Implement WCF Data Service using the Repository Pa

问题:

We are using the repository pattern in our ASP.NET MVC 3 application. This means that, although we use EF 4.1 Code First to access the data in the backend, all MVC controllers do that via a generic repository class rather than directly over the DbContext subclass.

Simplified code snippet:

public class MyEntityContext : DbContext, IMyEntityContext
{
    public IDbSet MyEntities { get; set; }
    ...
}

public class MyEntityRepository : IMyEntityRepository
{
    private IMyEntityContext _context;

    public IQueryable<MyEntity> MyEntities
    {
        return _context.MyEntities;
    }
    ...
}

public class MyEntityController : Controller
{
    private MyEntityRepository _repository;
    ...
}

We use interfaces and dependency injection for every dependency. It works fine. Looks nice, doesn't it? But now for the caveat:

We also provide a WCF Data Service (CTP supporting Code First) to access the entities. We want to use the repository in that service, too. But this seems tricky. When using the MyEntityContext directly, the service looks like this:

public class MyEntityService : DataService<MyEntityContext>
{
    public static void InitializeService(DataServiceConfiguration config)
    {
        config.SetEntitySetAccessRule("MyEntities", EntitySetRights.All);
    }
}

But when I try to replace the MyEntityContext by the repository, there are two issues:

1. The type specified for the generic DataService<..> needs to be a class with a default constructor, which breaks the pretty design-by-contract and dependency injection design.
2. It even seems that the type provided has to be a DbContext class: I tried and used the MyEntityRepository instead, but failed (see details).

I seem lost... Can anyone bring me back on the proper track?

---

Details:

My first go was:

public class MyEntityService : DataService<MyEntityRepository>
{
    ...

However, when calling the service, it fails with the following error message:

The server encountered an error processing the request. The exception message is 'On data context type 'MyEntityRepository', there is a top IQueryable property 'MyEntities' whose element type is not an entity type. Make sure that the IQueryable property is of entity type or specify the IgnoreProperties attribute on the data context type to ignore this property.'.

I tried the following steps to fix this, but did not get rid of this error message:

• Adding a [DataServiceKey("MyEntityId")] to MyEntity, where MyEntityId is the correct key property of the entity.
• Replacing the type of Repository.MyEntities by IDbSet instead of IQueryable.

---

BTW: The following posts are not duplicates:

• WCF Repository Service pattern with entity framework
• Service Layer/Repository Pattern
• Best way to implement Repository Pattern?
• webservices with repository pattern in c# and WCF?
• WCF Service design pattern

回答1:

Why do you want to use repository? You have context so use it. Don't create onion architecture just because you want to use pattern. WCF data service already handles everything you need itself. No sorry, it sometimes offers even more (for example interceptors).

By using custom repository you are moving to reflection provider data source. If you also plan to modify your entities through WCF data service that is also against your repository because reflection provider is read only unless it also implements IUpdateable. Check also rules for reflection provider.

Btw. WCF Data Services in .NET 4 doesn't support DbContext directly (that support is only in CTPs of upcoming version) but you there is workaround for that. The link is for old CTP. In current version there is not UnderlyingContext property but you can use IObjectContextAdapter to get ObjectContext.

As you can also see in last link type provided to the service doesn't need to have default constructor - it is up to you what constructor you use when creating data source. If you need dependency injection you will probably have to check the way how to inject directly to the service itself (for example here for Unity and plain WCF) and use injected data in CreateDataSource.

回答2:

Here's how to make a WCF Data Service with whatever pattern you're using, even none at all.

Getting Started With OData Part 2: Building an OData Services from Any Data Source

Just make sure your entity, poco, model or whatever has a property public int ID , or has this class annotation provided by the System.Data.Services assembly in the System.Data.Services namespace:

[DataServiceKey("TheNameOfYourPrimaryKeyProperty")]

This will make it recognizable as an entity type by the WCF Data Service.

As others have pointed out though, just make sure that adding yet another layer in your stack is a good decision.