I have a class that handles events created by multiple member objects. Events from these objects spawn worker threads for the event, so that the various event handlers in my class are running on different threads (one is a serial handler, one is a timer event, etc.) I'm looking for a simple way to make my code thread-safe, preferably by forcing the event handlers to run on my object's thread.
If this were a Forms UI object, I could take advantage of its implementation of the ISynchronizeInvoke interface, and make calls to InvokeRequired, Invoke, etc. In WPF I could use a Dispatcher object. But my class needs to run *independently of any UI code.
Here's a simplified example of what I have:
public class MyClass
{
private SomeObject object1;
private AnotherObject object2;
public MyClass()
{
object1 = new SomeObject();
object2 = new AnotherObject();
object1.AThreadedEvent += ThreadedEventHandler1;
object2.AnotherThreadedEvent += ThreadedEventHandler2;
}
// This runs in its own thread!
private void ThreadedEventHandler1()
{
// DO STUFF HERE
}
// This runs in its own thread!
private void ThreadedEventHandler2()
{
// DO STUFF HERE
}
}
Because both event handlers access the same objects in the parent class (including each-other!), it would be awesome if there were a simple way to force the event handlers to run in the creating object's thread.
I've toyed with the idea of having my class implement the ISynchronizeInvoke interface, but it appears that doing so can get pretty complicated. Before I jump down that rabbit hole, I thought I'd ping the experts to see if there is a more simple solution.
Thoughts?
EDIT:
Part of the reason I want to run the event handlers in the parent object's thread is because the parent object has it's *own events that are triggered based on the events sent by its member objects. I'd like any threading functionality to be hidden by this class, so that code that uses the class doesn't have to worry about thread-related issues (ie. locks and so on). Simply locking shared data won't do the job, because I *still need to trigger events from within the threaded event handlers.
The ideea of invoking on another thread is hand to hand with having a while loop that from time to time it checks whether there is an "outside" message to be processed. For UI, there is the windows loop that does that. For an external thread, you must write manually a loop. Imagine a situation without a loop and that you have a relative long running thread right ? and sudently you want to interrupt this thread to invoke your message and resume what it was doing ON THE SAME shared stack memory. This interruption would destroy your stack. This is simply NOT possible. The other possibility is to use a synchronization mechanism such as ManualResetEvent and just wait for a signal (a signal that comes outside your thread). So, to resume, in order to process a message from another thread, you basically have only two options:
1) You have a while loop, eventually using a little sleep (to give some time / ticks to other threads to do their job)
while (true) {
Thread.Sleep (5);
if (someMessageArrived) { ... }
}
2) You just wait for a message implementing somehow the producer / consummer architecture:
On listening thread:
aManualResetEvent.WaitOne ();
On the "producer" thread:
aManualResetEvent.Set ();
There are advanced classes in .NET framework that might help such as BlockingCollection.
Hope this helps
Assumming, that your class runs in its own thread that the only logic is to execute the incomming calls from other threads, this would be the solution:
(comments inside)
public class MyClass
{
private SomeObject object1;
private AnotherObject object2;
public MyClass()
{
object1 = new SomeObject();
object2 = new AnotherObject();
object1.AThreadedEvent += ThreadedEventHandler1;
object2.AnotherThreadedEvent += ThreadedEventHandler2;
}
// This runs in its own thread!
// Only add the real function call to the queue
public void ThreadedEventHandler1()
{
tasks.Add(ThreadedEventHandler1_really);
}
private void ThreadedEventHandler1_really()
{
// DO STUFF HERE
}
// This runs in its own thread!
// Only add the real function call to the queue
public void ThreadedEventHandler2()
{
tasks.Add(ThreadedEventHandler2_really);
}
// here is the actual logic of your function
private void ThreadedEventHandler2_really()
{
// DO STUFF HERE
}
// the queue of the tasks
BlockingCollection<Action> tasks = new BlockingCollection<Action>();
// this method never returns, it is blocked forever
// and the only purpose of i is to do the functions calls when they added to the queue
// it is done in the thread of this instance
public void StartConsume()
{
foreach (Action action in tasks.GetConsumingEnumerable())
{
// add logic before call
action();
// add logic after call
}
}
}
The solution based on that the caller threads tat calls the functions: ThreadedEventHandler1 and ThreadedEventHandler2, actually add the real call to the queue and emediately continue with their run.
