When do you dispose an Entities object context objects in entity framework and MVC?
For example if I have a persons table and I select a record in a controller method, dispose it and pass it back to my view, then the record won't be usable in the view.
Should I be disposing it somehow after my view is processed? or not disposing it at all?
One option is to create it in Global.asax's begin request event, and dispose of it in Global.asax's end request event. Every page simply uses that one (stored and obtained in HttpContext.Current.Items or in thread local storage) without disposing it. That lets it be available to your view to do lazy loading but still disposes of it after the request is completed.
The other option is to make sure everything you need is already loaded before calling your view (via .First(), .ToList(), and .Include(property) to include navigation property data) and dispose of it immediately. Both methods work.
I assume you're talking about disposing the Entity Framework "Contexts," since the objects themselves aren't disposable.
We've found it best to leave the entities themselves in our data layer and map them to POCOs/DTOs that contain all the information we need for a given view. That way we're not trying to lazy-load data while we render our view. We wrap the data-access code in a using(var context = contextFactory.Get()), so that the context will automatically be disposed before the method ends, but after we have loaded all the data we're retrieving into an in-memory collection.
Let's consider typical usage pattern of user, you will never just open one item and go away, in fact we move back and forth between items, search and review items again, modify and save them.
If you keep your ObjectContext alive for entire session, you will use little more memory per user, but you will reduce your application to database transfers, and you will be able to accumulate changes. And save changes at once. Since EF implements Identity Pattern, you will not be loading multiple copies of same object.
Otherwise if you dispose ObjectContext, will reduce memory but will increase overhead of loading objects again and again. You might be loading multiple copies of same object again and again over views and increasing query load on database server.
Related
I have an MVC app where I am getting a XML file and loading into an object.
In my view I have several client/server operations and since MVC is stateless I stored the object into a MemoryCache (to prevent the constant loading of the XML file). While this seems to work nicely, however my concern is how to remove this MemoryChache item when I am done?
If the user goes to different view then I want to remove this object.
However I don't know what event I could use to ensure this object gets removed.
It appears that I can handle it with JQuery:
$(window).unload(function () {
// Server call to clear MemoryCache
});
My MVC4 app uses code-first Entity Framework 5.0. I want to access my SQL Server data from a timer thread. Is there any reason why I can't instantiate, use, and dispose an instance of the same derived DbContext class that I also use on the main ASP.NET worker thread? (Yes, I use the same using() pattern to instantiate, use, and dispose the object on the main thread.)
A little problem context: My website has a WebsiteEnabled field in a table in the database. Currently, I incur a database fetch for each GET request to read that value. I want to change the code to read the value once every 15 seconds on a background thread, and store the value in a static variable that the GET request can read. I know that you run into problems if you try to instantiate multiple instances of the same DbContext on the same thread; I'm not sure if the same restrictions apply to instances of the same DbContext on different threads.
We use a background thread as well to check for emails and do cleanups every so often in one of our larger MVC applications. As long as you create a new context (and dispose of it) on the background thread and not try and use the one from your main application thread, you will be fine. The DbContext is not thread safe, meaning you cannot share it across multiple threads safely. This does not mean you cannot have multiple threads each with their own copy of the db context. The only caution is beware of concurrency issues (trying to update a row at the same time).
Statics and EF are recipe for a mess. Under asp.net, 1 app pool, many threads. Store statics if you must but not the context. So always make sure each thread gets its own context.
but given your problem, there is a simple out of the box solution I would use
In thr controller that should have cached values. On the GET method
You can cache per ID, for a specific period of time...
Worth checking out. Let IIS, ASP.NET do work for you. :-)
[OutputCache(Duration = int.MaxValue, VaryByParam = "id", Location = OutputCacheLocation.ServerAndClient)]
public ActionResult Get(string id) {
// the value that can be cached is collected with a NEW CONTEXT !
In my project I use entity framework 4.0 as ORM to persist data in a SQL Server.
My project is a ribbon from application with a grid view and navigation tree in the main form with ribbon panel on top of it. My app basically acts a CRUD UI with very little business logic.
Being first time with EF, I developed this project by creating and holding an instance of objectContext in the orchestrating form (main form or the one that shows up as application to user) as a member variable and bound a query to a grid view.
For various events like ribbon panel button clicks, grid view row clicks etc, I open another windows form. In that window form I create another object context and stored in member variable of that form class.
