I'm kind of new to threading in C# and had a couple of questions about what is out there:
What are the ways to implement threads in C#? (i.e. I can think of two off the top: backgroundWorker, Thread, etc)
How do you cause deadlock and if there is deadlock how do you get out of it (in C#)?
How does backgroundworker get implemented? It seems to have an underlying set of methods, but I'd like to know what those methods and instantiations are...
Thanks!
The definitive beginner's guide to threading in C# is here:
http://www.albahari.com/threading/
The documentation on BackgroundWorker, with a complete working example, is here: http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.aspx
Deadlocks are explained here: http://www.albahari.com/threading/part2.aspx
Threads can be implemented in many ways. You can use them directly, pull them from a ThreadPool, or use them indirectly using the Task Parallel Library.
What are the ways to implement threads
in C#?
There are various different ways to take advantage of threading; some involving the explicit creation of threads while others take advantage of already running threads.
The Thread class itself.
Queue a work item in the thread pool.
Use the BackgroundWorker class.
Use the Task Parallel Library (TPL).
Use Parallel LINQ.
Use asynchronous delegates.
Use timers like System.Threading.Timer and System.Timers.Timer.
How do you cause deadlock and if there is deadlock how do you get out
of it (in C#)?
Here are 3 different ways you can cause a deadlock. This list is not exhaustive.
Call a blocking method from within a lock section.
In this example thread A acquires a lock and then immediately calls a blocking method while at the same time thread B attempts to acquire the same lock, but gets hung because thread A is waiting for thread B to signal the event before it will release the lock.
public class Example
{
ManualResetEvent m_Event = new ManualResetEvent(false);
void ThreadA()
{
lock (this)
{
m_Event.WaitOne();
}
}
void ThreadB()
{
lock (this)
{
m_Event.Set();
}
}
}
Acquire two locks out of order.
No explanation is needed here since this is a well known problem.
public class Example
{
private object m_LockObjectA = new object();
private object m_LockObjectB = new Object();
void ThreadA()
{
lock (m_LockObjectA) lock (m_LockObjectB) { }
}
void ThreadB()
{
lock (m_LockObjectB) lock (m_LockObjectA) { }
}
}
The lock-free deadlock.
This is one my favorite illustrations of a deadlock because no lock or blocking method is involved. The subtlety of the problem is enough to confound even those who are familiar with threading. The issue here is related to the absence of memory barriers. Thread A waits for thread B to set the signal flag while at the same time thread B waits for thread A to reset it, all the while neither thread is seeing the changes the other is making because the compiler, JIT, and hardware are free to optimize the reads and writes of the flag in manner that is non-intuitive.
public class Example
{
private bool m_Signal = false;
void ThreadA()
{
while (!m_Signal);
m_Signal = false;
}
void ThreadB()
{
m_Signal = true;
while (m_Signal);
}
}
How does backgroundworker get
implemented?
Here is a very simple step-by-step procedure to get you started.
Add an event handler that performs the actual work to the DoWork event.
Add an event handler to receive progress information to the ProgressChanged event.
Add an event handler that will be executed upon completion to the RunWorkerCompleted event.
Call RunWorkerAsync from the UI thread to start the background operation. This raises the DoWork event on a separate thread.
Call ReportProgress periodically from the DoWork event handler to publish new progress information. This raises the ProgressChanged event on the UI thread.
.net 4 offers parallel LINQ. This is very nice if you want to parallelize a side-effect free calculation which is easily expressible in functional/linq style.
For all common uses and purposes, use Thread. If you want to communicate from some thread to GUI, you may think of using BackgroundWorker, because it will automatically serialize (with Invoke() ) calls to GUI methods so you won't have GUI locking issues.
And as the deadlocks are concerned, don't worry about them. Deadlocks are possible only if you have 2 threads competing for the same set of resources, and I guess you won't tackle that just yet.
I would classify the answer into 3 sections. So with .net 4.0 all the examples above fall under 3 major categories:
1. Threads managed by .net thread pool (asynchronous delegate invocation, backgroundworker, etc)
2. The thread class - you have to manage the lifetime of the thread yourself
and finally Parallel Linq which requires multi core CPU.
