Migrating from LINQ to SQL to Entity Framework 4.0 - Tips, Documentation, etc_问答_开发者

I tried out EF back in .NET 3.5 SP1, and I was one of the many who got frustrated and decided to learn LINQ to SQL instead. Now that I know EF is the "chosen" path forward, plus EF 4.0 has some exciting new features, I'd like to migrate my app to EF 4.0.

Can anyone suggest any good resources that are specifically targeted towards 4.0 and L2S migration? NOTE: I ca开发者_如何学Pythonn find plenty of blogs and articles related to migrating from L2S to EF on .NET 3.5, but I feel like many of those were obviously dated and unhelpful to someone using 4.0.

I'd really like as much deep, under-the-hood stuff as I can get; I want to really come away feeling like I know EF 4.0 the way I currently know L2S 3.5.

TIA!

I have done loads of this very type of conversion and FWIW, I would say there are more similarities than differences. I don't think there is any definitive documentation that will make you feel like an expert in EF4, beyond the stuff that is already out there...

http://msdn.microsoft.com/en-us/library/ex6y04yf(VS.100).aspx

What I can give you are the more obvious "gotchas." Specifically, Linq2Sql wanted to combine the business layer and the data layer a lot more obviously. It really pushed you to create your own partial classes. I could go on and on about way, but the most specific reason is the way the one-to-one mapper will create public parent and child properties for all relations.

If you attempt to use any type of serialization against this model, you will like run into circular reference problems as a serializer moves from a parent to a child and then back to the parent as the Linq2Sql serialization behavior automatically includes all children in the graph. This can also be really annoying when you try to grab a customer record to check the "Name" property and automatically get all the related order records included in the graph. You can set these parent and child navigation properties to be either "public" or "internal" which means if you want access to them, but don't want the serializers to automatically create circular references, you pretty much have to access them in partial classes.

Once you start down the partial class path you generally just continue the pattern and eventually will start to add helper methods for accessing your data into your individual entity classes. Also, with the Linq2Sql DataContext being more lightweight, you often find people using some kind of Singleton pattern or Repository pattern for their context. You don't see this as much at all with EF 3.5 / 4.

So let's say you have some environment similar to the one described and you want to start converting. Well, you need to find out when your DataContext is going to be create/destroyed...some people will just start each Business Layer method with a using() statement and let the context pretty much live for the lifetime of the method. Obviously this means you can get into some hairy situations that require adding .ToList() or some other extension method to the ends of your questions you can have a fully in-memory collection of your objects to pass to a child method or whatever and even then you can have problems with attempting to update entities on a context that they weren't originally retrieved from.

You'll also need to figure out how to much of the BusinessLogic incorporated in your Linq2Sql partial classes out into another layer if it doesn't deal explicitly with the data operations. This will not be painless as you figure out when you need/don't need your context, but it is for the best..

Next, you will want to deal with the object graph situation. Because of the difference in the way lazy-loading works (they made this configurable in EF 4.0 to make it behave more like Linq2Sql for those who wanted it) you will probably need to check any implied uses of child objects in the graph from your Linq2Sql implementation and verify that it doesn't now require an explicit .Include() or a .Load() to get the child objects in the graph.

Finally, you will need to decide on a serialization solution in general. By default, the DataContracts and DataMember attributes that are generated as part of an EF model work great with WCF, but not at all great with the XmlSerializer used for things like old .asmx WebServices. Even in this circumstance you might be able to get away with it if you never need to serialize child objects over the wire. Since that usually isn't the case, you are going to want to move to WCF if you have a more SOA, which will add a whole new host of opportunies, yet headaches.

In order to deal with the partial classes situation, and the hefty DataContext and even the serialization issues, there are a number of new code-generation templates available with EF 4.0. The POCO-Entity template has a lot of people excited as it creates POCO classes, just as you'd expect (the trouble is that excludes any class or member attributes for WCF etc etc). Also, the Self-Tracking Entities model pretty much solves the context issue, because you can pass your entities around and let them remember when and how they were updated, so you can create/dispose your contexts much more freely (like Linq2Sql). As another bonus, this template is the go-to template for WCF or anything that builds on WCF like RIA Services or WCF Data Services, so they have the [DataContract], [DataMember], and [KnownType] attributes already figured out.

Here is a link to the POCO template (not included out of the box): (EDIT: I cannot post two hyperlinks, so just visit the visualstudio gallery website and search for "ADO.NET C# POCO Entity Generator")

Be sure to read the link on the ADO.net team blog about implementing this. You might like the bit about splitting your context and your entities into separate projects/assemblies if you fall into the WebService vs. WCF Service category. The "Add Service Reference..." proxy generation doesn't do namespaces the same way "Add Web Reference..." used to, so you might like to actually reference your entity class assembly in your client app so you can "exclude types from reference libraries" or whatever on your service references so you don't get a lot of ambiguous references from multiple services which use the same EF model and expose those entities...

I know this is long and rambling, but these little gotchas were waaay more of an issue for me than remembering to use context.EntityCollection.AddObject() instead of context.EntityCollection.InsertOnSubmit() and context.SaveChanges() instead of context.SubmitChanges()...

For EF Code First, it's mostly about reverse engineering the existing tables into EF classes. EF Power Tools now does this for you:

http://msdn.microsoft.com/en-us/data/jj200620.aspx

The rest is the obvious work of modifying your existing code to use those generated classes to talk to the database instead of LINQ to SQL.