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In an application that I am developing I will be using 2 threads to do various operations. (I will not go into detail here.) These threads work in loops, checking if there is work to be done, doing work, calculating the time they need to wait and waiting. (See below)
public Global : System.Web.HttpApplication
{
private static Thread StartingDateThread;
private static Thread DeadlineDateThread;
private static object o1;
private static object o2;
public static Thread GetStartingDateThreadInstance
{
get
{
if(StartingDateThread==null)
{
StartingDateThread=new Thread(new ThreadStart(MonitorStartingDates));
}
return StartingDateThread;
}
}
public static Thread GetDeadlineThreadInstance
{
get
{
if(DeadlineDateThread==null)
{
DeadlineDateThread=new Thread(new ThreadStart(MonitorDeadlines));
}
return DeadlineDateThread;
}
}
public static object GetFirstObjectInstance
{
get
{
if(o1==null)
{
o1=new object();
}
return o1;
}
}
public static object GetSecondObjectInstance
{
get
{
if(o2==null)
{
o2=new object();
}
return o2;
}
}
protected void Application_Start(object sender, EventArgs e)
{
GetStartingDateThreadInstance.Start();
GetDeadlineThreadInstance.Start();
//////////////////////
////Do other stuff.
}
public void MonitorStartingDates()
{
while(true)
{
//Check if there is stuff to do.
//Do stuff if available.
//Check if there will be stuff to do in the future and if there is, check
//the time to wake up.
//If there is nothing to do, sleep for a pre-determined 12 hours.
if(StuffToDoInFuture)
{
Monitor.Enter(GetFirstObjectInstance);
Monitor.Wait(WaitingTime);
Monitor.Exit(GetFirstObjectInstance);
}
else
{
Monitor.Enter(GetFirstObjectInstance);
Monitor.Wait(new TimeSpan(12, 0, 0));
Monitor.Exit(GetFirstObjectInstance);
}
}
}
public void MonitorDeadlines()
{
while(true)
{
//Check if there is stuff to do.
//Do stuff if available.
//Check if there will be stuff to do in the future and if there is, check
//the time to wake up.
//If there is nothing to do, sleep for a pre-determined 3 days and 12 hours.
if(StuffToDoInFuture)
{
Monitor.Enter(GetSecondObjectInstance);
Monitor.Wait(WaitingTime);
Monitor.Exit(GetSecondObjectInstance);
}
else
{
Monitor.Enter(GetSecondObjectInstance);
Monitor.Wait(new TimeSpan(3, 12, 0, 0));
Monitor.Exit(GetSecondObjectInstance);
}
}
}
As you can see these two threads are started in the Application_Start method in the asax file. They operate if there is stuff available to do and then they calculate the time period they need to wait and then they wait. However, as users of the web application do operations new records will be inserted into the database and there will be circumstances where any of the two threads will have to resume operation sooner than planned. So, say I have a method in my DataAccess class which inserts into the database new data. (See below)
public class DataAccess
{
///////////////
//
public void InsertNewAuction()
{
///Insert new row calculate the time
Monitor.Pulse(Global.GetFirstObjectInstance);
Monitor.Pulse(Global.GetSecondObjectInstance);
///
}
}
It seems like this is an invalid operation, because at the stage where the Monitor.Pulse is called from the InsertNewAuction method I get an exception. Something like "Object synchronization method was called from an unsynchronized block of code." Is there any way of doing this? Thanks for your help
As to the specific error you're seeing, this is because Monitor.Pulse must be called inside the Monitor lock, like this (I've used lock rather than Enter/Exit, as it's safer for making sure the lock is always released, since it uses a proper try/finally block):
lock (Global.GetFirstObjectInstance)
{
Monitor.Pulse(Global.GetFirstObjectInstance);
}
In regard to the more general design question here, it's often dangerous to expose lock objects as public (or even worse, global) fields. In particular, it can be a recipe for deadlocks when multiple global locks are exposed and acquired in differing orders or when you have cases like blocking dispatches to the UI thread while holding a lock. Consider looking into alternate ways to accomplish what you're after.
As noted in the other answer, you have to acquire the lock before you can call Monitor.Pulse() on the monitor object.