Related
The situation
I'll try to explain my Problem with a small example. I have a class that manages a view-stack. Replaces views shows and initializes them etc.
class ViewManager()
{
void ReplaceView()
{
RemoveView(...);
InitializeView(...); // this code may call Application.DoEvents()
AddNewView(...);
}
void ShowModalView()
{
ShowView();
Dispatcher.PushFrame()
....
// wait until view can be removed
RemoveView();
}
void RemoveView()
{
...
}
// ... more functions
}
Because the ViewManager does a lot with UI elements, other Threads uses the ViewManager through Dispatcher.Invoke.
Dispatcher.Invoke(new Action(() => m_viewManager.Remove(someView)));
If now multiple threads Invoke actions on the ViewManager, and one of them calls deep down somewhere in the code Application.DoEvents or DispatcherPushFrame, a second MessageLoop is spawned that will invoke another method o n the view Manager.
The Problem:
ShowModalView is called and calls PushFrame.
During ShowModalView another thread calls ShowModalView or ReplaceView.
Because the new message loop executes all tasks queued to the dispatcher this method are also executed.
Therefore two methods of the ViewManager are executed "at the same time" - or better because it is the same thread - nested in each other.
Locking inside the ViewManager is useless, because it is all the same thread. A semaphore might freeze the UI thread, because during the Message loop of the "ShowModalView" another thread can invoke ShowModalView on the dispatcher and that will freeze the UI thread.
The Questions:
During the "ShowModalView" the UI thread should handle input / paint etc but should not handle other tasks invoked through "Dispatcher.Invoke" Is this possible?
Do you have other Ideas to solve this problem?
Thank you for your hints and answers
Manuel
EDIT:
One solution might be to get rid of all DoEvents and PushFrame code. This is very hard to achieve but probably the only right solution. This post explains a part of my Problem
Use of Application.DoEvents
As Servy said, get rid of Application.DoEvents() and Dispatcher.PushFrame is the only clean solution. Sometimes this will cause a lot of refactoring but it is worth the effort.
I asked the question below couple of weeks ago. Now, when reviewing my question and all the answers, a very important detail jumped into my eyes: In my second code example, isn't DoTheCodeThatNeedsToRunAsynchronously() executed in the main (UI) thread? Doesn't the timer just wait a second and then post an event to the main thread? This would mean then that the code-that-needs-to-run-asynchronously isn't run asynchronously at all?!
Original question:
I have recently faced a problem multiple times and solved it in different ways, always being uncertain on whether it is thread safe or not: I need to execute a piece of C# code asynchronously. (Edit: I forgot to mention I'm using .NET 3.5!)
That piece of code works on an object that is provided by the main thread code. (Edit: Let's assume that object is thread-safe in itself.) I'll present you two ways I tried (simplified) and have these four questions:
What is the best way to achieve what I want? Is it one of the two or another approach?
Is one of the two ways not thread-safe (I fear both...) and why?
The first approach creates a thread and passes it the object in the constructor. Is that how I'm supposed to pass the object?
The second approach uses a timer which doesn't provide that possibility, so I just use the local variable in the anonymous delegate. Is that safe or is it possible in theory that the reference in the variable changes before it is evaluated by the delegate code? (This is a very generic question whenever one uses anonymous delegates). In Java you are forced to declare the local variable as final (i.e. it cannot be changed once assigned). In C# there is no such possibility, is there?
Approach 1: Thread
new Thread(new ParameterizedThreadStart(
delegate(object parameter)
{
Thread.Sleep(1000); // wait a second (for a specific reason)
MyObject myObject = (MyObject)parameter;
DoTheCodeThatNeedsToRunAsynchronously();
myObject.ChangeSomeProperty();
})).Start(this.MyObject);
There is one problem I had with this approach: My main thread might crash, but the process still persists in the memory due to the zombie thread.
Approach 2: Timer
MyObject myObject = this.MyObject;
System.Timers.Timer timer = new System.Timers.Timer();
timer.Interval = 1000;
timer.AutoReset = false; // i.e. only run the timer once.
timer.Elapsed += new System.Timers.ElapsedEventHandler(
delegate(object sender, System.Timers.ElapsedEventArgs e)
{
DoTheCodeThatNeedsToRunAsynchronously();
myObject.ChangeSomeProperty();
});
DoSomeStuff();
myObject = that.MyObject; // hypothetical second assignment.