From the other hand, StartConsume function iterates the queue and makes the calls of the added method calls. If you want to add another logic before and after call, you can add it in this function.
Hope it helped to achieve your goal.
Without completely understanding the rational behind your design. I can say that the problem you are trying to solve was solved many times before.
I will assume your main object is like a service which expects calls (in this case events) from itself and other services (the sub objects). If you would think about it in terms of services (which you arguably should) WCF solves that problem for you doing all the heavy lifting #Rami suggested.
You define the main service with the following behavior:
Instance Context Mode - Single
Concurrency Mode - Single
More about these here.
And every event handler would call that main service notifying it about the event.
I am pretty sure you would not go that far and implement every class as a service, but thought it is worth offering anyway as an option.
OK, based on all of your feedback (thanks!) I have a solution to my problem. The short answer: what I wanted to do isn't possible.
Here are more details for those who asked. I'm writing a DLL that manages a device attached to a serial port. This includes basic serial port COM (packet TX and RX, including parsing), and higher-level protocol behavior (TX, Ack, retry on timeout, etc.) The serial port event handlers provided by .NET are obviously asynchronous, as are the System.Timers.Timer objects that I use to handle timeouts, etc.
I am building the code around an MVVM architecture, so that my UI doesn't have any logic in it whatsoever. Hence my need to avoid exploiting Dispatcher or Invoke functionality provided by the UI.
What I was looking for was a way to handle asynchronous events within my DLL in the same simple manner provided by WinForms and WPF. But as has been pointed out, and as I learned when digging deeper, what you are *really doing when you call BeginInvoke or a Dispatcher is pushing something onto a queue, to be consumed later by a different thread polling the queue. Outside the context of a UI, no such polling architecture exists.
SO. My options are to lock the shared objects in my class to make it thread safe, or to implement my own polling architecture within another thread (to avoid blocking the program that uses the DLL) that emulates what the UI code already does.
In either case, the UI code will still need to use its Invoke or equivalent tools when handling events from the DLL class. I suppose that's OK.
Related
I'm working through my first attempt to thread an application. The app works with a large data set that is split up into manageable chunks which are stored on disk, so the entire data set never has to reside in memory all at once. Instead, a subset of the data can be loaded piecemeal as needed. These chunks were previously being loaded one after the other in the main thread. Of course, this would effectively pause all GUI and other operation until the data was fully loaded.
So I decided to look into threading, and do my loading while the app continues to function normally. I was able to get the basic concept working with a ThreadPool by doing something along the lines of the pseudo-code below:
public class MyApp
{
List<int> listOfIndiciesToBeLoaded; //This list gets updated based on user input
Dictionary<int,Stuff> loadedStuff = new Dictionary<int,Stuff>();
//The main thread queues items to be loaded by the ThreadPool
void QueueUpLoads()
{
foreach(int index in listOfIndiciesToBeLoaded)
{
if(!loadedStuff.ContainsKey(index))
loadedStuff.Add(index,new Stuff());
LoadInfo loadInfo = new LoadInfo(index);
ThreadPool.QueueUserWorkItem(LoadStuff, loadInfo);
}
}
//LoadStuff is called from the worker threads
public void LoadStuff(System.Object loadInfoObject)
{
LoadInfo loadInfo = loadInfoObject as LoadInfo;
int index = loadInfo.index;
int[] loadedValues = LoadValuesAtIndex(index); /* here I do my loading and ...*/
//Then I put the loaded data in the corresponding entry in the dictionary
loadedStuff[index].values = loadedValues;
//Now it is accessible from the main thread and it is flagged as loaded
loadedStuff[index].loaded = true;
}
}
public class Stuff
{
//As an example lets say the data being loaded is an array of ints
int[] values;
bool loaded = false;
}
//a class derived from System.Object to be passed via ThreadPool.QueueUserWorkItem
public class LoadInfo : System.Object
{
public int index;
public LoadInfo(int index)
{
this.index = index;
}
}
This is very primitive compared to the quite involved examples I've come across while trying to learn this stuff in the past few days. Sure, it loads the data concurrently and stuffs it into a dictionary accessible from the main thread, but it also leaves me with a crucial problem. I need the main thread to be notified when an item is loaded and which item it is so that the new data can be processed and displayed. Ideally, I'd like to have each completed load call a function on the main thread and provide it the index and newly loaded data as parameters. I understand that I can't just call functions on the main thread from multiple other threads running concurrently. They have to be queued up in some way for the main thread to run them when it is not doing something else. But this is where my current understanding of thread communication falls off.