I had read through few blogs and questions like:
How to decide on a lifetime for your objectcontext
Entity Framework and ObjectContext n-tier architecture etc.
One set of authors suggests to share the object context while other suggest to short lived and non-shared.
I reached this state of confusion because I am now in a state where the changes I made to objectContext in one of the child form is not reflecting the parent form that showed it. I attempted to refresh but still nothing useful. Just for an experiment, I shared the objectContext that I first created in the most parent class through constructor injection and my problem of change reflection is solved.
It is a huge work for me to convert all my child forms to share objectContext. But I am ready if it is worth. I am not sure what will be the lurking problems of sharing it?
I may opt for a static instance of objectContext as I am not using this for Web and not planning for multi threaded scenarios. If required I can rise to as a singleton.
My Questions:
To share or not to share ObjectContext for my situation?
If not to share, how can I solve my present problem of updating one objectContext with the changes made in other?
If to share - which would be better way? Static or singleton or something else?
The details of the project and environment are as below:
Winforms
C#
VS 2012
EF 4.0, model created with data first approach.
I am posting this after searching and reading through many questions and blog posts. The more I read, the more confusing it becomes :) Please bear with me if I am leaving someone to assume something to answer. I will try to update the question if such clarifications are asked through comments.
Your Questions
To share or not to share ObjectContext for my situation?
Do not share your context. The EntityFramework context should follow a UnitOfWork pattern. Your object context should be as short lived as possible without unnecessarily creating/destroying too many contexts. This usually translates to individual "operations" in your app as units of work. For a web app/api this might be per HttpWebRequest, or you might do it per logical data operation (for each of your implemented pieces of "Business Logic").
For example:
LoadBusinssObjects() would create a context, load your list of data plus any related data you want then dispose of the context.
CreateBusinessObject() would create a context, create an instance of some entity, populate it with data, attached it to a collect in the context, save changes and then dispose of the context.
UpdateBusinessObject() would read some object from the context, update it, save changes, and dispose of the context.
DeleteBusinessObject() would find a business object in the context, remove it from the collection in the context, save changes and dispose of the context.
If not to share, how can I solve my present problem of updating one objectContext with the changes made in other?
This is a job for a pub/sub architecture. This can be as simple as a few static event handlers on your objects for each operation you implemented above. Then in your code for each business operation, you fire the corresponding events.
If to share - which would be better way? Static or singleton or something else?
This is incorrect. The EF context will continue to grow in memory footprint as the context's state manager continuously collects up cached objects (both attached and not-attached) for every single interaction you do in your application. The context is not designed to work like this.
In addition to resource usage, the EF context is not thread safe. For example what if you wanted to allow one of your editor forms to save some changes at the same time as the tree list is loading some new data? With one static instance you better make sure this is all running on the UI thread or synchronized with a semaphore (yuck, and yuck - bad practices).
Example
Here's an example using C# and code first approach as per your post. Note, I'm not addressing things like data concurrency or threading to keep the example short. Also in a real application this concept is implemented with generics and reflection so that ALL of our models have basic events on them for Creating, Updating, Deleting.
public class MyCodeFirstEntityChangedArgs : EventArgs
{
/// <summary>
/// The primary key of the entity being changed.
/// </summary>
public int Id {get;set;}
/// <summary>
/// You probably want to make this an ENUM for Added/Modified/Removed
/// </summary>
public string ChangeReason {get;set;}
}
public class MyCodeFirstEntity
{
public int Id {get;set;}
public string SomeProperty {get;set;}
/// <summary>
/// Occurs when an instance of this entity model has been changed.
/// </summary>
public static event EventHandler<MyCodeFirstEntityChangedArgs> EntityChanged;
}
public class MyBusinessLogic
{
public static void UpdateMyCodeFirstEntity(int entityId, MyCodeFirstEntity newEntityData)
{
using(var context = new MyEFContext())
{
// Find the existing record in the database
var existingRecord = context.MyCodeFirstEntityDbSet.Find(entityId);
// Copy over some changes (in real life we have a
// generic reflection based object copying method)
existingRecord.Name = newEntityData.Name;
// Save our changes via EF
context.SaveChanges();
// Fire our event handler so that other UI components
// subscribed to this event know to refresh/update their views.
// ----
// NOTE: If SaveChanges() threw an exception, you won't get here.