The local variable myObject is what I'm talking about in question 4. I've added a second assignment as an example. Imagine the timer elapses after the second assigment, will the delegate code operate on this.MyObject or that.MyObject?
Whether or not either of these pieces of code is safe has to do with the structure of MyObject instances. In both cases you are sharing the myObject variable between the foreground and background threads. There is nothing stopping the foreground thread from modifying myObject while the background thread is running.
This may or may not be safe and depends on the structure of MyObject. However if you haven't specifically planned for it then it's most certainly an unsafe operation.
I recommend using Task objects, and restructuring the code so that the background task returns its calculated value rather than changing some shared state.
I have a blog entry that discusses five different approaches to background tasks (Task, BackgroundWorker, Delegate.BeginInvoke, ThreadPool.QueueUserWorkItem, and Thread), with the pros and cons of each.
To answer your questions specifically:
What is the best way to achieve what I want? Is it one of the two or another approach? The best solution is to use the Task object instead of a specific Thread or timer callback. See my blog post for all the reasons why, but in summary: Task supports returning a result, callbacks on completion, proper error handling, and integration with the universal cancellation system in .NET.
Is one of the two ways not thread-safe (I fear both...) and why? As others have stated, this totally depends on whether MyObject.ChangeSomeProperty is threadsafe. When dealing with asynchronous systems, it's easier to reason about threadsafety when each asynchronous operation does not change shared state, and rather returns a result.
The first approach creates a thread and passes it the object in the constructor. Is that how I'm supposed to pass the object? Personally, I prefer using lambda binding, which is more type-safe (no casting necessary).
The second approach uses a timer which doesn't provide that possibility, so I just use the local variable in the anonymous delegate. Is that safe or is it possible in theory that the reference in the variable changes before it is evaluated by the delegate code? Lambdas (and delegate expressions) bind to variables, not to values, so the answer is yes: the reference may change before it is used by the delegate. If the reference may change, then the usual solution is to create a separate local variable that is only used by the lambda expression,
as such:
MyObject myObject = this.MyObject;
...
timer.AutoReset = false; // i.e. only run the timer once.
var localMyObject = myObject; // copy for lambda
timer.Elapsed += new System.Timers.ElapsedEventHandler(
delegate(object sender, System.Timers.ElapsedEventArgs e)
{
DoTheCodeThatNeedsToRunAsynchronously();
localMyObject.ChangeSomeProperty();
});
// Now myObject can change without affecting timer.Elapsed
Tools like ReSharper will try to detect whether local variables bound in lambdas may change, and will warn you if it detects this situation.
My recommended solution (using Task) would look something like this:
var ui = TaskScheduler.FromCurrentSynchronizationContext();
var localMyObject = this.myObject;
Task.Factory.StartNew(() =>
{
// Run asynchronously on a ThreadPool thread.
Thread.Sleep(1000); // TODO: review if you *really* need this
return DoTheCodeThatNeedsToRunAsynchronously();
}).ContinueWith(task =>
{
// Run on the UI thread when the ThreadPool thread returns a result.
if (task.IsFaulted)
{
// Do some error handling with task.Exception
}
else
{
localMyObject.ChangeSomeProperty(task.Result);
}
}, ui);
Note that since the UI thread is the one calling MyObject.ChangeSomeProperty, that method doesn't have to be threadsafe. Of course, DoTheCodeThatNeedsToRunAsynchronously still does need to be threadsafe.
"Thread-safe" is a tricky beast. With both of your approches, the problem is that the "MyObject" your thread is using may be modified/read by multiple threads in a way that makes the state appear inconsistent, or makes your thread behave in a way inconsistent with actual state.
For example, say your MyObject.ChangeSomeproperty() MUST be called before MyObject.DoSomethingElse(), or it throws. With either of your approaches, there is nothing to stop any other thread from calling DoSomethingElse() before the thread that will call ChangeSomeProperty() finishes.
Or, if ChangeSomeProperty() happens to be called by two threads, and it (internally) changes state, the thread context switch may happen while the first thread is in the middle of it's work and the end result is that the actual new state after both threads is "wrong".
