how to get a list of running tasks in .net 4.0

Question

I'm trying to get a list of all currently running tasks. Does the .net 4.0 tasks api provide such functionality? Or the only option is explicitly to store tasks in a separate collection?

Answer 1

I suppose you need TaskScheduler.GetScheduledTasks method, but:

1. It's protected
2. MSDN says it should be used only for debugging
3. As far as I know this method has implemented only by ThreadPoolTaskScheduler, SynchronizationContextTaskScheduler always returns null

So I think you should try to implement own TaskScheduler to achieve your goals.

Answer 2

Why would you want to find the list of running tasks? Apart from debugging, you shouldn't have to use this information. In any case, there is a difference between the list of the tasks that are scheduled for execution and the tasks that are actually executing.

As Rusted writes, you can get the number of scheduled tasks from the TaskScheduler.GetScheduledTasks method. The method is abstract so it has to be implemented by all TaskSchedulers.

How many tasks actually execute depends on the TaskScheduler implementation. The default task scheduler uses the threadpool, in which case you should check ThreadPool.GetAvailableThreads and ThreadPool.GetMaxThreads to approximate the number of executing tasks.

There is no actual list of running tasks, even when you use the default TaskScheduler. The scheduler essentially assigns a task to a ThreadPool and leaves the actual execution to the pool (actually, it calls the Task.ExecuteEntry private method). It has no need to keep a list of the running tasks.

If you want the running task information for debugging purposes, you can take advantage of the Event Trace for Windows events in TPL. Unfortunately, the Task Started and Task Finished events are not documented. I found them while browsing the definition of Task.ExecuteEntry with dotPeek.

I've found one article exploring the TPL events but I'm not sure it's worth the trouble. Unless you are writing your own debugger, it seems too much trouble.

If you just have to get to the list of running tasks, perhaps you should write your own TaskScheduler and override TryExecute and TryExecuteInternal to intercept the execution of each task and place each task in a list. This could get expensive though, and you would have to do some cleanup periodically to remove completed tasks from the list without using continuations (which would end up in the list themselves).

Answer 3

When you create a Task, by default the task is scheduled to run on a thread pool thread. So, you can get number of running tasks by using ThreadPool.GetMaxThreads and ThreadPool.GetAvailableThreads methods.

    private static int GetWorkingThreads() {
        int maxThreads;
        int completionPortThreads;
        ThreadPool.GetMaxThreads(out maxThreads, out completionPortThreads);

        int availableThreads;
        ThreadPool.GetAvailableThreads(out availableThreads, out completionPortThreads);

        return maxThreads - availableThreads;
    }