Question

It seems like I am getting threading errors as a result of an inconsistent state of my IEnumerable. Specifically, List<T>.

Here’s my workflow:

List<string> IDs = getIDs();
RunTask(IDs);
RunTask(IDs);

public void RunTask(List<string> IDs)
{
    TaskScheduler scheduler = new TaskScheduler();
    TaskFactory factory = new TaskFactory(scheduler);
    var tasks = IDs.Select(name => factory.StartNew(() => action(name))).ToArray();
    Task.WaitAll(tasks);
}

I am getting a generic error on my WaitAll() call.

My thinking is that I am getting some sort of unsynchronized IEnumerator, and that I need to make a copy of my List via Array.CopyTo(), then pass it into my multiple threads.

This is my exception:

Error: One or more errors occurred.
One or more errors occurred.
at System.Threading.Tasks.Task.WaitAll(Task[] tasks, Int32 millisecondsTimeout, CancellationToken cancellationToken)
at System.Threading.Tasks.Task.WaitAll(Task[] tasks, Int32 millisecondsTimeout)
at System.Threading.Tasks.Task.WaitAll(Task[] tasks)

Any thoughts on this matter?

Answer 1

I don’t think this is a threading error – it should be safe as long as each task does not modify the IDs list. Task.WaitAll() throws an AggregateException if any of the waited tasks throw an exception or are cancelled.

You can access the InnerExceptions on the exception to see what the error(s) are.

try
{
    Task.WaitAll(tasks);
}
catch(AggregateException ex)
{
    foreach(Exception in ex.InnerExceptions) { ... }
}