Entity Framework ObjectContext re-usage

Question

I'm learning EF now and have a question regarding the ObjectContext:

Should I create instance of ObjectContext for every query (function) when I access the database?

Or it's better to create it once (singleton) and reuse it?

Before EF I was using enterprise library data access block and created instance of dataacess for DataAccess function...

Answer 1

Definitely for every query. It's a lightweight object so there's not much cost incurred creating one each time you need it.

Besides, the longer you keep an ObjectContext alive, the more cached objects it will contain as you run queries against it. This may cause memory problems. Therefore, having the ObjectContext as a singleton is a particularly bad idea. As your application is being used you load more and more entities in the singleton ObjectContext until finally you have the entire database in memory (unless you detach entities when you no longer need them).

And then there's a maintainability issue. One day you try to track down a bug but can't figure out where the data was loaded that caused it.

Answer 2

Don't use a singleton.. everyone using your app will share that and all sorts of crazy things will happen when that object context is tracking entities.

I would add it as a private member

Answer 3

I think the most common way is to use it per request. Create it at the beginning, do what you need (most of the time these are operation that require common ObjectContext), dispose at the end. Most of DI frameworks support this scenario, but you can also use HttpModule to create context and place it in HttpContext.Current.Items. That is simple example:

public class MyEntitiesHttpModule : IHttpModule
{
    public void Init(HttpApplication application)
    {
        application.BeginRequest += ApplicationBeginRequest;
        application.EndRequest += ApplicationEndRequest;
    }

    private void ApplicationEndRequest(object sender, EventArgs e)
    {
        if (HttpContext.Current.Items[@"MyEntities"] != null)
            ((MyEntities)HttpContext.Current.Items[@"MyEntities"]).Dispose();
    }

    private static void ApplicationBeginRequest(Object source, EventArgs e)
    {
        var context = new MyEntities();
        HttpContext.Current.Items[@"MyEntities"] = context;
    }
}

Answer 4

Like Luke says this question has been asked numerous times on SO.

For a web application, per request cycle seems to work best. Singleton is definitely a bad idea.

Per request works well because one web page has a User, maybe some Projects belonging to that user, maybe some Messages for that user. You want the same ObjectContext so you can go User.Messages to get them, maybe mark some messages as read, maybe add a Project and then either commit or abandon the whole object graph at the completion of the page cycle.