That said, your code has at least one other serious bug: you are not initializing the synchronization object in a thread-safe way, which could easily lead to two different threads using two different object instances, resulting in no synchronization between those threads:
public static object GetFirstObjectInstance
{
get
{
if(o1==null)
{
o1=new object();
}
return o1;
}
}
If two threads call this getter simultaneously, they each may see o1 as null and try to initialize it. Then each might return a different value for the object instance.
You should simply initialize the object in a initializer:
private static readonly object o1 = new object();
And then return it from the getter:
public static object GetFirstObjectInstance { get { return o1; } }
That addresses the thread-safety issue. But you still have other issues with the code. First, you should encapsulate synchronization in an object, not expose the actual synchronization object instance. Second, assuming you are going to expose the synchronization object, I don't understand why you bother with the property, since you made the field public. The field should be private if you want to use a property as well.
It would also be better if the property followed normal .NET naming conventions. A method that returned the object would have "Get" in the name, but a property would not. Just name it "FirstObjectInstance".
Also as noted by Dan, use lock everywhere you want to acquire the lock.
There may be other issues in the code as well...I didn't do a thorough review. But the above you need to fix for sure.
This question already has answers here:
Why does my destructor never run?
(6 answers)
Closed 9 years ago.
I have a class Class that creates a Thread in it's constructor. This thread runs a while(true) loop that is reading non-critical data from a NetStream. The thread will be aborted by the destructor:
~Class()
{
_thread.Abort();
_thread = null;
}
When the program wants to end the use of Class's instance - ClassInstance, it calls:
ClassInstance = null;
GC.Collect;
I thought that means that ~Class() will be caller automatically at that point - but it's not.
This thread keeps running even after Application.Exit() and returning from Main().
The crucial bit of your code is not included; how the thread is started and what method it is running. If I had to make a guess I would say it is likely you started the thread by passing an instance method of Class. So basically your class instance is still rooted by the running of the thread. You attempt to stop the thread in the finalizer, but the finalizer will never run because the instance is still rooted leading to a catch-22 situation.
Also, you mentioned that the thread is running non-critical code and that was your justification for using Thread.Abort. That really is not a good enough reason. It is very hard to control where that ThreadAbortException will get injected into the thread and as a result it may corrupt critical program data structures you did not anticipate.
Use the new cooperative cancellation mechanisms included with the TPL. Change the while (true) loop to poll a CancellationToken instead. Signal the cancellation in the Dispose method when you implement IDisposable. Do not include a finalizer (destructor in C# terminology). Finalizers are intended to be used to clean up unmanaged resources. Since you have not indicated that unmanaged resources are in play then it is pointless to have a finalizer. You do not have to include a finalizer when implementing IDisposable. In fact, it is considered bad practice to have one when it is not really needed.
public class Class : IDisposable
{
private Task task;
private CancellationTokenSource cts = new CancellationTokenSource();
Class()
{
task = new Task(Run, cts.Token, TaskCreationOptions.LongRunning);
task.Start();
}
public void Dispose()
{
cts.Cancel();
}
private void Run()
{
while (!cts.Token.IsCancellationRequested)
{
// Your stuff goes here.
}
}
}
If you implement IDisposable, and dispose the object, then the code in Dispose will run, but there is no guarantee that Destructor will also be called.
Garbage Collector forms an opinion that it is a waste of time. So if you want to have a predictable dispose you can use IDisposable.
Check this Thread
Slightly off-topic: You can use Tasks instead of naked threads to run functions without worrying about disposal.
There are multiple issues here:
Setting the variable to null doesn't delete anything, it simply removes a reference to your instance.
The destructor will only get called when the garbage collector decides to collect your instance. The garbage collector runs infrequently, typically only when it detects that there is memory pressure.
The garbage collector collects ONLY orphaned collections. Orphaned means that any references pointed to by your object are invalid.
You should implement the IDisposable interface and call any cleanup code in the Dispose method. C# and VB offer the using keyword to make disposal easier even in the face of exception.
A typical IDisposable implementation is similar to the following:
class MyClass:IDisposable
{
ClassB _otherClass;
...
