James Kovacs' Weblog

When you try to install SQL Server 2005 Reporting Services on Windows Vista, the installer apparently fails to recognize that IIS7 is installed (assuming that you have installed it). The error message is misleading and is due to the fact that IIS7 components required by Reporting Services are not installed. IIS7 has been much more componentized than IIS6 and there is a magical list of IIS7 features that must be installed in order for Reporting Services to install successfully. The required IIS7 features are documented in KB article 920201.

Once you have SQL Server installed, you will need to install SQL Server 2005 SP2 November 2006 CTP or later as it addresses some Vista-compatibility issues. You can find the latest SQL Server 2005 service pack information here. You should now have a working Reporting Services install on Vista.

On a recent project, I was trying to return an object (with child objects) from an ASMX Web Service and was using the XmlSerializer for serialization. (When doing ASMX Web Services, you really have no choice but to use the XmlSerializer.) The child objects contained a circular reference, which ended up being the bane of my existence because the XmlSerializer can’t handle circular references. The XmlSerializer reports, “Circular reference detected” and gives up. It just can’t serialize it. The amount of code (and changes to the domain model) to work around this limitation was just plain stupid, not to mention the subtle bugs that needed to be fixed when object identity wasn’t respected in other areas of the object graph.

So what does this have to do with Windows Communication Foundation (WCF)? Given how excited I’ve been about WCF, my fellow developers (and the PM) asked if WCF would have avoided the XmlSerializer problems that we had experienced. My curiosity was piqued and I had to find out. As it turns out, WCF can handle circular references and preserve object identity. Or more correctly, the DataContractSerializer can. This would have saved me untold effort if I had been able to use WCF on the project. (Unfortunately release dates didn’t match up. Ironically the project released into production the day before WCF went RTM.) Just a pronouncement that WCF can handle circular references wouldn’t make for a very interesting blog entry. The reality is that out-of-the-box, WCF cannot handle circular references even though the DataContractSerializer can. With a bit of digging, I was able to configure WCF to serialize circular references and properly preserve object identity.

Let’s take a deeper look… Consider a simple object graph with a circular reference. The simplest is a parent-child relationship where the child has a back-reference to the parent. (My situation was more complex, but this example suffices.) Here is a Person class, which has a list of Children, as well as a reference to its parent.

<Aside>One of the nice things about the DataContractSerializer is that serialized members are opt-in. If you don’t decorate a member with the [DataMember] attribute, it is not serialized. For instance, m_name is not decorated and is therefore ignored by the DataContractSerializer. This is in stark contrast to the XmlSerializer where all public read/write properties are serialized unless they are marked with [XmlIgnore]. Another point to note is that I can serialize both properties and fields, which allows me to have read-only properties, such as m_parent and m_children, while still serializing them at the field level. (N.B. You can still configure WCF to use the XmlSerializer for compatibility, though the preferred serializer is the DataContractSerializer.)</Aside>

Now why can’t WCF handle circular references out-of-the-box. The reason is that there is no industry-accepted, interoperable way of expressing anything but parent-child relationships in XML. You can use the ID/IDREF feature of XML or the key/keyref feature of XML Schema, but a lot of serializers don’t respect these attributes or handle them properly. So if you want to serialize circular references, you need to stray out of the realm of safe interoperability. I found some very intriguing information in Aaron Skonnard’s MSDN article about the DataContractSerializer, Serialization in Windows Communication Foundation. When instantiating the DataContractSerializer, you can tell it to respect object identity (and thus handle circular references) by passing true for preserveObjectReferences constructor parameter.

