If you're a regular reader of my blog, you'll probably remember my pithy blog post where I stated that "It all depends..." to the question "Which Data Access Should I Use for Silverlight 3?" The reality is that much like the similar question I am confronted with at user groups for the past decade ("What data access should I use in my .NET app?"). The reasons for picking a strategy are wide and varied so I will not try to analyze all possible outcomes, but I think the different strategies need to be explained better.
The three major candidates in Silverlight 3 are Web Services (WCF/ASMX), ADO.NET Data Services and RIA Services. In any situation, any of these will work. But they are suited to different styles and requirements. Here's the abridged differences between the stacks:
The goal of Project Niagara is to democratize the validation support. The project wants to help developers add validation support to ADO.NET Data Services as well as Web Services in Silverlight. In addition, it has the goal of allowing multiple ways to supply the validation metadata to the different data access strategies. As it is my opinion that there are scenarios where attributes are not the best idea.
As RIA Services is plodding towards a release, many people are looking at it to help them with validation of data in Silverlight. Using this validation in Silverlight 3 is pretty straightforward but there are some caveats. I want to show you under the covers so you understand what is happening. In this first part of the series, let's look at what it means to use validation from the outside.
Back when Dynamic Data was being developed, a set of attributes was created to help tell the Dynamic Data folks about validation and other metadata so they could create smart scaffolds. These include:
If you've never had the chance to visit my sister site (http://www.silverlightdata.com), now's a good time. I've updated my examples there to include my MVVM, Prism and Declarative UI examples (to go with the skinning/switchable Astoria example). Take a look if you're doing Silverlight data-based applications.
I have been diving pretty deep into ADO.NET Data Services (see an upcoming article about ADO.NET Data Services and Silverlight 2 coming soon). I've been looking at the story around non-Entity Framework models through a Data Service and thought that NHibernate through a Data Service would be a great example.
So I tried to get it to work with the NHibernate LINQ project. The example model that the project uses is a simple Northwind model. I thought I'd just take that model and expose it via ADO.NET Data Services. I crufted up a simple Data Context object for ADO.NET Data Model but it didn't work. ADO.NET Data Services was complaining about the fact that the end-points (e.g. IQueryable<Customer>) was pointing at an Entity. This was a bug...a but in ADO.NET Data Services. I hacked together a fix to get around it (and reporting it). If you're interest, the problem is that if the key of the entity is in a base class and *not* named "ID", it fails to find it. Again, this is a bug not a feature of ADO.NET Data Services.
After spending time creating my own caches of reflection data I found the NHibernate type information to be more complete and faster. Go figure. At this point I am using the SessionFactory's GetClassMetadata and GetCollectionMetadata to return IClassMetadata and ICollectionMetadata interfaces. So far this has given me every piece of runtime information I need and means that I don't need to do any nasty (and potentially fragile) walking through the property interface of the context object. Whew...
Now that my ADO.NET Data Services support has been merged into the trunk of NHibernate.LINQ, I do have some caveats about using NHibernate.LINQ with ADO.NET Data Services. ADO.NET Data Services is a beta 1 product so there are some bugs and issues that you will either need to avoid or work around.
After some discussion, Mike Flasko (Program Manager, ADO.NET Data Services) has clarified the relationship between the Silverlight 2 Beta 2 Data Services Library and the SP1 of VS2008/.NET 3.5. Specifically he says:
I had a great time at the SQL Saturday in Atlanta today. I did two talks: Using MSchema and ADO.NET Data Services for DBAs. If you were there, thanks for attending. If you wanted to grab the code and slides, follow the link above!
If you are thinking about using SQL Data Services (the data part of the Azure stack) in your Silverlight 2 project, think again. As you might know, ADO.NET Data Services (Astoria) will not work cross-domain regardless of a security policy file (because of some limitations in the two networking stacks that Silverlight 2 uses). Its a problem but in most use-cases ADO.NET Data Services (Astoria) is used on the same domain so no biggie...but...
I've finally had a chance to update my Silverlight 2-ADO.NET Data Services example. In this new sample I show how to create a Line-of-Business application (an XBox Game editor) using ADO.NET Data Services against both an Entity Framework model and NHibernate. Unlike earlier examples, this one includes implementation against the ADO.NET Data Service Silverlight 2 library to support saving of changed entities. In addition, I show some techniques for paging, retrieving simple types over an ADO.NET Data Service and full styling of the application. I hope to add support for Forms Authentication in the coming weeks.
I just found out that my changes to the article and code for Silverlight 2 RTW are now live. The article makes the small change that it no longer requires you to use EdmGem to build your proxy (using the Service Reference instead). I've also updated the code example to be compatible with .NET 3.5 SP1 and Silverlight 2.
If you missed me in Bulgaria's DevReach conference, the video of my Silverlight Data Access talk is now available. The original talk's description was:
This all started with an innocent question by Bob Archer on Twitter. Bob wondered whether he could use ADO.NET Data Services in an application that was being touted as "Software as a Service" (SaaS). His concern was the apparent hard wiring of the Data Source in the DataService definition. This design might assume that you had to connect to a single database for all requests.
In his case, the Entity Framework metadata was the same but the connection string might choose a different physical server and database name. I thought that it shouldn't be too difficult. My first attempt to help him was to suggest interceptors for every entity type and manually change the database string based on some data in the request. This felt hacky so I asked Pablo Castro (at Microsoft) for some advice on the problem. He of course had the solution.
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