为什么WPF似乎在实现INotifyPropertyChanged时绕过TypeDescriptionProviderAttribute？

Question

我正在尝试使用它[TypeDescriptionProviderAttribute]来为我的类提供自定义类型描述符.这是有效的,但是当我实现INotifyPropertyChangedWPF时,似乎忽略了自定义类型描述符并直接进入CLR属性(如果存在).这是一个片段,稍后我将粘贴完整的示例:

//[TypeDescriptionProvider(typeof(MyProvider))]
class MyModel : Object
    //, INotifyPropertyChanged
    //, ICustomTypeDescriptor
{
    public string TheProperty { get { return "CLR - TheProperty"; } }

我将TextBlock绑定到TheProperty.当我...

• 留下所有评论

  我按预期看到了"CLR - TheProperty".

• 使用 [TypeDescriptionProvider]

  我按预期看到了"MyPropertyDescriptor - TheProperty".

• 使用 ICustomTypeDescriptor

  我按预期看到了"MyPropertyDescriptor - TheProperty".

• 使用ICustomTypeDescriptor和INotifyPropertyChanged

  我按预期看到了"MyPropertyDescriptor - TheProperty".

• 使用[TypeDescriptionProvider]和INotifyPropertyChanged

  我看到"CLR - TheProperty".为什么是这样？奇怪的是,正常显示没有 CLR属性的自定义属性.我的自定义类型描述符也返回"MyPropertyDescriptor - AnotherProperty",它在所有情况下都有效,因为没有AnotherProperty定义CLR .

总之,鉴于此XAML

<StackPanel>
    <TextBlock Text="{Binding TheProperty}" />
    <TextBlock Text="{Binding AnotherProperty}" />
</StackPanel>

AnotherProperty始终按预期工作,因为模型没有名为"AnotherProperty"的CLR属性.TheProperty按预期工作,除非使用[TypeDescriptionProvider]和INotifyPropertyChanged使用.

这是完整的代码.它有点长,但大部分都是无关紧要的,它只是System.ComponentModel所要求的

public partial class TestWindow : Window
{
    public TestWindow()
    {
        InitializeComponent();
        DataContext = new MyModel();
    }
}

//[TypeDescriptionProvider(typeof(MyProvider))]
class MyModel : Object
    //, INotifyPropertyChanged
    //, ICustomTypeDescriptor
{
    public string TheProperty { get { return "CLR - TheProperty"; } }

    public event PropertyChangedEventHandler PropertyChanged;

    public AttributeCollection GetAttributes()
    {
        return TypeDescriptor.GetAttributes(this);
    }

    public string GetClassName()
    {
        return TypeDescriptor.GetClassName(this);
    }

    public string GetComponentName()
    {
        return TypeDescriptor.GetComponentName(this);
    }

    public TypeConverter GetConverter()
    {
        return TypeDescriptor.GetConverter(this);
    }

    public EventDescriptor GetDefaultEvent()
    {
        return TypeDescriptor.GetDefaultEvent(this);
    }

    public PropertyDescriptor GetDefaultProperty()
    {
        return TypeDescriptor.GetDefaultProperty(this);
    }

    public object GetEditor(Type editorBaseType)
    {
        return TypeDescriptor.GetEditor(this, editorBaseType);
    }

    public EventDescriptorCollection GetEvents(Attribute[] attributes)
    {
        return TypeDescriptor.GetEvents(this, attributes);
    }

    public EventDescriptorCollection GetEvents()
    {
        return TypeDescriptor.GetEvents(this);
    }

    public PropertyDescriptorCollection GetProperties(Attribute[] attributes)
    {
        return TypeDescriptor.GetProperties(this, attributes);
    }

    public PropertyDescriptorCollection GetProperties()
    {
        return MyTypeDescriptor.GetCustomProperties();
    }

    public object GetPropertyOwner(PropertyDescriptor pd)
    {
        return this;
    }
}


class MyProvider : TypeDescriptionProvider
{
    public override ICustomTypeDescriptor GetTypeDescriptor(Type objectType, object instance)
    {
        return new MyTypeDescriptor();
    }
}


class MyTypeDescriptor : CustomTypeDescriptor
{
    public override PropertyDescriptorCollection GetProperties()
    {
        return GetCustomProperties();
    }

    public static PropertyDescriptorCollection GetCustomProperties()
    {
        return new PropertyDescriptorCollection(
            new[] { 
                new MyPropertyDescriptor("TheProperty"),
                new MyPropertyDescriptor("AnotherProperty")
            });
    }
}


class MyPropertyDescriptor : PropertyDescriptor
{
    public MyPropertyDescriptor(string propName)
        : base(propName, null)
    {
    }

    public override bool CanResetValue(object component)
    {
        return false;
    }

    public override Type ComponentType
    {
        get { return typeof(MyModel); }
    }

    public override object GetValue(object component)
    {
        return "MyPropertyDescriptor - " + Name;
    }

    public override bool IsReadOnly
    {
        get { return true; }
    }

    public override Type PropertyType
    {
        get { return typeof(string); }
    }

    public override void ResetValue(object component)
    {
        throw new InvalidOperationException("cannot reset value");
    }

    public override void SetValue(object component, object value)
    {
        throw new InvalidOperationException("property is readonly");
    }

    public override bool ShouldSerializeValue(object component)
    {
        return true;
    }
}

Answer 1

老问题,但对于寻找答案的人来说..

问题出在System.Windows.PropertyPath.ResolvePropertyName(String,Object,Type,Object,Boolean)私有方法中.我在.NET 4.0中的PresentationFramework.dll中找到了它.

从.NET Reflector中提取:

object propertyHelper = DependencyProperty.FromName(str, ownerType);
if ((propertyHelper == null) && (item is ICustomTypeDescriptor))
{
    propertyHelper = TypeDescriptor.GetProperties(item)[str];
}
if ((propertyHelper == null) && ((item is INotifyPropertyChanged) || (item is DependencyObject)))
{
    propertyHelper = this.GetPropertyHelper(ownerType, str);
}
if (propertyHelper == null)
{
    propertyHelper = TypeDescriptor.GetProperties(item)[str];
}
if (propertyHelper == null)
{
    propertyHelper = this.GetPropertyHelper(ownerType, str);
}
if ((propertyHelper == null) && throwOnError)
{
    throw new InvalidOperationException(SR.Get("PropertyPathNoProperty", new object[] { ownerType.Name, str }));
}
return propertyHelper;

如您所见,检索属性标识符(DependencyProperty/PropertyDescriptor/PropertyInfo)如下所示:

1. 尝试获取DependencyProperty,
2. 如果item实现ICustomTypeDescriptor,请使用TypeDescriptor获取PropertyDescriptor,
3. 如果item实现了INotifyPropertyChanged或者是DependencyObject,请使用System.Reflection获取PropertyInfo,
4. 否则使用TypeDescriptor获取PropertyDescriptor,
5. 否则使用System.Reflection获取PropertyInfo,
6. 否则抛出异常或返回null.

因此,如果item实现了INotifyPropertyChanged接口,则System.Reflection/PropertyInfo优先于TypeDescriptor/PropertyDescriptor.我认为他们出于性能原因选择了这种策略,因为PropertyInfo比PropertyDescriptor轻得多.

您的问题的解决方案是实现ICustomTypeDescriptor(最好是显式的),以便它将ICustomTypeDescriptor方法调用传递给适当的TypeDescriptor方法调用,但不传输对象参数,但是Type参数(this.GetType()).这样就可以使用TypeDescriptionProvider.