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Can a method from a singleton object be called from multiple threads at the same time?

I have a component registered in Castle Windsor as a singleton. This object is being used in many other places within my application which is multithreaded.
Is it possible that the two objects will invoke the same method from that singleton at the same time or 'calling it' will be blocked until the previous object will get result?
Thanks

You can call a Singleton object method from different threads at the same time and they would not be blocked if there is no locking/ synchronization code. The threads would not wait for others to process the result and would execute the method as they would execute methods on separate objects.
This is due to the fact that each thread has a separate stack and have different sets of local variables. The rest of the method just describes the process as to what needs to be done with the data which is held the variables/fields.
What you might want to take care of is if the methods on the Singleton object access any static methods or fields/variables. In that case you might need to work on synchronization part of it. You would need to ensure multi-threaded access to shared resources for the execution of the method to be reliable.
To be able to synchronize, you might need to use lock statement or other forms of thread synchronization techniques.
You might want to refer to this article from Wikipedia which provides information on C# thread local storage as well.

You can call the same method or different methods on one object simultaneously from different threads. In the specific methods you'll need to know when sensitive variables are being accessed (mostly when member-variables are changing their values) and will need to implement locking on your own, in order to solve lost updates and other anomalies.
You can lock a part of a code with the lock-statement and here an article on how Thread-Synchronization works in .Net.

The normal version of Singleton may not be thread safe, you could see different implementation of thread safe singleton here.
http://tutorials.csharp-online.net/Singleton_design_pattern:_Thread-safe_Singleton

Linq-to-SQL DataContext across multiple threads

How do I handle a Linq-to_SQL DataContext across multiple threads?
Should I be creating a global static DataContext that all the threads use and commit changes at the very end or should I create a Context per thread and use that instance for everything inside that thread?

DataContext is not thread safe; using it directly from multiple threads would cause #fail; having a global static data-context would cause #fail and would cause uncontrolled memory growth (the data-context includes an identity manager and change tracker for every object fetched; this only grows over time, as more objects are touched)
Data context should ideally be used for a unit of work; spin one up; do something (that is bound in scope - i.e. not the entire app lifetime), and dispose it. So IMO the real answer here is "tie it to that unit of work". Only you can know what that is in your application; it could be a single method, it could a page request on a web page, it could be a timer "tick" in a service. Who knows...

Associate data with a thread: How do you do that?

While writing some aysnc controllers in ASP.NET MVC2, I ran up against a situation where I had to call AsyncManager.Sync. It got me wondering:
When I retrieve HttpContext.Current, what's going on? How does ASP.NET know which HttpContext I'm after? How is the current context associated with this thread, and how is it retrieved?

Thread scoped storage is used, this is called Thread Local Storage.
This mechanism allows data to be affinitized with a thread i.e. only the thread that allocates the data sees the data. This is useful for creating so called ambient programming models such as HttpContext.Current and TransactionScope. The mechanism allows data to be accessible at any time on the executing thread without having to "tramp" data through method parameters for instance. It's an elegant solution for certain context\orthogonal problems.
There are number of ways of using TLS including the ThreadStaticAttribute and Thread.SetData\GetData.

Singleton pattern in web applications

I'm using a singleton pattern for the datacontext in my web application so that I dont have to instantiate it every time, however I'm not sure how web applications work, does IIS open a thread for every user connected? if so, what would happend if my singleton is not thread safe? Also, is it OK to use a singleton pattern for the datacontext? Thanks.

I'm using a singleton pattern for the datacontext in my web application
"Singleton" can mean many different things in this context. Is it single-instance per request? Per session? Per thread? Per AppDomain (static instance)? The implications of all of these are drastically different.
A "singleton" per request (stored in the HttpContext) is fine. A singleton per session is discouraged, but can be made to work. A singleton per thread may appear to work but is likely to result in unexpected and difficult-to-debug behaviour. A singleton per Application or AppDomain is a disaster waiting to happen.
so that I dont have to instantiate it every time
Creating a DataContext is very, very cheap. The metadata is globally cached, and connections aren't created until you actually execute a query. There is no reason to try to optimize away the construction of a DataContext instance.
however I'm not sure how web applications work, does IIS open a thread for every user connected?
IIS uses a different thread for every request, but a single request may use multiple threads, and the threads are taken from the Thread Pool, which means that ultimately the same user will have requests on many different threads, and conversely, different users will share the same thread over multiple requests and an extended period of time. That is why I mention above that you cannot rely on a Thread-Local Singleton.
if so, what would happend if my singleton is not thread safe?
Very bad things. Anything that you cache globally in an ASP.NET application either needs to be made thread safe or needs to be locked while it is in use.
Also, is it OK to use a singleton pattern for the datacontext? Thanks.
A DataContext is not thread-safe, and in this case, even if you lock the DataContext while it is in use (which is already a poor idea), you can still run into cross-thread/cross-request race conditions. Don't do this.
DataContext instances should be confined to the scope of a single method when possible, using the using clause. The next best thing is to store them in the HttpContext. If you must, you can store one in the Session, but there are many things you need to be aware of (see this question I answered recently on the ObjectContext - almost all of the same principles apply to a DataContext).
But above all, do not create "global" singleton instances of a DataContext in an ASP.NET application. You will deeply regret it later.