I've read over a few in-depth explanations of how events and delegates can be set up using Control.Invoke(delegate) when working with Windows Forms. But I'm not working with Windows Forms and haven't been able to apply these ideas. I suppose I need a more universal approach that doesn't depend on the Control class. If you do respond, please be detailed and maybe use some of the naming in my pseudo-code. That way it will be easier for me to follow. Threading appears to be a pretty deep topic, and I'm just coming to grips with the basics. Also please feel free to make suggestions on how I can refine my question to be more clear.
If you aren't using a GUI framework with some kind of dispatcher or GUI thread (like WPF or WinForms) then you'll have to do this manually.
One way to do this is to use a SynchronizationContext.
It's somewhat tricky to manage but there are a few articles which go into how it works and how can you make your own:
http://www.codeproject.com/Articles/31971/Understanding-SynchronizationContext-Part-I
http://www.codeproject.com/Articles/32113/Understanding-SynchronizationContext-Part-II
However I would also consider using either a single 'DictionaryChanged' boolean which is regularly checked by your 'main thread' (when it is idle) to indicate that the dictionary is changed. The flag could then be reset on the main thread to indicate that this has been handled. Keep in mind that you'll need to do some locking there.
You could also queue messages using a thread safe queue which is written by the background thread and read from the main thread if a simple variable is not sufficient. This is essentially what most dispatcher implementations are actually doing under the hood.
I'm jusing Jurassic javascript compiler to run javascript in my C# application. Now, jurassic isn't thread-safe, and I call functions (in javascript) from threads all over the place, which I figured wasn't that smart. What I could do was ofcause just to create a simple lock on the javscript-engine, however, what I want is a programming model similar to the one you have when working with a GUI thread in WPF or WinForms. So, I spawned a thread, and created my javascript-engine inside that thread, and what I would like is that no other threads are allowed to edit the objects created in that thread (which will just be the javascript-engine and all the js-objectes). And then, to call js-code from other thread I'd like to use a dispatcher, or something similar, to make the js-thread run the code.
Is this possible in C#?
It doesn't quite answer your question but you may want to take a look at this MSDN article. It talks about the approach that WPF took with their objects and the Dispatcher model, as well as the Dispatcher, DispatcherObject and DispatcherSynchronizationContext classes.
What they recommend for individual objects is to inherit from DispatcherObject (which may not be feasible for your situation) and call the inherited VerifyAccess() method on public access.
public class ThreadOwnedObject : DispatcherObject
{
private string field;
public string ExposedProperty
{
get { return field; }
set
{
VerifyAccess();
field = value;
}
}
}
And the invocation would use the inbuilt Dispatcher.
ThreadOwnedObject ownedInstance = new ThreadOwnedObject();
ownedInstance.Dispatcher.Invoke(new Action(() => ownedInstance.ExposedProperty = "foo"));
Alternately, if that or the DispatcherSynchronizationContext in the article doesn't fit your needs, I imagine that you could probably create a mechanism that mimics the DispatcherObject by holding onto the Thread.CurrentThread.ManagedThreadId when an object is created and perform checks against that value for every exposed access. Beyond that or an equivalent, I don't think that there's a built-in mechanism that will associate a random object with a given thread.
I'm not quite sure what you are asking, but I'll try to help anyway.
Can you use a semaphore to lock the thread running the javascript engine? Even if you don't use the Semaphore class, you could use a boolean or "lock" keyword to protect the code block with the executing engine. The objects produced by that thread could be hidden by the class with the engine until you are ready. You could also expose a method in that class that would allow code injection or object fetching from behind the protected code.
I am quite new to C# and threading, and I have this problem to resolve:
I have a thread which processes some data and from time to time (when necessary) it fires my event method (DataProcessor) which was set before starting the thread. This thread is in proprietary dll. So I cannot affect DataProcessor calling anymore.
public void DataProcessor(object sender)
{
//...
