Question

I helped create a background task system for an ASP.NET web site.

This is my root Task

Task.Factory.RunNew(RunTimer);

This is called from the root Task.

private void RunTimer() 
{ 
    while (!cancellationToken.IsCancellationRequested) 
    { 
        var backgroundTasks = _tasks.Values.ToArray(); 
        var tplTasks = new List<Task>(); 
        foreach (var backgroundTask in backgroundTasks) 
        { 
            var newTask = new Task(() => backgroundTask.Run()); 
            tplTasks.Add(newTask); 
            newTask.Start(); 
        } 
        Task.WaitAll(tplTasks.ToArray()); 
        for (int i = 0; i < NumberOfSecondsToWait && 
            !cancellationToken.IsCancellationRequested; i++)
        { 
            Thread.Sleep(new TimeSpan(0, 0, 1)); 
        } 
    } 
}

_tasks is a ConcurrentDictionary<string, IBackgroundTask>. For what ever reason, newTask is executed 2 times on separate threads — namely backgroundTask.Run() is called twice. RunTimer is only called once. NumberOfSecondsToWait is 60. I’ve verified that tplTasks only has 2 items in it.

Anyone have any idea?

Answer 1

This is because lambdas (in particular, the newTask lambda) bind to variables, not values.

You need:

...
foreach (var backgroundTask in backgroundTasks)
{
  var localBackgroundTask = backgroundTask;
  var newTask = new Task(() => localBackgroundTask.Run());
  ...
}
...