Why does my delegate only use the last item from my foreach loop?

问题

Scenario: I am building a scheduling system and each timer event I wanted to run a custom method instead of the usual Timer.Elapsed event.

So I wrote something like this.

foreach (ScheduleElement schedule in schedules) {
    TimeSpan timeToRun = CalculateTime(schedule);
    schedule.Timer = new Timer(timeToRun.TotalMilliseconds);
    schedule.Timer.Elapsed += delegate { Refresh_Timer(schedule); };
    schedule.Timer.AutoReset = true;
    schedule.Timer.Enabled = true;
}

Ok so simple enough that actually did create my timers. However, I wanted each elapsed event to run using the schedule element that it passed in. My question is, why does the Elapsed event only pass in the last ScheduleElement in the for loop for every single Timer.Elapsed event.

Now I know what fixes it, I am just not sure why. If I roll back to the original Timer.Elapsed event and extend the Timer class with my own class I can work around it. Like so.

The Fix:

foreach (ScheduleElement schedule in schedules) {
    TimeSpan timeToRun = CalculateTime(schedule);
    schedule.Timer = new TimerEx(timeToRun.TotalMilliseconds);
    schedule.Timer.Elapsed +=new System.Timers.ElapsedEventHandler(Refresh_Timer);
    schedule.Timer.Tag = schedule;
    schedule.Timer.AutoReset = true;
    schedule.Timer.Enabled = true;
}

I then cast the object sender back into its original object, and thieve the Tag property off of it which gives me my correct schedule for each unique timer.

So again, why does using a delegate { } only pass in the last ScheduleElement in the foreach loop for all Timers?

EDIT 1

The Timer class

public TimerEx : Timer {

    public TimerEx(double interval) : base(interval) { }

    private Object _Tag;

    public Object Tag {
        get { return _Tag; }
        set { _Tag = value; }
    }
}

回答1:

This is because you're using closure in your delegate, and it closes over the same variable, which is shared for each iteration of the foreach loop.

For details, see Eric Lippert's article Closing over the loop variable considered harmful.

In this case, you can easily fix it with a temporary:

foreach (ScheduleElement schedule in schedules) {
    TimeSpan timeToRun = CalculateTime(schedule);
    schedule.Timer = new Timer(timeToRun.TotalMilliseconds);

    // Make a temporary variable in the proper scope, and close over it instead
    var temp = schedule;
    schedule.Timer.Elapsed += delegate { Refresh_Timer(temp); };

Note that C# 5 changes this behavior for foreach loops. If you compile this with the latest compilers, the issue no longer exists.

来源：https://stackoverflow.com/questions/17688790/why-does-my-delegate-only-use-the-last-item-from-my-foreach-loop

标签

timer

delegates