I've solved what I believe to be your exact problem by using a wrapper class around 1+ BackgroundWorker instances.
Unfortunately, I'm not able to post my entire class, but here's the basic concept along with it's limitations.
Usage:
You simply create an instance and call RunOrReplace(...) when you want to cancel your old worker and start a new one. If the old worker was busy, it is asked to cancel and then another worker is used to immediately execute your request.
public class BackgroundWorkerReplaceable : IDisposable
{
BackgroupWorker activeWorker = null;
object activeWorkerSyncRoot = new object();
List<BackgroupWorker> workerPool = new List<BackgroupWorker>();
DoWorkEventHandler doWork;
RunWorkerCompletedEventHandler runWorkerCompleted;
public bool IsBusy
{
get { return activeWorker != null ? activeWorker.IsBusy; : false }
}
public BackgroundWorkerReplaceable(DoWorkEventHandler doWork, RunWorkerCompletedEventHandler runWorkerCompleted)
{
this.doWork = doWork;
this.runWorkerCompleted = runWorkerCompleted;
ResetActiveWorker();
}
public void RunOrReplace(Object param, ...) // Overloads could include ProgressChangedEventHandler and other stuff
{
try
{
lock(activeWorkerSyncRoot)
{
if(activeWorker.IsBusy)
{
ResetActiveWorker();
}
// This works because if IsBusy was false above, there is no way for it to become true without another thread obtaining a lock
if(!activeWorker.IsBusy)
{
// Optionally handle ProgressChangedEventHandler and other features (under the lock!)
// Work on this new param
activeWorker.RunWorkerAsync(param);
}
else
{ // This should never happen since we create new workers when there's none available!
throw new LogicException(...); // assert or similar
}
}
}
catch(...) // InvalidOperationException and Exception
{ // In my experience, it's safe to just show the user an error and ignore these, but that's going to depend on what you use this for and where you want the exception handling to be
}
}
public void Cancel()
{
ResetActiveWorker();
}
public void Dispose()
{ // You should implement a proper Dispose/Finalizer pattern
if(activeWorker != null)
{
activeWorker.CancelAsync();
}
foreach(BackgroundWorker worker in workerPool)
{
worker.CancelAsync();
worker.Dispose();
// perhaps use a for loop instead so you can set worker to null? This might help the GC, but it's probably not needed
}
}
void ResetActiveWorker()
{
lock(activeWorkerSyncRoot)
{
if(activeWorker == null)
{
activeWorker = GetAvailableWorker();
}
else if(activeWorker.IsBusy)
{ // Current worker is busy - issue a cancel and set another active worker
activeWorker.CancelAsync(); // Make sure WorkerSupportsCancellation must be set to true [Link9372]
// Optionally handle ProgressEventHandler -=
activeWorker = GetAvailableWorker(); // Ensure that the activeWorker is available
}
//else - do nothing, activeWorker is already ready for work!
}
}
BackgroupdWorker GetAvailableWorker()
{
// Loop through workerPool and return a worker if IsBusy is false
// if the loop exits without returning...
if(activeWorker != null)
{
workerPool.Add(activeWorker); // Save the old worker for possible future use
}
return GenerateNewWorker();
}
BackgroundWorker GenerateNewWorker()
{
BackgroundWorker worker = new BackgroundWorker();
worker.WorkerSupportsCancellation = true; // [Link9372]
//worker.WorkerReportsProgress
worker.DoWork += doWork;
worker.RunWorkerCompleted += runWorkerCompleted;
// Other stuff
return worker;
}
} // class
Pro/Con:
This has the benefit of having a very low delay in starting your new execution, since new threads don't have to wait for old ones to finish.
This comes at the cost of a theoretical never-ending growth of BackgroundWorker objects that never get GC'd. However, in practice the code below attempts to recycle old workers so you shouldn't normally encounter a large pool of ideal threads. If you are worried about this because of how you plan to use this class, you could implement a Timer which fires a CleanUpExcessWorkers(...) method, or have ResetActiveWorker() do this cleanup (at the cost of a longer RunOrReplace(...) delay).
The main cost from using this is precisely why it's beneficial - it doesn't wait for the previous thread to exit, so for example, if DoWork is performing a database call and you execute RunOrReplace(...) 10 times in rapid succession, the database call might not be immediately canceled when the thread is - so you'll have 10 queries running, making all of them slow! This generally tends to work fine with Oracle, causing only minor delays, but I do not have experiences with other databases (to speed up the cleanup, I have the canceled worker tell Oracle to cancel the command). Proper use of the EventArgs described below mostly solves this.
Another minor cost is that whatever code this BackgroundWorker is performing must be compatible with this concept - it must be able to safely recover from being canceled. The DoWorkEventArgs and RunWorkerCompletedEventArgs have a Cancel/Cancelled property which you should use. For example, if you do Database calls in the DoWork method (mainly what I use this class for), you need to make sure you periodically check these properties and take perform the appropriate clean-up.