
       <TextBlock Text="{Binding Name}" 
          FontSize="{Binding ItemCount, Converter={StaticResource CountToBrushConverter}}" 
          Foreground="{Binding ItemCount, Converter={StaticResource CountToFontSizeConverter}}"> 
      </DataTemplate> 
     </Setter.Value> 
    </Setter> 
    <Setter Property="Template"> 
     <Setter.Value> 
      <ControlTemplate TargetType="{x:Type local:TagCloudControl}"> 
       <Grid> 
        <WrapPanel Orientation="Horizontal" Margin="2" IsItemsHost="True"> 
        </WrapPanel> 
       </Grid> 
      </ControlTemplate> 
     </Setter.Value> 
    </Setter> 
</Style> 

最後，我想你可能會在你的'標記'類中缺少ItemCount。