Many people keep the DataContext around for the duration of the request by keeping it in the HttpContext.Current.Items Thereby it is also private to the request.
Have a look at this blogpost by Steve Sanderson, and the UnitOfWork pattern.

Static variables are visible to all users on the per app domain, not per session. Once created, the variable will sit in memory for the lifetime of the app domain, even if there are no active references to the object.
So if you have some sort of stateful information in a web app that shouldn't be visible to other users, it should absolutely not be static. Store that sort of information in the users session instead, or convert your static var to something like this:
public static Data SomeData
{
get
{
if (HttpContext.Session["SomeData"] == null)
HttpContext.Session["SomeData"] = new Data();
return (Data)HttpContext.Session["SomeData"];
}
}
It looks like a static variable, but its session specific, so the data gets garbage collected when the session dies and its totally invisible to other users. There safety is not guaranteed.
Additionally, if you have stateful information in a static variable, you need some sort of syncronization to modify it, otherwise you'll have a nightmare of race conditions to untangle.

#ryudice the web server creates a new thread for each request. I think the best approach is to have a datacontext bound to each request, meaning that you should create a new datacontext every time you serve a request. A good way of achieving this is by using a DI tool, such as StructureMap. These kind of tools allow you to setup the lifecycle of the instances you configure, so for example in your case you would configure your XDataContext class to be HttpContext scoped.
Regards.

here are Microsoft's examples on how to do multi-tier with LINQ-To-SQL.
http://code.msdn.microsoft.com/multitierlinqtosql

Server-side equivalent of HttpContext?

I have a web app that currently uses the current HttpContext to store a LINQ Data Context. The context is persisted for the current request, on a per user basis, per Rick Strahl's blog:
string ocKey = "ocm_" + HttpContext.Current.GetHashCode().ToString("x")
Thread.CurrentContext.ContextID.ToString();
if (!HttpContext.Current.Items.Contains(ocKey))
{
// Get new Data Context and store it in the HTTP Context
}
However, I have some scripts that execute from the global.asax file, that don't have an HttpContext. The HttpContext.Current is NULL, because the server is the one making the "request".
Is there an equivalent object that I can use to store the Data Context? So I don't have to worry about re-creating it, and attaching/detaching objects? I only want to persist the context for the lifetime of my processes.
UPDATED:
I am currently trying to use a static variable in my DAL helper class. on the first call to one of the methods in the class the DataContext is instantiated, and stored in the static variable. At the end of my process, I call another method that calls Dispose on the DataContext, and sets the static variable to NULL.

Can you not just use a static variable specifically for those scripts? That will have the same life-time as the AppDomain. You should probably think carefully about any concurrency concerns, but it sounds like the simplest way to keep a value around.
(I've just checked, and although one instance of HttpApplication can be used to service multiple requests, each one only serves one request at a time - which suggests that multiple instances are created for concurrent request processing. I haven't validated this, but it does sound like it wouldn't be safe to keep it in an instance variable.)
EDIT: Josh's answer suggests that you want this to be per-thread. That sounds slightly odd to me, as unless you've got a lot of these events occurring, you're quite likely to only ever see them execute on different threads, making the whole sharing business pointless. If you really do want that sort of thing, I'd suggest just using an instance variable in the HttpApplication-derived class - for exactly the reason described in the paragraph above :)

Why not use the current HttpContext? The scripts in your global.asax file are all the result of a request coming into the server, so there should be a context associated with that request which you can grab.
I don't understand the need for generating the key based on the hashcode or the thread. There is going to be a separate instance of HttpContext for each request that comes in, and that instance is going to be specific to the thread that is processing the request. Because of that, the key is pretty much worthless when it's based on the instance of HttpContext and the thread.
Also, how do you dispose of the DataContext when you are done? It implements IDisposable for a reason, so I would recommend against a shared instance like this.
UPDATE
In the comments, it indicates that there is a timer that is running that is executing the scripts. Instead of the timer, I would recommend setting up a Scheduled Task which will call a webservice or predetermined page on the site which will perform the task. Then you will always have an HttpContext to work with.

HttpContext.Current is a static method and should be available from anywhere as long as the code is executing within the context of a request.
In your case your not executing within the context of a request, You could look at using Application.Cache but I would caution against holding a DataContext open. I am not very famillar with linq to entities, so I could be wrong, but generally caching data base related items such as connections is bad.
I would also recommend that you consider moving the logic out of your global.asax and to a windows service. This would let you have more control over these tasks, for example you can shut them down seperatley of the web site.
Edit
As JS points out you could use a static variable. You could also define an instance variable marked with ThreadLocal attribute. This will give each thread its own copy of the variable, and can eliminate contention. Since you want each thread to have its own copy anyways.

Is there a reason why these need to be handled the same way as the other DataContexts? It seems to me that if the context is only needed inside the event handling routine, you shouldn't need to keep it around. Especially if it is in Application_Start (as per your comment), I wouldn't bother caching it anywhere -- just use it locally and pass it to the other methods as needed.

Set the DataContext as the state parameter when creating the timer. Based on the info you posted on the comments, it seems to me that your DataContext is more related to the timers than anything else.
Also avoid using the same DataContext for different timers, because you would end up with mixed modifications from the different timers. Also make sure your same timer logic isn't run twice, since it would cause the same i.e. too short period with no control.