//do some stuff which take some time
}
My problem is that from time to time when DataProcessor is fired and starts doing some stuff and it's not finished yet, it's fired again and I have a conflict.
What I need is, when DataProcessor is doing some stuff all other data is unnecessary so all other attempts to process another data MUST be ignored/skipped - are not relevant for processing.
Event handlers should be used to perform very quick operations. The way I've handled similar situations is when the event fires, to put some "to-do" item inside a queue. I then run a continuous thread that handles items from the queue.
A good way to simplify this operation is to use BlockingCollection. Your thread simply runs a foreach over that collection, which will block waiting on enqueued items.
You can also unsubscribe from the event when you are inside your handler, and resubscribe when you are done processing, but you will lose events this way.
I have an application wherein I would like a function to be executed in the same thread when an event is fired. For example:
SomeCode()
{
// Do something...
// Fire event to run SomeOtherCode().
}
SomeOtherCode()
{
// Do something else...
}
I do not want to simply call the function because it will hold things up. SomeOtherFuction() needs to be executed in the same thread because it needs to access the form controls, and I need it to begin execution from an event trigger firing. I am using Microsoft Visual C# 2008 Express Edition. Thanks.
::: EDIT:::
Additional Details: The bottom line is that the contrustor of my form application is taking far too long to complete, and it is causing a significant delay, from when the user launches the application to when the application window appears on the display. This is not a problem on faster computers, but on slower computers it is a big problem. I need to exit the contrustor as soon as possible, thus allowing the framework to draw the application window, and continue initialization outside the constructor. (All essential items would still be initialized inside the constructor.)
An event-triggered function call would be ideal. I would prefer not to use a timer. Interlacing the affected code with Invokes is impractical in my situation and would require much more time to implement than I have to work on this. A simple example of an event-driven function call is all I'm really looking for. Thanks.
From your posts it's seems like you're confusing a few issues. The standard pattern in .Net is for events to run synchronously. The following lines are essentially identical in terms of when they execute.
Option #1
SomeCode();
SomeOtherCode();
Option #2
SomeEvent += delegate { SomeOtherCode(); }
...
SomeCode();
SomeEvent(this,EventArgs.Empty);
If you want to unblock the UI thread and run the code later you'll need to use some mechanism to delay the running of the SomeOtherCode function. The easiest way to do this in a WinForms application is to use a WinForms Timer instance. This will raise an event on the UI thread at a later point in time that you can respond to. It also won't block the UI thread during this time allowing your form to continue processing.
You seem to be asking to run SomeOtherCode() later.
You can call BeginInvoke (either from the UI thread or from any other thread) to queue a function to run during the next message loop:
BeginInvoke(new Action(SomeOtherCode));
It seems that you would want to add an event to the class that exposes the SomeCode method. Then, the class that implements the SomeOtherCode method would attach an event handler that calls the SomeOtherCode method.
It's completely viable to have this done in one class, in case you have some sort of state model where you want to add/remove the call depending on some other logic.
I think you want to put SomeOtherCode into a Task or BackgroundWorker, which would then synchronize with the UI thread to send it updates.
I recently posted on my blog a class that makes updating the UI from a Task as easy as from a BGW. I do recommend using Task rather than BackgroundWorker.
Simialr to what Stephen said, I would recommend that you move as much of that initialization code to a background thread or task. Let the background thread do as much work as possible, then send the necessary window updates to your UI thread via Action<>'s. Here's some quick psuedo-sample code:
protected void LoadMyListInBackground(object state)
{
List<string> myList = Databse.FetchMyList(myParameters); // This take a while, so the UI thread isn't waiting
ShowMyList(myList);
}
protected void ShowMyList(List<string> theList)
{
if(InvokeRequired)
Invoke(new Action<List<string>>(ShowMyList, theList);
else
{
foreach(string item in theList)
myListBox.Items.Add(item);
}
}
In this example the UI thread is free to keep drawing your window while the background thread does the lengthy database work. The problem is, even if you fire an event outside of your constructor, and that event occurs on the UI thread and takes a long time, the user might see the window but that window is going to 'freeze' and possibly appear to be 'crashed' to the user. This technique prevents that and provides a better user experience.
If I have Thread A which is the main Application Thread and a secondary Thread. How can I check if a function is being called within Thread B?
