wpfdata-bindinginotifypropertychangedicustomtypedescriptorcustomtypedescriptor

Why does WPF seem to bypass TypeDescriptionProviderAttribute when INotifyPropertyChanged is implemented?

I'm trying to use the [TypeDescriptionProviderAttribute] in order to give my class a custom type descriptor. This works, but when I implement INotifyPropertyChanged WPF seems to ignore the custom type descriptor and go straight for the CLR property (if it exists). Here's a snippet, I'll paste the full example later on:

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

I bind a TextBlock to TheProperty. When I...

In summary, given this XAML

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

AnotherProperty always works as expected because the model does not have a CLR property named "AnotherProperty". TheProperty works as expected except when [TypeDescriptionProvider] and INotifyPropertyChanged are both used.

Here's the full code. It's a bit long but most of it is irrelevant, it's just required by System.ComponentModel

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;
    }
}
Solution

  • Old question, but for people looking for an answer..

    Problem is in System.Windows.PropertyPath.ResolvePropertyName(String, Object, Type, Object, Boolean) private method. I have found it in PresentationFramework.dll in .NET 4.0.

    Extracted from .NET Reflector:

    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;

    As you can see, retrieving property identifier (DependencyProperty / PropertyDescriptor / PropertyInfo) goes like this:

    1. Try get DependencyProperty,
    2. If item implements ICustomTypeDescriptor, use TypeDescriptor to get PropertyDescriptor,
    3. If item implements INotifyPropertyChanged or is DependencyObject, use System.Reflection to get PropertyInfo,
    4. Else use TypeDescriptor to get PropertyDescriptor,
    5. Else use System.Reflection to get PropertyInfo,
    6. Else throw exception or return null.

    So System.Reflection/PropertyInfo gets precedence over TypeDescriptor/PropertyDescriptor if item implements INotifyPropertyChanged interface. I believe they choose this strategy for performance reasons because PropertyInfo is much more lighter than PropertyDescriptor.

    Solution to your problem would be to implement ICustomTypeDescriptor (preferably explicitly) so that it transfers ICustomTypeDescriptor method calls to appropriate TypeDescriptor method calls but not with object parameter, but Type parameter (this.GetType()). This way your TypeDescriptionProvider will be used.