将异常或消息从线程/任务传递到主线程

Question

We are dealing with processing and uploading of large files in windows service (.net 4.0). Process and upoload steps can take minutes to complete. Admin has the ability to mark job as cancelled, directly in database but to clear in memory queue it requires service restart. Goal is to abandon that job and pick next job in queue without service restart. Here's what I want to do:

In main entry point, start two tasks:

Task processTask = Task.Factory.StartNew(ProcessJob);

Task monitorTask = Task.Factory.StartNew(MonitorDB);

ProcessJob would call multiple long running steps like ProcessFile, UploadFile. We are checking for job status between the steps but job may be stuck in one of these long running steps.

If monitorTask detects job status change in DB, it should communicate that to main thread (through exception or message), so that main thread can quit, removing itself from the processing queue and allow next job in queue to start. Without Wait cannot get to exception but cannot wait because need to run these tasks in parallel. At this time we are not concerned with the fact that some synch step in ProcessJob may still be going on and it may eventually complete. We would handle that in code.

So far in all of my applications, I have used Task.ContinueWith for success and failure, but never had to communicate back to main thread.

Answer 1

You could communicate between the monitor thread and the main thread through a BlockingCollection<T> .

Here's a simple example I wrote.

BlockingCollection<string> queue =
    new BlockingCollection<string>();

// monitor thread.
Task.Factory.StartNew(() =>
{
    while (true)
    {
        Thread.Sleep(1000);
        queue.Add("event occured.");
    }
});

// main thread.
while (true)
{
    // blocks when no messages are in queue.
    string message = queue.Take();

    // kill process thread here.
}

Answer 2

You can use a CancellationToken for this. If you look at the Task class, there is an overload where you can pass in a CancellationToken . Store a reference to this token in your service and simply have your upload/process routine periodically check if the token has been cancelled like so:

if (yourToken.IsCancellationRequested)
    break;