~MyClass()
{
//Call Dispose from constructor
Dispose(false);
}
public void Dispose()
{
//Call Dispose Explicitly
Dispose(true);
//Tell the GC not call our destructor, we already cleaned the object ourselves
GC.SuppressFinalize(this);
}
protected virtual Dispose(bool disposing)
{
if (disposing)
{
//Clean up MANAGED resources here. These are guaranteed to be INvalid if
//Dispose gets called by the constructor
//Clean this if it is an IDisposable
_otherClass.Dispose();
//Make sure to release our reference
_otherClass=null;
}
//Clean UNMANAGED resources here
}
}
You can then use your class like this :
using(var myClass=new MyClass())
{
...
}
Once the using block terminates, Dispose() will be called even if an exception occurs.
CLR maintains all the running threads. You will have passed the InstanceMethod of your class to the thread's constructor as either ThreadStart or ParameterizedThreadStart delegate. Delegate will hold the MethodInfo of the method you passed and the Instance of your class in Target Property.
Garbage collector collects and object which should not have any Strong References but your instance is still alive inside Delegate of Thread. So your class is still having the Strong Reference hence it is not eligible for garbage collection.
To prove what I stated above
public class Program
{
[STAThread]
static void Main(string[] args)
{
GcTest();
Console.Read();
}
private static void GcTest()
{
Class cls = new Class();
Thread.Sleep(10);
cls = null;
GC.Collect();
GC.WaitForPendingFinalizers();
}
}
public class Class
{
private Thread _thread;
~Class()
{
Console.WriteLine("~Class");
_thread.Abort();
_thread = null;
}
public Class()
{
_thread = new Thread(ThreadProc);
_thread.Start();
}
private void ThreadProc()
{
while (true)
{
Thread.Sleep(10);
}
}
}
}
Try the above code. Destructor Will not be called. To make it work mark the ThreadProc method as static and run again Destructor will be called
In .NET (C#) I follow some custom conventions and patterns that require Constructors, Initialization functions and IDisposable implementations. A typical class is illustrated below. No initialization is done directly in the constructor but rather through a dedicated function that is supposed to make the object reusable. However, I am not sure what happens when Dispose gets called. If the GC calls it, the reference to the object is lost anyways so no worries there. If it is explicitly called, are there any drawbacks simply calling Initialize and treating the class as a fresh object since GC.SupressFinalize has been called? Lol, I'm sure I could have asked this in an easier way.
public abstract class Thread: System.IDisposable
{
protected bool Disposed { get; set; }
protected bool Terminate { get; private set; }
public bool IsRunning { get; private set; }
private System.Threading.Thread ThreadObject { get; set; }
public Thread ()
{
this.Initialize();
}
~Thread ()
{
this.Dispose(false);
}
public virtual void Initialize ()
{
this.Stop();
this.Disposed = false;
this.Terminate = true;
this.IsRunning = false;
this.ThreadObject = null;
}
//====================================================================================================
// Functions: Thread
//====================================================================================================
public void Start ()
{
if (!this.IsRunning)
{
this.IsRunning = true;
this.Terminate = false;
this.ThreadObject = new System.Threading.Thread(new System.Threading.ThreadStart(this.Process));
this.ThreadObject.Start();
}
}
/// <summary>
/// Override this method to do thread processing.
/// [this.Terminate] will be set to indicate that Stop has been called.
/// </summary>
/// <param name="template"></param>
protected abstract void Process ();
public void Stop (System.TimeSpan timeout)
{
if (this.IsRunning)
{
this.Terminate = true;
try
{
if (timeout.TotalMilliseconds > 1D)
{
this.ThreadObject.Join(timeout);
}
else
{
this.ThreadObject.Join();
}
}
catch
{
try
{
this.ThreadObject.Abort();
}
catch
{
}
}
this.ThreadObject = null;
this.IsRunning = false;
}
}
//====================================================================================================
// Interface Implementation: System.IDisposable
//====================================================================================================
public void Dispose ()
{
this.Dispose(true);
System.GC.SuppressFinalize(this);
}
protected virtual void Dispose (bool disposing)
{
if (!this.Disposed)
{
if (disposing)
{
// Dispose managed resources.
this.Stop(System.TimeSpan.FromSeconds(1));
}
// Dispose unmanaged resources here.