Basically I am trying to implement the following code snippit:
public void ensureRunningOnCorrectThread()
{
if( function is being called within ThreadB )
{
performIO()
}
else
{
// call performIO so that it is called (invoked?) on ThreadB
}
}
Is there a way to perform this functionality within C# or is there a better way of looking at the problem?
EDIT 1
I have noticed the following within the MSDN documentation, although Im a dit dubious as to whether or not its a good thing to be doing! :
// if function is being called within ThreadB
if( System.Threading.Thread.CurrentThread.Equals(ThreadB) )
{
}
EDIT 2
I realise that Im looking at this problem in the wrong way (thanks to the answers below who helped me see this) all I care about is that the IO does not happen on ThreadA. This means that it could happen on ThreadB or indeed anyother Thread e.g. a BackgroundWorker. I have decided that creating a new BackgroundWorker within the else portion of the above f statement ensures that the IO is performed in a non-blocking fashion. Im not entirely sure that this is the best solution to my problem, however it appears to work!
Here's one way to do it:
if (System.Threading.Thread.CurrentThread.ManagedThreadId == ThreadB.ManagedThreadId)
...
I don't know enough about .NET's Thread class implementation to know if the comparison above is equivalent to Equals() or not, but in absence of this knowledge, comparing the IDs is a safe bet.
There may be a better (where better = easier, faster, etc.) way to accomplish what you're trying to do, depending on a few things like:
what kind of app (ASP.NET, WinForms, console, etc.) are you building?
why do you want to enforce I/O on only one thread?
what kind of I/O is this? (e.g. writes to one file? network I/O constrained to one socket? etc.)
what are your performance constraints relative to cost of locking, number of concurrent worker threads, etc?
whether the "else" clause in your code needs to be blocking, fire-and-forget, or something more sophisticated
how you want to deal with timeouts, deadlocks, etc.
Adding this info to your question would be helpful, although if yours is a WinForms app and you're talking about user-facing GUI I/O, you can skip the other questions since the scenario is obvious.
Keep in mind that // call performIO so that it is called (invoked?) on ThreadB implementation will vary depending on whether this is WinForms, ASP.NET, console, etc.
If WinForms, check out this CodeProject post for a cool way to handle it. Also see MSDN for how this is usually handled using InvokeRequired.
If Console or generalized server app (no GUI), you'll need to figure out how to let the main thread know that it has work waiting-- and you may want to consider an alternate implementation which has a I/O worker thread or thread pool which just sits around executing queued I/O requests that you queue to it. Or you might want to consider synchronizing your I/O requests (easier) instead of marshalling calls over to one thread (harder).
If ASP.NET, you're probably implementing this in the wrong way. It's usually more effective to use ASP.NET async pages and/or to (per above) synchronize snchronizing to your I/O using lock{} or another synchronization method.
What you are trying to do is the opposite of what the InvokeRequired property of a windows form control does, so if it's a window form application, you could just use the property of your main form:
if (InvokeRequired) {
// running in a separate thread
} else {
// running in the main thread, so needs to send the task to the worker thread
}
The else part of your snippet, Invoking PerformIO on ThreadB is only going to work when ThreadB is the Main thread running a Messageloop.
So maybe you should rethink what you are doing here, it is not a normal construction.
Does your secondary thread do anything else besides the performIO() function? If not, then an easy way to do this is to use a System.Threading.ManualResetEvent. Have the secondary thread sit in a while loop waiting for the event to be set. When the event is signaled, the secondary thread can perform the I/O processing. To signal the event, have the main thread call the Set() method of the event object.
using System.Threading;
static void Main(string[] args)
{
ManualResetEvent processEvent = new ManualResetEvent(false);
Thread thread = new Thread(delegate() {
while (processEvent.WaitOne()) {
performIO();
processEvent.Reset(); // reset for next pass...
}
});
thread.Name = "I/O Processing Thread"; // name the thread
thread.Start();
// Do GUI stuff...
// When time to perform the IO processing, signal the event.
processEvent.Set();
}
Also, as an aside, get into the habit of naming any System.Threading.Thread objects as they are created. When you create the secondary thread, set the thread name via the Name property. This will help you when looking at the Threads window in Debug sessions, and it also allows you to print the thread name to the console or the Output window if the thread identity is ever in doubt.