Entity Framework 4.0 POCO Classes and Data Services

If you've flipped on the POCO (Plain Ol' CLR Objects) code generation T4 templates for Entity Framework to enable testing or just 'cuz you like the code better, you might find that you lack the ability to expose that same model via Data Services as OData (Open Data). If you surf to the feed, you'll likely see something like this:

The XML page cannot be displayed

Cannot view XML input using XSL style sheet. Please correct the error and then click the Refresh button, or try again later.

The following tags were not closed: feed. Error processing resource 'http://localhost:10749/MyODataEndpoint.svc/Posts'

There are two problems. The first problem is that we're not reporting the problem very well. You can't see what's happening in IE8 with a simple View Source, as apparently IE won't show malformed XML. Instead, you have to use Fiddler or some other tool (I'm a big tcpTrace fan) to see the actual error in the HTTP response:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<feed ...>
<title type="text">Posts</title>
<id>http://localhost:8080/MyODataEndpoint.svc/Posts</id>
<updated>2010-05-11T22:48:13Z</updated>
<link rel="self" title="Posts" href="Posts" />
<m:error>
<m:code></m:code>
<m:message xml:lang="en-US">Internal Server Error. The type
'System.Data.Entity.DynamicProxies.Post_CF2ABE5AD0B93AE51D470C9FDFD72E780956A6FD7294E0B4205C6324E1053422'
is not a complex type or an entity type.</m:message>
</m:error>

It's in the creation of the OData feed that the error happens, so instead of clearing the response and just returning the error, we dump it into the middle of the output, making it very difficult to find. In this case, what we're telling you is that you've mistakenly left dynamic proxy creation on, which doesn't work with EF4 POCO objects and Data Services in .NET 4.0. To fix this, you need to override the CreateDataSource method in your DataService<T> derived class:

public class MyODataEndpoint : DataService<FooEntities> {
  public static void InitializeService(DataServiceConfiguration config) {
    ...
  }

  protected override sellsbrothersEntities CreateDataSource() {
    var dataSource = new FooEntities();
    dataSource.ContextOptions.ProxyCreationEnabled = false;
    return dataSource;
  }
}

This solution came from Shyam Pather, a Dev Manager on the EF team. He says that once you turn off proxy generation, you give up lazy loading and "immediate" change tracking. Instead, you'll get "snapshot" change tracking, which means the context won't be informed when the properites are changed, but the context still detects changes when you call DetectChanges() or SaveChanges(). For the internals of a Data Service, none of this matters, but any code you write in query interceptors, change interceptors, or service operations will have to be aware of this.

This limitations are only true when used from the OData endpoint, of course. The rest of your app will get proxy creation by default unless you turn it off.