// Note disposing has been done.
this.Disposed = true;
}
}
}
The GC never calls Dispose, it's up to the consuming code. The GC does however call the finalizer. This is used in the best practice IDisposable implementation to clean up unmanaged code only.
Where Dispose is used outside of the context of a finalizer, then there is no need for the GC to call the finalizer, and therefore SuppressFinalize is used as an optimisation to prevent it happening twice.
If the object is reused this causes an issue. Technically you can re-register the finalizer on initialization, but this would need to be made thread safe. Common practice is that an object is not reused after it has been Disposed, and typically the Dispose method should only execute exactly once. IMO the initializer method and object reuse introduces complexities to the pattern that move it away from it's intended purpose.
There's no technical reason why you can't reactivate a disposed object in this way, though I woudln't do it as it's against the principle of least surprise (most disposable objects are used once).
If you really do want to go this way, I'd avoid having a finalizer, which means your IDisposable class must not directly own any unmanaged resources. You can do this by wrapping any unmanaged resources your class uses in a manged wrapper (e.g. look at the SafeHandle class for an example).
I don't like all the precise details of your thread handling, but if you are going to have a class where each instance owns a thread, you should provide a Dispose method which will ensure that the instance's thread dies off in an orderly fashion.
If you want to allow for the thread to get cleaned up even when an object is abandoned, you'll probably have to create a wrapper object to which the outside application holds a reference but your thread does not. The Finalize() method for that wrapper object should nudge the thread in such a way that it will die off. The thread could simply poll a flag every few seconds to see if it should exit, or there could be a more sophisticated termination strategy.
I'm confused, though, why Initialize calls Stop()? I would have expected it to call Start().
Wrong language pattern appication sample is used in the code. I clearly see C++ backgroung for the C# code author. Unfortunately C++ coding techniques in not applicable in C# language.
Better not to allow object to get into garbage collector (GC), simply referencing it somewhere else, as in the Singleton pattern, rather that trying to resurrect disposed object, or use Dispose pattern in a language not allowing full control for the garbage collector and memory management, as is to be true, for example, in C++.
Simply, you should not use C++ idioms in C#, but the tips and tricks are:
Interfaces instead of pure virtual functions in C++,
Interface inheritancee instead of multiple class inheritance in C++,
No memory management (use weak references) instead of full controlled object lifetime in C++
I have a class that makes use of temporary files (Path.GetTempFileName()) while it is active. I want to make sure these files do not remain on the user's hard drive taking up space after my program is closed. Right now my class has a Close() method which checks if any temporary files used by the class still exist and deletes them.
Would it make more sense to put this code in the Dispose() or Finalize() methods instead?
Better yet would be to create the file with FileOptions.DeleteOnClose. This will ensure that the operating system forcibly deletes the file when your process exits (even in the case of a rude abort). Of course, you will still want to close/delete the file yourself when you are done with it, but this provides a nice backstop to ensure that you don't allow the files to be sit around forever
Example:
using (FileStream fs = File.Create(Path.GetTempFileName(), Int16.MaxValue,
FileOptions.DeleteOnClose))
{
// Use temp file
} // The file will be deleted here
I would do both; make the class disposable, and have the finalizer clean it up. There is a standard pattern for doing so safely and effectively: use it rather than attempting to deduce for yourself what the right pattern is. It is very easy to get wrong. Read this carefully:
http://msdn.microsoft.com/en-us/library/system.idisposable.aspx
Note that you've got to be really really careful when writing a finalizer. When the finalizer runs, many of your normal assumptions are wrong:
There are all kinds of potentials for race conditions or deadlocks because you are no longer on the main thread, you're on the finalizer thread.
In regular code, if you're running code inside an object then you know that all the things the object refers to are alive. In a finalizer, all the things the object refers to might have just been finalized! Finalizers of dead objects can run in any order, including "child" objects being finalized before "parent" objects.
In regular code, assigning a reference to an object to a static field could be perfectly sensible. In a finalizer, the reference you are assigning could be to an already dead object, and therefore the assignment brings a dead object back to life. (Because objects referred to by static fields are always alive.) That is an exceedingly weird state to be in and nothing pleasant happens if you do.
