我想要一个PropertyChangedCallback添加到UIElement.RenderTransformOriginProperty。当我尝试重写PropertyMetadata，则抛出异常。

我寻觅MSDN和谷歌，而我已经能够拿出的this. DependencyPropertyDescriptor.AddValueChanged建议在该职位的某个时刻，但不会解决我的问题，因为这不是每个实例的回调。

我不明白这是什么异常意味着在所有。有谁知道我在做什么错了？

 公共类Foo：FrameworkElement的
{
    私有静态无效Origin_Changed（DependencyObject的研发，
                                        DependencyPropertyChangedEventArgs五）
    {}

    静态FOO（）
    {
        PropertyMetadata OriginalMetaData =
            UIElement.RenderTransformOriginProperty.GetMetadata（
                typeof运算（FrameworkElement的））;



/ *当执行这一行，则抛出异常：
 无法更改属性的元数据后，它已经与物业相关的* /
        OriginalMetaData.PropertyChangedCallback + =
            新PropertyChangedCallback（Origin_Changed）;



        UIElement.RenderTransformOriginProperty.OverrideMetadata（
            typeof运算（富），OriginalMetaData）;
    }
}
 

解决方案

WPF将合并属性的元数据为你，当你调用OverrideMetadata，没有必要通过它原来的元数据对象。因此，所有你需要做的就是

  UIElement.RenderTransformOriginProperty.OverrideMetadata（typeof运算（富），新PropertyMetadata（新PropertyChangedCallback（Origin_Changed）））;
 

有一点需要注意的是有时候code以上抛出一个异常。这两种情况下，出现这种情况是

1 原来的元数据是PropertyMetadata的一个子类 - 我见过FrameworkPropertyMetadata和UIPropertyMetadata。你只需要使用适当的一个在每种情况下。

2 的依赖项属性是只读的，你不能做任何事情。

I'm trying to add a PropertyChangedCallback to UIElement.RenderTransformOriginProperty. An exception is thrown when I try to override the PropertyMetadata.

I have searched MSDN and Google, and all I have been able to come up with is this. DependencyPropertyDescriptor.AddValueChanged is suggested at some point in that post, but that won't solve my problem since this is not a per-instance callback.

I don't understand what this exception means at all. Does anyone know what I am doing wrong?

public class foo : FrameworkElement
{
    private static void Origin_Changed( DependencyObject d,
                                        DependencyPropertyChangedEventArgs e)
    { }

    static foo()
    {
        PropertyMetadata OriginalMetaData =
            UIElement.RenderTransformOriginProperty.GetMetadata(
                typeof(FrameworkElement));



/*An exception is thrown when this line is executed:
 "Cannot change property metadata after it has been associated with a property"*/
        OriginalMetaData.PropertyChangedCallback +=
            new PropertyChangedCallback(Origin_Changed);



        UIElement.RenderTransformOriginProperty.OverrideMetadata(
            typeof(foo), OriginalMetaData);
    }
}

WPF will merge the property metadata for you when you call OverrideMetadata, no need to pass it the original Metadata object. So all you have to do is

UIElement.RenderTransformOriginProperty.OverrideMetadata(typeof(foo), new PropertyMetadata(new PropertyChangedCallback(Origin_Changed)));

One thing to be aware of is sometimes the code above throws an exception. The two cases where that happens are

1. The original metadata is a subclass of PropertyMetadata - I have seen FrameworkPropertyMetadata and UIPropertyMetadata. You just have to use the appropriate one in each case.

2. The dependency property is read only and you can't do anything about it.