However, by itself, neither of your approaches is inherently thread-unsafe, they just need to make sure that changing state is serialized and that accessing state is always giving a consistent result.
Personally, I wouldn't use the second approach. If you're having problems with "zombie" threads, set IsBackground to true on the thread.
Your first attempt is pretty good, but the thread continued to exist even after the application exits, because you didn't set the IsBackground property to true... here is a simplified (and improved) version of your code:
MyObject myObject = this.MyObject;
Thread t = new Thread(()=>
{
Thread.Sleep(1000); // wait a second (for a specific reason)
DoTheCodeThatNeedsToRunAsynchronously();
myObject.ChangeSomeProperty();
});
t.IsBackground = true;
t.Start();
With regards to the thread safety: it's difficult to tell if your program functions correctly when multiple threads execute simultaneously, because you're not showing us any points of contention in your example. It's very possible that you will experience concurrency issues if your program has contention on MyObject.
Java has the final keyword and C# has a corresponding keyword called readonly, but neither final nor readonly ensure that the state of the object you're modifying will be consistent between threads. The only thing these keywords do is ensure that you do not change the reference the object is pointing to. If two threads have read/write contention on the same object, then you should perform some type of synchronization or atomic operations on that object in order to ensure thread safety.
Update
OK, if you modify the reference to which myObject is pointing to, then your contention is now on myObject. I'm sure that my answer will not match your actual situation 100%, but given the example code you've provided I can tell you what will happen:
You will not be guaranteed which object gets modified: it can be that.MyObject or this.MyObject. That's true regardless if you're working with Java or C#. The scheduler may schedule your thread/timer to be executed before, after or during the second assignment. If you're counting on a specific order of execution, then you have to do something to ensure that order of execution. Usually that something is a communication between the threads in the form of a signal: a ManualResetEvent, Join or something else.
Here is a join example:
MyObject myObject = this.MyObject;
Thread task = new Thread(()=>
{
Thread.Sleep(1000); // wait a second (for a specific reason)
DoTheCodeThatNeedsToRunAsynchronously();
myObject.ChangeSomeProperty();
});
task.IsBackground = true;
task.Start();
task.Join(); // blocks the main thread until the task thread is finished
myObject = that.MyObject; // the assignment will happen after the task is complete
Here is a ManualResetEvent example:
ManualResetEvent done = new ManualResetEvent(false);
MyObject myObject = this.MyObject;
Thread task = new Thread(()=>
{
Thread.Sleep(1000); // wait a second (for a specific reason)
DoTheCodeThatNeedsToRunAsynchronously();
myObject.ChangeSomeProperty();
done.Set();
});
task.IsBackground = true;
task.Start();
done.WaitOne(); // blocks the main thread until the task thread signals it's done
myObject = that.MyObject; // the assignment will happen after the task is done
Of course, in this case it's pointless to even spawn multiple threads, since you're not going to allow them to run concurrently. One way to avoid this is by not changing the reference to myObject after you've started the thread, then you won't need to Join or WaitOne on the ManualResetEvent.
So this leads me to a question: why are you assigning a new object to myObject? Is this a part of a for-loop which is starting multiple threads to perform multiple asynchronous tasks?
What is the best way to achieve what I want? Is it one of the two or another approach?
Both look fine, but...
Is one of the two ways not thread-safe (I fear both...) and why?
...they are not thread safe unless MyObject.ChangeSomeProperty() is thread safe.
The first approach creates a thread and passes it the object in the constructor. Is that how I'm supposed to pass the object?
Yes. Using a closure (as in your second approach) is fine as well, with the additional advantage that you don't need to do a cast.
The second approach uses a timer which doesn't provide that possibility, so I just use the local variable in the anonymous delegate. Is that safe or is it possible in theory that the reference in the variable changes before it is evaluated by the delegate code? (This is a very generic question whenever one uses anonymous delegates).
Sure, if you add myObject = null; directly after setting timer.Elapsed, then the code in your thread will fail. But why would you want to do that? Note that changing this.MyObject will not affect the variable captured in your thread.