The resulting XML looks like this:

You’ll notice that references to other objects are encoded using the z:Id attribute. So the DataContractSerializer can handle circular references, but how do we tell WCF to pass true for the preserveObjectReferences parameter when creating a DataContractSerializer? We can’t, but what we can do is do it ourselves and wedge our code into the WCF pipeline. Is that cool or what? All the details can be found in Ingo Rammer’s MSDN Library Article, From .NET Remoting to the Windows Communication Foundation (WCF). We create a class derived from DataContractSerializerOperationBehaviour, which we’ll call PreserveReferencesOperationBehavior. Here’s the important overload:

So we’ve wedged in the preserveObjectReferences = true. We then create an attribute, PreserveReferencesAttribute, which implements IOperationBehavior, to inject the PreserveReferencesOperationBehavior.

Finally, we apply this attribute to the OperationContract in our ServiceContract.

Our service is now able to serialize circular references, as well as preserve object identity. If we need to transmit objects with circular references back to the service, we could make similar changes to inject PreserveReferencesOperationBehavior into the client-side proxy.

To summarize, WCF is a very extensible framework for distributed communication. We were able to make some small modifications to the WCF pipeline that allowed us to return an object graph with circular references from a service. With the release of WCF, the life of the distributed application developer just got a whole lot easier. Full source code can be found here (16 KB).

N.B. The HTTP transport in WCF relies on http.sys for hosting HTTP ports outside of IIS. Opening up arbitrary HTTP ports cannot be done by non-admins unless they have been granted permission by an administrator. This means an administrator must run httpcfg to grant non-admin users permission to open these ports.

The .NET Framework 3.0, formerly known as WinFx, has gone gold. You can download the release bits now. Congrats to everyone involved in this release!

• .NET Framework 3.0 Readme
• .NET Framework 3.0 Runtime Components
• Windows SDK for Vista and the .NET Framework 3.0 
• Visual Studio 2005 Extensions for .NET Framework 3.0 (Windows Workflow Foundation)
• Visual Studio 2005 Extensions for .NET Framework 3.0 (WCF & WPF), November 2006 CTP

If you have a previous version installed (CTP or Beta), you’ll want to read the uninstall instructions found below the download link for the uninstall tool. (The uninstall tool is there in case Add/Remove Programs fails in a horrible way.)

I am now a card-carrying member of the MSDN Canada Speakers Bureau*. (OK, maybe they don’t issue cards, but it’s still a great honour.) The long and short of it is that user groups from across Canada can invite me to speak and Microsoft Canada will cover the expenses.

Here are some possible topics:

• Enterprise Architecture for Mere Mortals: Authentication
• Discussing advantages/disadvantages of different authentication models and how to make them work, including trusted subsystem, full delegation, constrained delegation, and protocol transition.
• Tools of the Trade: Must-Have .NET Utilities
• An overview of .NET tools that should be in your toolbox.
• ASP.NET Kung-Fu: Advanced Techniques and Idioms
• Solving problems in ASP.NET using techniques such as HttpHandlers and HttpModules.
• Introducing Windows CardSpace
• Looking at the technology, its architecture, and why you should care.

I’ll be adding more presentations in the coming months. So if you are looking for someone to present on a particular topic, don’t hesitate to ask. I might have something in my back pocket…

* Please note that the public page is horribly out-of-date. Some people have incorrect information. Others, such as Jean-Paul Boodhoo, are not yet listed. User Group leaders have up-to-date information on all speakers. Talk to them about inviting one of us to come speak at your user group.

BTW – If you’re looking for a fantastic presenter to talk about TDD, Agile, and patterns, I would highly recommend inviting Jean-Paul out to your user group.

A few months ago, Joanne Cummings of Redmond Magazine interviewed me regarding my thoughts on Microsoft Virtual PC and how it compares to VMWare Workstation. The article was recently published and you can find it here. Joanne did a good job capturing my thoughts on the overall advantages and disadvantages of the two products. While I’m overall quite favourable toward Microsoft software (I honestly think Microsoft does a good job in most cases and a great job in many), desktop-based virtualization is one area where they fall short. Read the article to find out more. Thanks, Joanne, for the opportunity to share my thoughts on VPC and VMWare.