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VB 9.0 Xml features - latest update

  • Article

For those who do not know me yet, my name is Avner Aharoni and I am a program manager in the WebData Xml team. I am working on the VB 9.0 Xml features that I believe will make VB the most productive language with which to program Xml.

  

We heard from many people who tried the VB 9.0 January preview that the late-bound properties we added to XLinq objects in VB 9.0 are confusing.  One reason is that it is hard to know if a certain property name refers to the underlying object property or to the elements with that name, which makes writing and reading code harder. In addition, the way we chose to disambiguate between XLinq object properties and the Xml late bound properties by calling the “Element” method was unsatisfactory, since the developer needs extra knowledge to use this feature, and needs to know which properties exist on the XLinq objects to get it right. Consider the following example:

Dim Person = <Person Dept="SQL Server">

      <Name>Erik Meijer</Name>

 </Person>

Console.WriteLine(Person.Name)

  Console.WriteLine(Person.Element("Name"))

 

The output is:

 

Person 'This is the name of the person element

<Name>Erik Meijer</Name> 'This is the "Name" element itself

 

To fix this issue we introduced a new syntax for these late-bound properties.  Angle brackets now wrap the Xml member name so it is clear that the name refers to the elements with that name. For example Person.<Name> returns the elements called Name in the above document. To unify the syntax we are also using angle brackets for the Descendants method. For example Person...<Name> returns all the descendants of the Person element with the element name “Name”. You can read more about it in Erik Meijer's post on lambda-the-ultimate . As a very nice side effect, we do not require any more double-colon for the namespace prefix, since the Xml name is enclosed in the angle brackets. We also renamed the feature to “Xml axis members” or just “axis members” to make it clearer.

 

In addition to this change in the axis members’ syntax, we are planning to change the binding of the Xml attribute member “@” to XLinq’s “Attributes” method as opposed to the “Attribute” method as the compiler did in the January CTP. As a result, all of the axis members now return IEnumerable(Of XElement) or IEnumerable(Of XAttribute). This aspect is important since we have added a .Value extension property on these IEnumerables that will get the first object in the enumerable (either XElement or XAttribute) and call the underlying .Value property on that object. If the result is an empty collection, we return Nothing. The following example illustrates how all these changes come together to provide a very nice programming interface with XLinq objects:

Dim Person = <Person Dept="SQL Server">

            <Name>Erik Meijer</Name>

 </Person>

Console.WriteLine(Person.<Name>.Value)

Console.WriteLine(Person.@Dept.Value>)

Person.<Name>.Value = "Mike Champion"

Console.WriteLine(Person.<Name>.Value)

 

The output is:

Erik Meijer

SQL Server

Mike Champion

We plan to include these changes in the next CTP. As always feedback is welcome.

 

Avner Aharoni

Comments

  • Anonymous
    February 21, 2006
    Bill McCarthy asked me to post Xml axis members example using namespaces. So I updated the sample from...

  • Anonymous
    February 21, 2006
    Why use the "<", ">" and "@" conventions?

    I've been playing with the XSD.EXE utility that has the /C option (generates managed classes from schemas/XML instances) and the classes it generates seems to work quite nicely despite the fact that it makes use of properties to represent elements/attributes.


  • Anonymous
    February 23, 2006
    Is it possible to make "value" the default member so instead of writing:

              Person.<Name>.Value
              Person.@Dept.Value


    you can optionally write:

              Person.<Name>
              Person.@Dept


  • Anonymous
    February 23, 2006
    To: Johan Myburg

    If you have an XSD and it happens to be one that XSD.exe can handle then surely there is no need to mess around with XML. The less I have to deal with XML explicitely the better!
    <p>
    However, the scenario that we are trying to enable is when you do care about XML, when you have no schema at all, or when you have a document centric schema.
    <p>
    Makes sense?

  • Anonymous
    February 23, 2006
    The comment has been removed

  • Anonymous
    February 24, 2006
    The comment has been removed

  • Anonymous
    February 27, 2006
    The comment has been removed

  • Anonymous
    February 28, 2006
    Hi Joshua, long time no seen, I am glad to see that you are following our VB Xml features!

    Our design goal is to make the Xml axis members experience as close as possible to regular CLR members, so users have a very easy learning curve. Our preference was to avoid the special syntax for Xml members, however because of the ambiguity and double colon problems we had to choose special syntax.

    We want users to continue using the most intuitive way to access members, which is the "Dot". Our plan is to add the known Xml members to the intellisense drop down so that the Xml members and the new syntax are easily discoverable. We hope that the new syntax will contribute the intellisense experience since the Xml members will show up in the same area due to the sorting of the intellisense members.
    Avner

  • Anonymous
    March 03, 2006
    Hi Avner,

    My URL is a general blog on the Visual Studio 9.0 Overview with some comments on the XML features.  Hope you find some of it useful and your feedback is welcome

    Cheers

    Dave Mc

  • Anonymous
    April 07, 2006
    Erik Meijer and Brian Beckman have put out a new paper on XML and LINQ and VB.&#160; It's a good short read...

  • Anonymous
    May 10, 2006
    TrackBack From:http://zc1984.cnblogs.com/archive/2006/05/11/397016.html

  • Anonymous
    May 11, 2006
    Latest LINQ CTP released for download.
    The list of goodies is here:

    What’s new?
    Enhanced DLinq...

  • Anonymous
    May 12, 2006
    I hope you already had the chance to download the latest LINQ CTP. This CTP contains important improvements...

  • Anonymous
    May 16, 2006
    Un nouveau CTP de LINQ (VB et C#) a &#233;t&#233; publi&#233; la semaine derni&#232;re. LINQ a &#233;t&#233; rendu public lors du PDC...

  • Anonymous
    November 19, 2006
    “ Typed XML programmer, where have you have been in the last few weeks? ” This series of blog posts has

  • Anonymous
    March 10, 2007
    By Amanda Silver I just wanted to let you know that we’re working furiously on turning the VB9 features