EF（实体框架）使用“使用”语句

Question

I have a project on MVC. We chose EF for our DB transactions. We created some managers for the BLL layer. I found a lot of examples, where " using " statement is used, ie

public Item GetItem(long itemId)
    {
        using (var db = new MyEntities())
        {
            return db.Items.Where(it => it.ItemId == itemId && !it.IsDeleted).FirstOrDefault();
        }
    }

Here we create a new instance of DBcontext MyEntities() . We using " using " in order to "ensure the correct use of IDisposable objects."

It's only one method in my manager. But I have more than ten of them. Every time I call any method from the manager I'll be using " using " statement and create another DBcontext in the memory. When will the garbage collector (GC) dispose them? Does anyone know?

But there is alternative usage of the manager methods. We create a global variable:

private readonly MyEntities db = new MyEntities();

and use DBcontext in every method without " using " statement. And method looks like this:

public Item GetItem(long itemId)
{
    return db.Items.Where(it => it.ItemId == itemId && !it.IsDeleted).FirstOrDefault();
}

Questions:

1. What is the proper way of using DBcontext variable?
2. What if we wouldn't use " usage " statement (because it affects the performance) - GC will do all for that?

I'm a "rookie" in EF usage and still haven't found the unequivocal answer for this question.

Answer 1

I think you will find many suggesting this style of pattern. Not just me or Henk DBContext handling

• Yes, Ideally Using statements for DBContext subtypes
• Even better Unit Of Work patterns that are managed with Using, that have a context and dispose the context Just 1 of many UoW examples, this one from Tom Dykstra
• The Unit Of Work Manager should be New each Http request
• The context is NOT thread safe so make sure each thread has its own context.
• Let EF cache things behind the scenes.
• Test Context creation times. after several Http request. Do you still have a concern?
• Expect problems if you store the context in static. any sort of concurrent access will hurt and if you are using parallel AJAX calls, assume 90+% chance of problems if using a static single context.

For some performance tips, well worth a read

Answer 2

The proper or best practice way of using DBContext variable is with the Using.

    using (var db = new MyEntities())
    {
        return db.Items.Where(it => it.ItemId == itemId && !it.IsDeleted).FirstOrDefault();
    }

The benefit is many things are done automatically for us. For example once the block of code is completed the dispose is called.

Per MSDN EF Working with DbContext

The lifetime of the context begins when the instance is created and ends when the instance is either disposed or garbage-collected. Use using if you want all the resources that the context controls to be disposed at the end of the block. When you use using, the compiler automatically creates a try/finally block and calls dispose in the finally block.