EntityFramework中的ObjectDisposedException

Question

I have this class and i often(but not always) get NullReferenceException or ObjectDisposedException when use method ExecuteQuery:

public class Dao
{
    protected StoreDbContext Context = new StoreDbContext();

    public IEnumerable<T> ExecuteQuery<T>(string query, params object[] parameters)
    {
        return  Context.Database.SqlQuery<T>(query, parameters).ToList();
    }

}

but, if if i will create Context in method i don't get any exception. Why?

public class Dao
{
    protected StoreDbContext Context = new StoreDbContext();

    public IEnumerable<T> ExecuteQuery<T>(string query, params object[] parameters)
    {
        return  new StoreDbContext().Database.SqlQuery<T>(query, parameters).ToList();
    }

}

Answer 1

Don't keep the StoreDbContext hanging around when doing updates. It will end up with stale data/entities in it. The accepted pattern is to fire up a new Context for a unit of work/scoped set of operations. The second example might be acceptable, but you need to ensure you dispose of the context by creating it with a using statement. Refactor the method to account for this.

I tend to keep search results in a separate context with no tracking (read only) but would use a fresh context for updates, as I'm using WCF Data Services.