MyCodeFirstEntity.EntityChanged(null, new MyCodeFirstEntityChangedArgs()
{
Id = existingRecord.Id,
ChangeReason = "Updated"
});
}
}
}
Now you can attach event handlers to your model from anywhere (its a static eventhandler) like this:
MyCodeFirstEntity.EntityChanged += new EventHandler<MyCodeFirstEntityChangedArgs>(MyCodeFirstEntity_LocalEventHandler);
and then have a handler in each view that will refresh local UI views whenever this event is fired:
static void MyCodeFirstEntity_LocalEventHandler(object sender, MyCodeFirstEntityChangedArgs e)
{
// Something somewhere changed a record! I better refresh some local UI view.
}
Now every UI component you have can subscribe to what events are important to it. If you have a Tree list and then some editor form, the tree list will subscribe for any changes to add/update/remove a node (or the easy way - just refresh the whole tree list).
Updates Between Applications
If you want to go a step further and even link separate instances of your app in a connected environment you can implement a pub/sub eventing system over the network using something like WebSync - a comet implementation for the Microsoft Technology Stack. WebSync has all the stuff built in to separate events into logical "channels" for each entity/event you want to subscribe to or publish to. And yes, I work for the software company who makes WebSync - they're paying for my time as I write this. :-)
But if you didn't want to pay for a commercial implementation, you could write your own TCP socket client/server that distributes notifications for the above events when an entity changes. Then when the subscribing app gets the notification over the network, it can fire its local event handlers the same way which will cause local views to refresh. You can't do this with a poorly architected static instance of your data context (you'd be bound to only ever have one instance of your app running). With some good setup early on, you can easily tack on a distributed pub-sub system later that works across multiple instances of native apps and web apps all at the same time! That gets very powerful.
I'm working on an enterprise application that leverages the repository pattern on top of EF 4.1 with eager loading to POCO entities. Typically, a call will look like this:
public IEnumerable<SomeEntity> List(DateTime date)
{
using (var context = ContextFactory.CreateContext()) // Returns a DbContext
{
return CreateQuery<SomeEntity>(context)
.Include("PATH1")
.Include("PATH2")
.Where(...)
.AsNoTracking()
.ToList();
}
}
At some point, the business layer translates these entities into DTOs that are then transmitted via WCF to a web application.
As eager loading is rather expensive, I'm trying to keep .Include's to a minimum, so sometimes related properties are (eagerly) loaded and sometimes they're not. However, the business layer has no idea when related properties are present in the entity, so I get an ObjectContextDisposedException, the reason for which is perfectly clear to me and I don't intend to change the basic strategy (i.e. dispose the context right after eager loading the entities).
However, I need to check whether a particular related property is loaded or not, checking if the related object is null doesn't work (ObjectContextDisposedException), nor is there any kind of IsLoaded() method I could use.
Am I stuck with a try/catch block here or are there any other options?
Turn off lazy loading and check for null will work. Your current solution cannot use lazy loading because you dispose context immediately after running the query:
context.Configuration.LazyLoadingEnabled = false;
I am not sure if that makes any sense, but here is an example.
I have a Category object, that my Service hands to the Controller, which uses AutoMapper to create a CategoryViewModel. Hand that off to the view, serve it to the client.
Now when that gets posted back, AutoMapper creates a Category from the Model sent back, and I hand it to the Service that gives it to the Repository to persist to the database.
My question is, what is the correct way of doing this? I assume the object is a detached object when posted back and I need to attach it to the context, mark it dirty and save changes?
Basically two ways of doing the update of the entity:
Attach the entity to the context, mark it as modified using ObjectStateManager.ChangeObjectState Method, call ObjectContext.SaveChanges Method
Load the original entity from DB, apply changes to the original using ObjectContext.ApplyCurrentValues<TEntity> Method, call ObjectContext.SaveChanges Method
Each of those have their own pros and cons. For example the 1st one does not make round trip to get the original entity but fails to address concurrency as well as tries to update every property of the entity, while the 2nd one works best when employing optimistic concurrency, updates only changed properties, but it does make extra trip to Db to get the original entity.
"I assume the object is a detached object when posted back and I need to attach it to the context, mark it dirty and save changes?"
Yes.
Any one of the links on this page should help:
http://www.google.com/search?rlz=1C1CHFX_enUS410US410&sourceid=chrome&ie=UTF-8&q=working+with+dicsonnected+entities+entity+framework