wpf - 使用标记扩展绑定(bind)时出错 : Unknown property encountered while parsing a Markup Extension

Question

原则上，我开发了一种将 RadioButtons 绑定(bind)到几乎任何东西的简洁方法:

/// <summary>Converts an value to 'true' if it matches the 'To' property.</summary>
/// <example>
/// <RadioButton IsChecked="{Binding VersionString, Converter={local:TrueWhenEqual To='1.0'}}"/>
/// </example>
public class TrueWhenEqual : MarkupExtension, IValueConverter
{
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }

    public object To { get; set; }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return object.Equals(value, To);
    }
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if ((bool)value) return To;
        throw new NotSupportedException();
    }
}

例如，您可以使用它来将 RadioButtons 绑定(bind)到字符串属性，如下所示(这是一个众所周知的错误，您必须为每个 RadioButton 使用唯一的 GroupName):

<RadioButton GroupName="G1" Content="Cat"
    IsChecked="{Binding Animal, Converter={local:TrueWhenEqual To='CAT'}}"/>
<RadioButton GroupName="G2" Content="Dog"
    IsChecked="{Binding Animal, Converter={local:TrueWhenEqual To='DOG'}}"/>
<RadioButton GroupName="G3" Content="Horse"
    IsChecked="{Binding Animal, Converter={local:TrueWhenEqual To='HORSE'}}"/>

现在，我想使用 public static readonly名为 Filter1 的对象和 Filter2作为我的 RadioButtons 的值。所以我尝试了:

<RadioButton GroupName="F1" Content="Filter Number One"
    IsChecked="{Binding Filter, Converter={local:TrueWhenEqual To='{x:Static local:ViewModelClass.Filter1}'}}"/>
<RadioButton GroupName="F2" Content="Filter Number Two"
    IsChecked="{Binding Filter, Converter={local:TrueWhenEqual To='{x:Static local:ViewModelClass.Filter2}'}}"/>

但这给了我一个错误:

Unknown property 'To' for type 'MS.Internal.Markup.MarkupExtensionParser+UnknownMarkupExtension' encountered while parsing a Markup Extension.

如果我删除引号，错误仍然会发生。我究竟做错了什么？

Answer 1

WPF 不能很好地处理嵌套标记扩展。为了克服这个问题，您可以使用您的标记扩展作为一个元素。它有点笨拙且难以阅读，但它有效:

<RadioButton GroupName="F1" Content="Filter Number One">
    <RadioButton.IsChecked>
        <Binding Path="Filter">
            <Binding.Converter>
                <local:TrueWhenEqual To={x:Static local:ViewModelClass.Filter1} />
            </Binding.Converter>
        </Binding>
    </RadioButton.IsChecked>
</RadioButton>

另一种方法是声明您的转换器并将其用作静态资源:

<Window.Resources>
    <local:TrueWhenEqual To={x:Static local:ViewModelClass.Filter1} x:Key="myConverter" />
</Window.Resources>

<RadioButton GroupName="F1" Content="Filter Number One"
             IsChecked="{Binding Filter, Converter={StaticResource myConverter}}" />