And so on. Be careful. You are expected to fully understand the operation of the garbage collector if you write a non-trivial finalizer.
A file is an unmanaged resource, and you implement IDisposable to clean up unmanaged resources that your classes are dependent upon.
I have implemented similar classes, although never in production code.
However, I understand your tentativeness about this - user interaction with the files outside of your application could screw things up and cause problems during disposal. However, that is the same for any file created/deleted by an application, regardless of whether or not it's tidied up by a Dispose() method or not.
I'd have to say that implementing IDisposable would be a reasonable choice.
A nice way is suggested by David M. Kean on the MSDN entry on Path.GetTempFileName. He creates a wrapper class implementing IDisposable that will automatically remove the file:
public class TemporaryFile : IDisposable
{
private bool _isDisposed;
public bool Keep { get; set; }
public string Path { get; private set; }
public TemporaryFile() : this(false)
{
}
public TemporaryFile(bool shortLived)
{
this.Path = CreateTemporaryFile(shortLived);
}
~TemporaryFile()
{
Dispose(false);
}
public void Dispose()
{
Dispose(false);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!_isDisposed)
{
_isDisposed = true;
if (!this.Keep)
{
TryDelete();
}
}
}
private void TryDelete()
{
try
{
File.Delete(this.Path);
}
catch (IOException)
{
}
catch (UnauthorizedAccessException)
{
}
}
public static string CreateTemporaryFile(bool shortLived)
{
string temporaryFile = System.IO.Path.GetTempFileName();
if (shortLived)
{
// Set the temporary attribute, meaning the file will live
// in memory and will not be written to disk
//
File.SetAttributes(temporaryFile,
File.GetAttributes(temporaryFile) | FileAttributes.Temporary);
}
return temporaryFile;
}
}
Using the new class is easy, just type the following:
using (TemporaryFile temporaryFile = new TemporaryFile())
{
// Use temporary file
}
If you decide, after constructing a TemporaryFile, that you want to prevent it from being deleted, simply set the TemporaryFile.Keep property to true:
using (TemporaryFile temporaryFile = new TemporaryFile())
{
temporaryFile.Keep = true;
}
Absolutely. This way you can ensure cleanup with exceptions present.
You should definitely use Dispose to clean up resources, but make sure you implement the IDisposable interface. You don't want to just add a method named Dispose.
I always make my classes that point to temp files IDisposable, and usually implement a finalizer that calls my dispose method there as well. This seems to be the paradigm suggested by the IDisposable MSDN page.
Related code below:
public void Dispose()
{
Dispose(true);
// This object will be cleaned up by the Dispose method.
// Therefore, you should call GC.SupressFinalize to
// take this object off the finalization queue
// and prevent finalization code for this object
// from executing a second time.
GC.SuppressFinalize(this);
}
// Dispose(bool disposing) executes in two distinct scenarios.
// If disposing equals true, the method has been called directly
// or indirectly by a user's code. Managed and unmanaged resources
// can be disposed.
// If disposing equals false, the method has been called by the
// runtime from inside the finalizer and you should not reference
// other objects. Only unmanaged resources can be disposed.
private void Dispose(bool disposing)
{
// Check to see if Dispose has already been called.
if(!this.disposed)
{
// If disposing equals true, dispose all managed
// and unmanaged resources.
if(disposing)
{
// Dispose managed resources.
}
// Call the appropriate methods to clean up
// unmanaged resources here.
// If disposing is false,
// only the following code is executed.
// Note disposing has been done.
disposed = true;
}
}
// Use C# destructor syntax for finalization code.
// This destructor will run only if the Dispose method
// does not get called.
// It gives your base class the opportunity to finalize.
// Do not provide destructors in types derived from this class.
~MyResource()
{
// Do not re-create Dispose clean-up code here.
// Calling Dispose(false) is optimal in terms of
// readability and maintainability.
Dispose(false);
}
If you wish to re-use your temporary files e.g. open\close\read\write\etc, then clearing them up at the AppDomain unload level can be useful.
This can be used in combination with putting temp files in a well known sub-directory of a temp location and making sure that the directory is deleted on application startup to ensure unclean shut-downs are taken care of.
