How entity framework track the loaded entities? what are their life cycle?

Question

I am relatively new to entity framework, all the documents or books I can find are talking about how to use the framework, or which model should be used, but short of explanation how the framework works in depth.

For instance, when I load the entities from the database via either LINQ query or framework methods, are those entities thread safe? In another words can they be shared with other threads? If so how EF controls the consistency?

When control goes out of context, are those entities gone or still in memory? After .SaveChanges are those entities gone? What is the life cycle?

Can an expert in EF explain the above points in details please.

Thanks in advance.

Answer 1

The life cycle of loaded entities is more-or-less tied to that of the Entity Context which loaded them. Hence in many examples you will see:

using (var ctx = new Context())
{
    // ... do work
}   // The context gets disposed here.

Once the context is disposed (at the end of the using statement, e.g.), you should no longer treat entities that were loaded inside the context as if you can load additional information from them. For example, don't try accessing navigation properties on them. To avoid problems, I usually find it best to create a DTO that has only the exact data that I expect people to be able to use, and have that be the only value that leaves the using statement.

using (var ctx = new Context())
{
    var q = from p in ctx.People
            select new PersonSummary{Name = p.Name, Email = p.Email};
    return q.ToList(); // This will fully evaluate the query, 
                       // leaving you with plain PersonSummary objects.
}

Entity Contexts are not thread-safe, so you shouldn't be trying to load navigation properties and such from multiple threads for objects tied to the same context, even within the context's lifecycle.

Answer 2

For instance, when I load the entities from the database via either LINQ query or framework methods, are those entities thread safe? In another words can they be shared with other threads? If so how EF controls the consistency?

The ObjectContext class is not tread safe. You must have one object context per thread or to create you own thread synchronization process. This way the consistency is managed by the ObjectContext since it tracks all the objects' state.

When control goes out of context, are those entities gone or still in memory? After .SaveChanges are those entities gone? What is the life cycle?

ObjectContext class inherit from IDisposable interface so you can, and should, use USING statement when using Entity Framework. This way they're gone after you close the using statement. If you DO NOT dispose the context they keep being tracked, only their states are changed. Disposing ObjectContext instances will also make sure that the database connection is properly disposed and you are not leaking database connections.

So, the big question is:

Where and when should EF live?

Theses ORM should be treated as the Unit of Work pattern, that is, the ORM object should live until the business task is done.

In my specific scenarios I use an IoC container like Windsor that does the heavy lifting for me. In an ASP.NET MVC app for example, Windsor can create a Context per Web Request. With this you don't have to write a lot of using statements throughout your code. You can read more about it here:

Windsor Tutorial - Part Seven - Lifestyles

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Here's a link that explains it in more details directly from the guy that helps build the framework at Microsoft:

Entity Framework Object Context Life Cycle compared to Linq to Sql Data Context Life Cycle