Question

I’m using the XamlServices.Transform to take an object model and serialize it to Xaml.

I’ve implemented a class which inherits from XamlXmlWriter which overrides WriteValue. I’m using this to reinstantiate a custom MarkupExtension back into the rendered Xaml. My code works fine except when the value of the property is null, in which case the WriteValue doesn’t fire and I don’t get chance to “swap out” the value in the overriden class.

A related issue is where a property has the same value as that specified by the System.ComponentModel.DefaultValue() attribute. For example say i’ve got a property in my object model decorated like this:

[DefaultValue(true)]
public Boolean IsVisible {get; set;}

Then the WriteValue method only fires if the IsVisible property is false (which kind of makes sense).

The Remarks section in the documentation (http://msdn.microsoft.com/en-us/library/system.xaml.xamlxmlwriter.writevalue.aspx) mentions something about null values, but I don’t understand it:

The input value may be null, which
supports explicitly writing out null
as a serialized value. This behavior
uses the XamlLanguage.Null definition
as WriteStartObject input and then
immediately calls WriteEndObject.

How to I a) make the “WriteValue” fire when the property is null, and b) make the “WriteValue” fire when the property is the same as the DefaultValue Attribute?

I’m not sure if they are related, a solution for either of them would be very welcome.

Thanks,

Daniel

Answer 1

To answer ‘b’ first: The XamlObjectReader’s intended behavior is to skip properties whose value is the declared “default value” and we have no configuration feature to override that. Note that “default value” here is the one declared with the [DefaultValue()] attribute, not the C# language default(T), so things may not be as bad as you fear. I mean not every “0” in an “Int” property is skipped because it is the “default value”.

‘a’: The XamlXmlWriter’s output Node stream for a null value is not “WriteValue(null)”, but instead is “WriteStartObject(nullExtensions); WriteEndObject()”. This is the behavior the documentation you quoted was describing. So you should be fine. Look for StartObject “nullExtension” instead of value “null”.