So, how to make this thread-safe? The problem is that myObject.ChangeSomeProperty(); might run in parallel with some other code that modifies the state of myObject. There are basically two solutions to that:
Option 1: Execute myObject.ChangeSomeProperty() in the main UI thead. This is the simplest solution if ChangeSomeProperty is fast. You can use the Dispatcher (WPF) or Control.Invoke (WinForms) to jump back to the UI thread, but the easiest way is to use a BackgroundWorker:
MyObject myObject = this.MyObject;
var bw = new BackgroundWorker();
bw.DoWork += (sender, args) => {
// this will happen in a separate thread
Thread.Sleep(1000);
DoTheCodeThatNeedsToRunAsynchronously();
}
bw.RunWorkerCompleted += (sender, args) => {
// We are back in the UI thread here.
if (args.Error != null) // if an exception occurred during DoWork,
MessageBox.Show(args.Error.ToString()); // do your error handling here
else
myObject.ChangeSomeProperty();
}
bw.RunWorkerAsync(); // start the background worker
Option 2: Make the code in ChangeSomeProperty() thread-safe by using the lock keyword (inside ChangeSomeProperty as well as inside any other method modifying or reading the same backing field).
The bigger thread-safety concern here, in my mind, may be the 1 second Sleep. If this is required in order to synchronize with some other operation (giving it time to complete), then I strongly recommend using a proper synchronization pattern rather than relying on the Sleep. Monitor.Pulse or AutoResetEvent are two common ways to achieve synchronization. Both should be used carefully, as it's easy to introduce subtle race conditions. However, using Sleep for synchronization is a race condition waiting to happen.
Also, if you want to use a thread (and don't have access to the Task Parallel Library in .NET 4.0), then ThreadPool.QueueUserWorkItem is preferable for short-running tasks. The thread pool threads also won't hang up the application if it dies, as long as there is not some deadlock preventing a non-background thread from dying.
One thing not mentioned so far: The choice of threading methods depends heavily on specifically what DoTheCodeThatNeedsToRunAsynchronously() does.
Different .NET threading approaches are suitable for different requirements. One very large concern is whether this method will complete quickly, or take some time (is it short-lived or long-running?).
Some .NET threading mechanisms, like ThreadPool.QueueUserWorkItem(), are for use by short-lived threads. They avoid the expense of creating a thread by using "recycled" threads--but the number of threads it will recycle is limited, so a long-running task shouldn't hog the ThreadPool's threads.
Other options to consider are using:
ThreadPool.QueueUserWorkItem() is a convienient means to fire-and-forget small tasks on a ThreadPool thread
System.Threading.Tasks.Task is a new feature in .NET 4 which makes small tasks easy to run in async/parallel mode.
Delegate.BeginInvoke() and Delegate.EndInvoke() (BeginInvoke() will run the code asynchronously, but it's crucial that you ensure EndInvoke() is called as well to avoid potential resource-leaks. It's also based on ThreadPool threads I believe.
System.Threading.Thread as shown in your example. Threads provide the most control but are also more expensive than the other methods--so they are ideal for long-running tasks or detail-oriented multithreading.
Overall my personal preference has been to use Delegate.BeginInvoke()/EndInvoke() -- it seems to strike a good balance between control and ease of use.
What is the most efficient way to create a “cancel” event in a C# program that is crunching a large set of data in a loop on a separate thread?
For now, I am simply using a cancel event that is triggered from my UI thread, which subsequently calls an “onCancel” function on the number crunching thread. That cancel function sets a variable to “true”, which the crunch loop checks periodically, e.g.
Class Cruncher {
private bool cancel = false;
public cruncher()
{
crunch();
}
private void crunch()
{
while(conditions AND !cancel) { crunch; }
dispose_resources;
}
private void onCancel()
{
cancel = true;
}
}
While I am not checking the cancel variable as often as my example above (and not actually performing a NOT cancel), I would still like to optimize this crunch method as much as possible. Any examples where this is done more efficiently would be very nice to see.
The cancel event/flag should be a volatile... I asked a very similar question to yours: Is it safe to use a boolean flag to stop a thread from running in C#
I would also recommend that when you cancel your threads you wait for all of them to cancel by using something similar to the C# version of CountDownLatch. It's useful when you want to guarantee that the thread is canceled.