A basic example of the technique (with exception handling removed around delete for brevity). I use this technique in file-based unit tests where it makes sense and is useful.
public static class TempFileManager
{
private static readonly List<FileInfo> TempFiles = new List<FileInfo>();
private static readonly object SyncObj = new object();
static TempFileManager()
{
AppDomain.CurrentDomain.DomainUnload += CurrentDomainDomainUnload;
}
private static void CurrentDomainDomainUnload(object sender, EventArgs e)
{
TempFiles.FindAll(file => File.Exists(file.FullName)).ForEach(file => file.Delete());
}
public static FileInfo CreateTempFile(bool autoDelete)
{
FileInfo tempFile = new FileInfo(Path.GetTempFileName());
if (autoDelete)
{
lock (SyncObj)
{
TempFiles.Add(tempFile);
}
}
return tempFile;
}
}
Let's say I have a class that implements the IDisposable interface. Something like this:
MyClass uses some unmanaged resources, hence the Dispose() method from IDisposable releases those resources. MyClass should be used like this:
using ( MyClass myClass = new MyClass() ) {
myClass.DoSomething();
}
Now, I want to implement a method that calls DoSomething() asynchronously. I add a new method to MyClass:
Now, from the client side, MyClass should be used like this:
using ( MyClass myClass = new MyClass() ) {
myClass.AsyncDoSomething();
}
However, if I don't do anything else, this could fail as the object myClass might be disposed before DoSomething() is called (and throw an unexpected ObjectDisposedException). So, the call to the Dispose() method (either implicit or explicit) should be delayed until the asynchronous call to DoSomething() is done.
I think the code in the Dispose() method should be executed in a asynchronous way, and only once all asynchronous calls are resolved. I'd like to know which could be the best way to accomplish this.
Thanks.
NOTE: For the sake of simplicity, I haven't entered in the details of how Dispose() method is implemented. In real life I usually follow the Dispose pattern.
UPDATE: Thank you so much for your responses. I appreciate your effort. As chakrit has commented, I need that multiple calls to the async DoSomething can be made. Ideally, something like this should work fine:
using ( MyClass myClass = new MyClass() ) {
myClass.AsyncDoSomething();
myClass.AsyncDoSomething();
}
I'll study the counting semaphore, it seems what I'm looking for. It could also be a design problem. If I find it convenient, I will share with you some bits of the real case and what MyClass really does.
It looks like you're using the event-based async pattern (see here for more info about .NET async patterns) so what you'd typically have is an event on the class that fires when the async operation is completed named DoSomethingCompleted (note that AsyncDoSomething should really be called DoSomethingAsync to follow the pattern correctly). With this event exposed you could write:
var myClass = new MyClass();
myClass.DoSomethingCompleted += (sender, e) => myClass.Dispose();
myClass.DoSomethingAsync();
The other alternative is to use the IAsyncResult pattern, where you can pass a delegate that calls the dispose method to the AsyncCallback parameter (more info on this pattern is in the page above too). In this case you'd have BeginDoSomething and EndDoSomething methods instead of DoSomethingAsync, and would call it something like...
var myClass = new MyClass();
myClass.BeginDoSomething(
asyncResult => {
using (myClass)
{
myClass.EndDoSomething(asyncResult);
}
},
null);
But whichever way you do it, you need a way for the caller to be notified that the async operation has completed so it can dispose of the object at the correct time.
Since C#8.0 you can use IAsyncDisposable.
using System.Threading.Tasks;
public class ExampleAsyncDisposable : IAsyncDisposable
{
public async ValueTask DisposeAsync()
{
// await DisposeAllTheThingsAsync();
}
}
Here is the reference to the official Microsoft documentation.
Async methods usually have a callback allowing you to do do some action upon completition. If this is your case it would be something like this:
// The async method taks an on-completed callback delegate
myClass.AsyncDoSomething(delegate { myClass.Dispose(); });
An other way around this is an async wrapper:
ThreadPool.QueueUserWorkItem(delegate
{
using(myClass)
{
// The class doesn't know about async operations, a helper method does that
myClass.DoSomething();
}
});
I consider it unfortunate that Microsoft didn't require as part of the IDisposable contract that implementations should allow Dispose to be called from any threading context, since there's no sane way the creation of an object can force the continued existence of the threading context in which it was created. It's possible to design code so that the thread which creates an object will somehow watch for the object becoming obsolete and can Dispose at its convenience, and so that when the thread is no longer needed for anything else it will stick around until all appropriate objects have been Disposed, but I don't think there's a standard mechanism that doesn't require special behavior on the part of the thread creating the Dispose.
