Entity Framework Context in Singleton

问题

I'm building a App that use Context of EF in Singleton Pattern like NHibernate work with Session:

public class DbContextFactory
{
    private static volatile DbContextFactory _dbContextFactory;
    private static readonly object SyncRoot = new Object();
    public DbContext Context;

    public static DbContextFactory Instance
    {
        get
        {
            if (_dbContextFactory == null)
            {
                lock (SyncRoot)
                {
                    if (_dbContextFactory == null)
                        _dbContextFactory = new DbContextFactory();
                }
            }
            return _dbContextFactory;
        }
    }

    public DbContext GetOrCreateContext()
    {
        if (this.Context == null)
            this.Context = new DbContext(ConfigurationManager.AppSettings["DefaultConnectionString"]);

        return Context;
    }
}

I'm using Ninject to Inject the Context:

public class DbContextModule : NinjectModule
{
    public override void Load()
    {
        Bind<IDbContext>().ToConstant(DbContextFactory.Instance.GetOrCreateContext());
    }
}

I'm reading about that approach and some people are saying that is bad practice and I'll have problems.

Someone that know about this with EF can explain me in more details?

回答1:

NHibernate doesn't use Session as singleton ... Such scenario has only meaning in very rare cases where your application is very short batch representing single transaction / unit of work.

Here are described reasons why you should not use shared / long living context. In case of multithreaded or server application serving multiple clients you must not use shared context.

回答2:

A good pratice is to use a datacontext per unit of work To have more information about unit of work with EF you should read http://blogs.msdn.com/b/adonet/archive/2009/06/16/using-repository-and-unit-of-work-patterns-with-entity-framework-4-0.aspx

When sharing the same datacontext you are probably saving a few 10s of milliseconds. The word micro optimisation springs to mind - in which case you probably shouldn't be using Entity Framework.

Having a DataContext that is never closed allows you to lazy load whenever you want. You may have left your Service and now be in your Controller or worse still, your View. Accessing the database from a View is asking for performance problems as I'm sure you didn't allow that intentionally. It is most likely because you forgot to eager load all the data you need to populate your view.

回答3:

You read this article. Why you shouldn't use singleton DataContexts in Entity Framework? http://www.britishdeveloper.co.uk/2011/03/dont-use-singleton-datacontexts-entity.html

回答4:

public class Dock
{
    // Statik field
    private static Dock _dock;
    // lock object
    private static object lockObject = new object();

    // We prevent the constructive method from being modeled with new by 
    //making it secret.
    private Dock()
    {

    }
    // Class in Instance
    public static Dock Instance()
    {   
        if (_dock == null)
        {
            lock (lockObject)
            {
                if (_dock == null)
                    _dock = new Dock();
            }
        }
        return _dock;
    }

}

来源：https://stackoverflow.com/questions/7499233/entity-framework-context-in-singleton

标签

.net

entity-framework

entity-framework-4

ninject