Software Development

Task status state changes

Over the course of the last couple of months I’ve blogged a lot about the Task Parallel Library and mentioned a number of statuses that a task can have, but nowhere is a nice handy-dandy chart to show what those statuses are and how the transition from one to another. The green status show the normal line that most tasks will take. The purple and red show alternative paths that may be taken.

When a task is first created it status is Created – however you will almost never see this. This status only exists when you create a task using its constructor. Most of the time you would create tasks with Task.Factory.StartNew(…)

When you start a task the status will be WaitingToRun until such time as the task scheduler can actually run the task. For some tasks the transition will be almost instant, for others it will mean a wait while other tasks complete.

From WaitingToRun a task can transition to either Running or Cancelled. The latter transition happens if the cancellation token is signalled before the task gets a chance to start.

Once a task is running there are three possible exits for it. The normal exit is for the status to transition to RanToCompletion which means that the task completed without incident. The other two exits require exception handling in the calling code.

If an error happens that the task cannot (or does not) handle internally then it will transition to Faulted (see Tasks that throw exceptions). The AggregateException will contain details of all the exceptions in Faulted tasks as part of its InnerExceptions collection.

If the tasks are cancelled, any task that has not already completed will transition immediately to Cancelled. An AggregateException will contain details of all tasks that were cancelled (whether they had a chance to run or not) with TaskCanceledException inner exceptions. The exception to this is any task that was running already when the cancellation token was signalled and either ran to completion normally, or did not respond correctly to the cancellation request (see Cancelling parallel tasks).

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