Your best bet is probably to have all the objects of interest created within a common thread (perhaps the UI thread), try to guarantee that the thread will stay around for the lifetime of the objects of interest, and use something like Control.BeginInvoke to request the objects' disposal. Provided that neither object creation nor cleanup will block for any length of time, that may be a good approach, but if either operation could block a different approach may be needed [perhaps open up a hidden dummy form with its own thread, so one can use Control.BeginInvoke there].
Alternatively, if you have control over the IDisposable implementations, design them so that they can safely be fired asynchronously. In many cases, that will "just work" provided nobody is trying to use the item when it is disposed, but that's hardly a given. In particular, with many types of IDisposable, there's a real danger that multiple object instances might both manipulate a common outside resource [e.g. an object may hold a List<> of created instances, add instances to that list when they are constructed, and remove instances on Dispose; if the list operations are not synchronized, an asynchronous Dispose could corrupt the list even if the object being disposed is not otherwise in use.
BTW, a useful pattern is for objects to allow asynchronous dispose while they are in use, with the expectation that such disposal will cause any operations in progress to throw an exception at the first convenient opportunity. Things like sockets work this way. It may not be possible for a read operation to be exit early without leaving its socket in a useless state, but if the socket's never going to be used anyway, there's no point for the read to keep waiting for data if another thread has determined that it should give up. IMHO, that's how all IDisposable objects should endeavor to behave, but I know of no document calling for such a general pattern.
I wouldn't alter the code somehow to allow for async disposes. Instead I would make sure when the call to AsyncDoSomething is made, it will have a copy of all the data it needs to execute. That method should be responsible for cleaning up all if its resources.
You could add a callback mechanism and pass a cleanup function as a callback.
var x = new MyClass();
Action cleanup = () => x.Dispose();
x.DoSomethingAsync(/*and then*/cleanup);
but this would pose problem if you want to run multiple async calls off the same object instance.
One way would be to implement a simple counting semaphore with the Semaphore class to count the number of running async jobs.
Add the counter to MyClass and on every AsyncWhatever calls increment the counter, on exits decerement it. When the semaphore is 0, then the class is ready to be disposed.
var x = new MyClass();
x.DoSomethingAsync();
x.DoSomethingAsync2();
while (x.RunningJobsCount > 0)
Thread.CurrentThread.Sleep(500);
x.Dispose();
But I doubt that would be the ideal way. I smell a design problem. Maybe a re-thought of MyClass designs could avoid this?
Could you share some bit of MyClass implementation? What it's supposed to do?
Here's a more modern spin on this old question.
The real objective is to track the async Tasks and wait until they finish...
public class MyExample : IDisposable
{
private List<Task> tasks = new List<Task>();
public async Task DoSomething()
{
// Track your async Tasks
tasks.Add(DoSomethingElseAsync());
tasks.Add(DoSomethingElseAsync());
tasks.Add(DoSomethingElseAsync());
}
public async Task DoSomethingElseAsync()
{
// TODO: something else
}
public void Dispose()
{
// Block until Tasks finish
Task.WhenAll(tasks);
// NOTE: C# allows DisposeAsync()
// Use non-blocking "await Task.WhenAll(tasks)"
}
}
Consider turning it into a base class for re-usability.
And sometimes I use a similar pattern for static methods...
public static async Task MyMethod()
{
List<Task> tasks = new List<Task>();
// Track your async Tasks
tasks.Add(DoSomethingElseAsync());
tasks.Add(DoSomethingElseAsync());
tasks.Add(DoSomethingElseAsync());
// Wait for Tasks to complete
await Task.WhenAll(tasks);
}
So, my idea is to keep how many AsyncDoSomething() are pending to complete, and only dispose when this count reaches to zero. My initial approach is:
public class MyClass : IDisposable {
private delegate void AsyncDoSomethingCaller();
private delegate void AsyncDoDisposeCaller();
private int pendingTasks = 0;
public DoSomething() {
// Do whatever.
