我有一些类型层次结构:
public class Base {}
public class Derived_1 : Base {}
public class Derived_2 : Base {}
// more descendants...
public class Derived_N : Base {}
此层次结构中的类型用作视图模型中的查找列表:
public class SomeViewModel
{
// available items
public IEnumerable<Derived_N> SomeItems { get; }
// currently selected item
public Derived_N SelectedItem { get; set; }
// there could be several property pairs as above
}
要从查找列表中选择值,我已经创建了用户控件(某种选择器).从选择过程开始,所有Base后代看起来都相似,用户控件操作Base类型属性:
public IEnumerable<Base> ItemsSource
{
get { return (IEnumerable<Base>)GetValue(ItemsSourceProperty); }
set { SetValue(ItemsSourceProperty, value); }
}
public static readonly DependencyProperty ItemsSourceProperty =
DependencyProperty.Register("ItemsSource", typeof(IEnumerable<Base>), typeof(BaseSelector), new PropertyMetadata(null));
public Base SelectedItem
{
get { return (Base)GetValue(SelectedItemProperty); }
set { SetValue(SelectedItemProperty, value); }
}
public static readonly DependencyProperty SelectedItemProperty =
DependencyProperty.Register("SelectedItem", typeof(Base), typeof(BaseSelector), new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
XAML通常看起来像:
<myCtrls:BaseSelector ItemsSource="{Binding SomeItems}"
SelectedItem="{Binding SelectedItem}"/>
这按预期工作,但有这样的绑定错误:
Cannot create default converter to perform ‘two-way’ conversions
between types ‘Derived_N’ and ‘Base’
我知道,为什么它们在输出窗口中 – 理论上,SelectedItem可以是任何类型,派生自Base,但实际上这不是我的情况.
如果这个转换器错误消失:
public class DummyConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return value;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
return value;
}
}
用于绑定:
<myCtrls:BaseSelector ItemsSource="{Binding SomeItems}"
SelectedItem="{Binding SelectedItem, Converter={StaticResource DummyConverterKey}}"/>
但我根本不想使用它 – 正如你所看到的那样,转换器中没有任何有效载荷(虽然有很多这样的属性).
还有其他解决方法吗?
最佳答案 现在,我已经解决了将用户控件属性的属性类型分别替换为IEnumerable / object的问题(IEnumerable< object> / object也是一个解决方案):
public static readonly DependencyProperty ItemsSourceProperty =
DependencyProperty.Register("ItemsSource", typeof(IEnumerable), typeof(BaseSelector), new PropertyMetadata(null));
public static readonly DependencyProperty SelectedItemProperty =
DependencyProperty.Register("SelectedItem", typeof(object), typeof(BaseSelector), new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
这导致在用户控件内进行额外的类型检查,但不会产生任何绑定错误(我真的不明白,为什么 – 对象的情况与Base,IMO相同).