为什么在MultiBinding中转换值时会得到DependencyProperty.UnsetValue？ [英] Why do I get a DependencyProperty.UnsetValue when converting a value in a MultiBinding?

问题描述

我有一个非常简单的IMultiValueConverter，可以对两个值进行或运算。在下面的示例中，我想使用一个同样简单的布尔值反相器来反转第一个值。

 < MultiBinding Converter = { StaticResource multiBoolToVis}> 
< Binding Path = ConditionA Converter = {StaticResource boolInverter} /> 
< Binding Path = ConditionB /> 
< / MultiBinding> 
  

和逆变器：

 公共类BoolInverterConverter：IValueConverter 
 {
 #region IValueConverter成员
公共对象Convert（对象值，类型targetType，对象参数，System.Globalization.CultureInfo文化）
 {
 if（值是布尔值）
 {
 return！（（bool）value）; 
} 
返回null； 
} 
公共对象ConvertBack（对象值，类型targetType，对象参数，System.Globalization.CultureInfo文化）
 {
 throw new NotImplementedException（）; 
} 
 #endregion 
} 
  

当我包含boolInverter时，MultiValueConverter中的第一个值将成为 DependencyProperty.UnsetValue。当我不使用转换器时，没有任何问题（当然，除了我要使用的逻辑之外）。

我错过了什么吗？单步调试器显示InverseBoolConverter正确反转了我传递的值，但随后该值未发送到MultiValueConverter。

解决方案

从MSDN：

UnsetValue是一个哨兵值，用于WPF属性系统无法确定请求的DependencyProperty值的情况。使用UnsetValue而不是使用null引用（在Visual Basic中为Nothing），因为null引用可以是有效的属性值，也可以是有效的（并且经常使用的）DefaultValue。

这意味着以下内容之一：

• 您使用模板（ ControlTemplate 或 DataTemplate ），并且该值在加载时未设置数据源。因此它将两次对您的转换器产生影响，首先是UnsetValue，其次是布尔值；因此不必担心；


• 您的 Binding 是错误的，这意味着Binding无法确定值，从而导致UnsetValue。

此外，您不能像您那样合并Converters。 >

在内部绑定中删除转换器，它应该是固定的！ ：）

希望这会有所帮助！

I have an extremely simple IMultiValueConverter that simply OR's two values. In the example below, I want to invert the first value using an equally simple boolean inverter.

<MultiBinding Converter="{StaticResource multiBoolToVis}">
    <Binding Path="ConditionA" Converter="{StaticResource boolInverter}"/>
    <Binding Path="ConditionB"/>
</MultiBinding>

and the inverter:

public class BoolInverterConverter : IValueConverter
{
    #region IValueConverter Members
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value is bool)
        {
            return !((bool)value);
        }
        return null;
    }
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
    #endregion
}

When I include the boolInverter, the first value in the MultiValueConverter becomes a "DependencyProperty.UnsetValue". There are no problems when I do not use the converter (other than not the logic I am aiming for, of course).

Am I missing something? Stepping through the debugger shows that the InverseBoolConverter is properly inverting the value I pass it, but that value is then not being 'sent' to the MultiValueConverter.

解决方案

From MSDN:

UnsetValue is a sentinel value that is used for scenarios where the WPF property system is unable to determine a requested DependencyProperty value. UnsetValue is used rather than null reference (Nothing in Visual Basic), because null reference could be a valid property value, as well as a valid (and frequently used) DefaultValue.

Which means one of the following things:

• You use a template (ControlTemplate or DataTemplate), and the value does not have a DataSource set at the time of being Loaded. So it will hit your converter twice, first with the UnsetValue, second with the boolean value; so nothing to worry about;
• Your Binding is incorrect, meaning the Binding cannot determine a value, thus resulting in the UnsetValue.. You should propbably see a warning..

Also, you cannot combine Converters like you do.. So its probably that.

Remove the Converter in the inner Binding, and it should be fixed! :)

Hope this helps!

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