}
public AsyncDoSomething() {
pendingTasks++;
AsyncDoSomethingCaller caller = new AsyncDoSomethingCaller();
caller.BeginInvoke( new AsyncCallback( EndDoSomethingCallback ), caller);
}
public Dispose() {
AsyncDoDisposeCaller caller = new AsyncDoDisposeCaller();
caller.BeginInvoke( new AsyncCallback( EndDoDisposeCallback ), caller);
}
private DoDispose() {
WaitForPendingTasks();
// Finally, dispose whatever managed and unmanaged resources.
}
private void WaitForPendingTasks() {
while ( true ) {
// Check if there is a pending task.
if ( pendingTasks == 0 ) {
return;
}
// Allow other threads to execute.
Thread.Sleep( 0 );
}
}
private void EndDoSomethingCallback( IAsyncResult ar ) {
AsyncDoSomethingCaller caller = (AsyncDoSomethingCaller) ar.AsyncState;
caller.EndInvoke( ar );
pendingTasks--;
}
private void EndDoDisposeCallback( IAsyncResult ar ) {
AsyncDoDisposeCaller caller = (AsyncDoDisposeCaller) ar.AsyncState;
caller.EndInvoke( ar );
}
}
Some issues may occur if two or more threads try to read / write the pendingTasks variable concurrently, so the lock keyword should be used to prevent race conditions:
public class MyClass : IDisposable {
private delegate void AsyncDoSomethingCaller();
private delegate void AsyncDoDisposeCaller();
private int pendingTasks = 0;
private readonly object lockObj = new object();
public DoSomething() {
// Do whatever.
}
public AsyncDoSomething() {
lock ( lockObj ) {
pendingTasks++;
AsyncDoSomethingCaller caller = new AsyncDoSomethingCaller();
caller.BeginInvoke( new AsyncCallback( EndDoSomethingCallback ), caller);
}
}
public Dispose() {
AsyncDoDisposeCaller caller = new AsyncDoDisposeCaller();
caller.BeginInvoke( new AsyncCallback( EndDoDisposeCallback ), caller);
}
private DoDispose() {
WaitForPendingTasks();
// Finally, dispose whatever managed and unmanaged resources.
}
private void WaitForPendingTasks() {
while ( true ) {
// Check if there is a pending task.
lock ( lockObj ) {
if ( pendingTasks == 0 ) {
return;
}
}
// Allow other threads to execute.
Thread.Sleep( 0 );
}
}
private void EndDoSomethingCallback( IAsyncResult ar ) {
lock ( lockObj ) {
AsyncDoSomethingCaller caller = (AsyncDoSomethingCaller) ar.AsyncState;
caller.EndInvoke( ar );
pendingTasks--;
}
}
private void EndDoDisposeCallback( IAsyncResult ar ) {
AsyncDoDisposeCaller caller = (AsyncDoDisposeCaller) ar.AsyncState;
caller.EndInvoke( ar );
}
}
I see a problem with this approach. As the release of resources is asynchronously done, something like this might work:
MyClass myClass;
using ( myClass = new MyClass() ) {
myClass.AsyncDoSomething();
}
myClass.DoSomething();
When the expected behavior should be to launch an ObjectDisposedException when DoSomething() is called outside the using clause. But I don't find this bad enough to rethink this solution.
I've had to just go old-school. No, you can't use the simplified "using" block. But a Using block is simply syntactic sugar for cleaning up a semi-complex try/catch/finally block. Build your dispose as you would any other method, then call it in a finally block.
public async Task<string> DoSomeStuffAsync()
{
// used to be a simple:
// using(var client = new SomeClientObject())
// {
// string response = await client.OtherAsyncMethod();
// return response;
// }
//
// Since I can't use a USING block here, we have to go old-school
// to catch the async disposable.
var client = new SomeClientObject();
try
{
string response = await client.OtherAsyncMethod();
return response;
}
finally
{
await client.DisposeAsync();
}
}
It's ugly, but it is very effective, and much simpler than many of the other suggestions I've seen.