It will ultimately always result in something like this - although it's important that you make your cancel variable volatile, as otherwise the worker threads may not see the change from the cancelling thread.
You've got to check something periodically unless you want to go the more drastic route of interrupting the thread (which I don't recommend). Checking a single Boolean flag isn't likely to be exactly costly... if you can do a reasonable chunk of work in each iteration of the loop (enough to dwarf the cost of the check) then that's fine.
If you ever need to perform any waiting, however (in the worker thread), then you may be able to improve matters, by using a form of waiting (e.g. Monitor.Wait) which allows the cancelling thread to wake any waiting threads up early. That won't make normal operation more efficient, but it will allow the threads to terminate more quickly in the event of cancellation.
Especially since it's UI-triggered, I would recommend just leveraging the BackgroundWorker that's already in the framework, especially since it'll nicely have the progress and done events happen on the UI thread for you (so you don't have to invoke it over yourself).
Then you can just use the CancelAsync() call. Admittedly, it's not much different than what you're already doing, just done in the framework already (and including the thread synchronization logic)
As Jon mentioned, you're still going to want to do cooperative cancellation (checking CancellationPending in your DoWork for use of BackgroundWorker) since the 'interrupt/abort the thread' option is something you want to avoid if possible.
If in .NET 4 you can use TPL and the new Cancellation support, but again it's focused on cooperative cancellation.
I recommend using the unified cancellation model that was introduced in .NET 4.0 (if .NET 4.0 is an option).
It is very efficient, and allows integrated cancellation with Task objects and Parallel LINQ.
i would do it the same way. i would also add Thread.Sleep in to the loop to yield control to the main thread.
http://msdn.microsoft.com/en-us/library/7a2f3ay4%28VS.80%29.aspx
I have a console app that I'm porting to WPF. The application has 3 worker threads, that are all joined to the main thread before some output results are printed to the screen. My understanding is that, if I try and do the same thing in a WPF application, the GUI will be blocked and will not be reponsive to the user. How then can I notify the parent thread that all the threads have completed their work? I think the solution is going to involve delegates and events (or maybe BackgroundWorker?), but it was not clear to me how to get the callback invoked when the thread terminated.
Original Code:
foreach (Thread t in threadList)
{
t.Start();
}
foreach (Thread t in threadList)
{
t.Join();
}
// print some results here
If you are using three BackgroundWorkers, you can use the event RunWorkerCompleted to notice that one of the workers is completed: Before starting the workers set a counter to 3 then decrement and check this counter in the method called by RunWorkerCompleted if it hits 0 you are finished.
You should use three BackgroundWorkers.
You can then handle their RunWorkerCompleted events to find out when the operations finish.
Take a look at this article in MSDN magazine that gives you an example on using BackgroundWorker. Start at Figure 7.
Depends on what you would like to accomplish. What form of communication are you trying to facilitate?
If I were to guess, what you really want is to simply report [or display] your worker results in your application. If this is the case, then in a typical WPF application you have a view model, say
public class AwesomeViewModel : INotifyPropertyChanged
{
// if small fixed number, otherwise, you could use
// an ObservableCollection<T>
public string WorkerResultA { ... }
public string WorkerResultB { ... }
public string WorkerResultC { ... }
}
which is data-bound to your WPF controls. You can simply pass a reference of your view model to each worker thread and they update the class without requiring blocking\waiting on Gui thread. In this manner, each worker reports its results when it completes without intervention from anyone else. This is optimal.
Of course, if you go ahead and do just this, you run into another completely different issue. Which, fyi, is resolvable via Dispatcher. One possible solution here.
As for BackgroundWorker versus explicit Thread control, that is up to you. There are advantages to both, but remember you already have functional code written. That, and in my personal opinion, BackgroundWorker isn't particularly useful.
If you really absolutely positively must implement a more sophisticated synchronization model, then I highly recommend you brush up on ManualResetEvent its cousin AutoResetEvent, Semaphore, keyword lock and concurrent programming in general. Sorry, no shortcuts there :)
Hope this helps!
If you just want to poll the worker threads, you could use something like
bool threadWasDone = thread.Join(0);
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.