[英]WPF Error: Could not create an instance of type… when creating custom control
我正在嘗試創建自定義標簽雲控件。 它應該起作用的方式是,用戶可以在itemsSource中為其提供字符串集合,並且轉換后的字符串將顯示在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TagCloudControl類型的實例”。 有人可以幫忙嗎?
后面的代碼
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>
標簽類別
public class Tag
{
public Tag(string name)
{
Name = name;
}
public string Name { get; set;}
}
您需要實例化標簽嗎?
我猜這是一個空引用異常,您在構造函數中將項源設置為Tag,但是尚未實例化Tag。
public TagCloudControl()
{
Tags = new ObservableCollection<Tags>();
ItemsSource = Tags;
...
}
編輯:
在玩了代碼之后...我認為標記可能不需要是DependencyProperty。 似乎您想將ItemsSource綁定到Tags ...,但是您可以將Tags作為一個簡單的屬性,然后在其中將ItemsSource設置為傳入的值:
public TagCloudControl()
{
//this is now empty.
}
public IEnumerable<Tag> Tags
{
get { return (this.ItemsSource as IEnumerable<Tag>); }
set { this.ItemsSource = value; }
}
另外,我認為您的樣式可能希望更像這樣:
<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>
最后,我認為您可能在“標記”類中缺少ItemCount。
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