Visiting the WCF Data Services Toolkit April 2011 Update

Looking for a way to expose data from Windows Azure Table Storage via an OData webservice I quickly discovered that in its current form, WCF Data Services are primarily meant to expose relational data when using the Entity Framework. Since working with Azure Storage – such as TableStorage – typically does not involve a relational data model, OData seemed to be a difficult path. Not using Entity Framework means you will have to implement your own LINQ provider to get the job done. This is not a trivial task as it involves juggling – or rather messing – around with expression trees.

At this point I discovered the WCF Data Services Toolkit on Codeplex which circumvents the need to write your own LINQ provider by using a repository pattern architecture for you to hook into. I also found documentation to get me started, however I discovered later that the documentation does not match the current (beta) release of the toolkit.

As a prerequisite, please read the author’s blog post about updating resources with the toolkit. Here he describes the IWriteableRepository interface. Please note that this documentation is partly out of date as with the april update:

 

IWriteableRepository interface is gone

But fear not. It is a breaking change, but simply removing it from the class declaration will do the trick. This is because methods on your repository will be called by convention (hence by reflection). This has multiple advantages which the author mentions at the end of the updating resources post.

CreateDefaultEntity has been renamed to CreateResource

While the previous change might have been obvious, this one is not. But again, this is easy to solve, just rename the method.

 

This is it for the changes. Some random things I discovered while using the Toolkit:

Make sure to implement CreateResource. In there, simply new up a new instance of your entity (or do more advanced stuff if necessary). Otherwise, the toolkit will try to use Activator.CreateInstance() to create an entity instance, which did not work for me because the type to create an instance from was in a different assembly. This is a known issue (#12 on Codeplex) and I sent a patch for this to the author.

 

Just for reference: The list of methods one can implement in the repository:

  1. public void Save(<YourType> entity)
  2. public void Remove(<YourType> entity)
  3. public void CreateRelation(<YourType> entity, <YourOtherType> relatedEntity)
  4. public object CreateResource()

So far, I really like the toolkit and how it helped me to work with Azure Storage. Keep up the good work!

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