How do I make many backgroundworkers in the same foreach loop?

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I run the code below and only the last square in the list gets animated. I want all the squares animated, how?

What I have tried:

private void animateSquaresAfterConvertin(List<Point> lstOfConvertedNowPoints)
        {
            foreach (Point p in lstOfConvertedNowPoints)
            {
                animateThisSquarePoint(p);
            }
        }

        private void animateThisSquarePoint(Point p)
        {
            BackgroundWorker bgw = new BackgroundWorker();
            lstBackgroundWorkersAnimate.Add(bgw);
            bgw.DoWork += Bgw_DoWork;
            bgw.RunWorkerCompleted += Bgw_RunWorkerCompleted;
            bgw.RunWorkerAsync(p);
        }

        List<BackgroundWorker> lstBackgroundWorkersAnimate = new List<BackgroundWorker>();

        private void Bgw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            lstBackgroundWorkersAnimate.Remove(sender as BackgroundWorker);
            (sender as BackgroundWorker).Dispose();
        }



        Pen penForBGWAnimate = new Pen(Color.Purple);
        private void Bgw_DoWork(object sender, DoWorkEventArgs e)
        {
            Point p1 =(Point) e.Argument;

            g.DrawRectangle(penForBGWAnimate, p.X + S.Width * p1.X, p.Y + S.Height * p1.Y, S.Width, S.Height);

            System.Threading.Thread.Sleep(300);

            drawSquare(p1.X, p1.Y);
            Debug.WriteLine(p1.X.ToString()+p1.Y.ToString());
        }

Posted 21-May-21 16:16pm

john1990_1

Updated 23-May-21 16:25pm

v4

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Dave Kreskowiak · Accepted Answer · 2021-05-21T16:58:00

Solution 1

You're already doing it, but you're doing drawing all wrong.

If you're drawing on a control, you can NOT do it from a background thread. Your "g" is defined elsewhere so it's impossible to say.

If you're going to draw on something, draw on a Bitmap. You do not need a bunch of BackgroundWorkers to do this. You just go through in a loop and draw on your Bitmap, then when the drawing is done, you can draw the entire Bitmap onto whatever control surface you're using.

Also, DO NOT CREATE GDI OBJECTS IN A LOOP, like your Pen! You're are creating GDI object but never Disposing it, so you're wasting system resources and will eventually crash your app and possibly Windows. Create your Pen once, possibly at the class level, and you can reuse it over and over again without creating new ones.

You don't need any BackgroundWorkers.

Posted 21-May-21 16:58pm

Dave Kreskowiak

Updated 21-May-21 16:59pm

v2

Comments

George Swan 22-May-21 4:33am

Dave is correct in what he says. There is another problem with your code, it seems to me that your Bgw_DoWork method is used by multiple threads all trying to share the same resources at the same time. This results in chaos. The only reason that the last worker in the list succeeds is that there is not a subsequent worker to interfere with it. Asynchronous coding is problematical -it is best avoided if possible.

john1990_1 22-May-21 6:09am

I'm sorry, you got the code wrong, I'm running a: System.Threading.Thread.Sleep(300); and it would freeze my program and my program's window woldn't be able to move if it's not another task, I wouldn't have used BackGroundWorkers if I didn't need them, I will make the use of "Pen pen" for all in the same instance, would there be a problem of accessing "g" at the same time if the program is used for a long time? How to make many simultaneous async threads that animate the squares on board without having the window freeze for 300 ms?

Dave Kreskowiak 22-May-21 10:46am

You don't use multiple threads. You build a "game loop" using a timer. On every tick, you update your data, use that data to draw on a Bitmap, then invalidate the control you're going to draw on. In the drawing code for that control, you just paint the Bitmap onto the control surface.

john1990_1 22-May-21 10:52am

This works, I'm anyway interested in knowing if it's possible to make many BackGroundWorkers in a for loop as this would change my life... the way you described is a little resource inefficient and not very professional code, with RunWorkerComplete I can do some stuff and timers are going to be very problematic as RunWorkerComplete doesn't exist in timers, I want to change my C# life with learnig threads, how to make each thread stand by itself and not have only the last one alive?

Dave Kreskowiak 22-May-21 13:48pm

:LAUGH: OK.

You're already creating a BackgroundWorker. All you have to do is wrap that code in a for loop to create as many as you want.

Keep in mind, your CPU only has so many cores, so creating more BackgroundWorkers than that is just creating jobs that will sit and wait for a CPU core to become available. So, if you create 100 BackgroundWorkers, only, say, 4 to 8 of them are going to run are any one time.

But, Graphics objects are NOT THREAD SAFE. That means only one thread can use it at a time.

Oh, you think you're going to create a new Graphics object per-thread? Think again. You can only have a single Graphics object open on a bitmap or other control surface at a time.

These limitation are imposed by GDI+.

You were saying something about "resource inefficient" and "not very professional code"? I'm sorry to tell you that's how game engines really work in the real world. You might want to go Google for "game loop" and start reading.

You can do all kinds of other work, gathering and updating data, in threads all you want. All the drawing has to be done in a single thread.

john1990_1 22-May-21 14:10pm

I can make a point:

repeat:
try
{ g.DrawXYZ();}
catch
{ if(e.CancelPending) return; goto repeat;}

And in the very improbable possibility that more than one thread is gonna access the Graphics then repeat is gonna fire, I know that's not a professional code but I'm a hobbyist and this isn't very much inefficient, do you know of any video that teaches how to do this job easily and professionally? also I think the threads don't mean that if I have 500 threads open only 2 is gonna work in DualCore and 1 in SingleCore and 498-499 are gonna wait 5 seconds for the first 1-2 to finish if the first thread has Sleep(5000), the cores are gonna exchange work on the threads I believe.

I'm creating a foreach loop and my problem is that in my code only the last square in the foreach loop gets animated, and if I make a wait in the foreach loop of 2,000 ms, then the squares get animated one by one from beginning to end, my question is why in my loop when many are made and added to a global list, only one of them which is the last fires? the last bgw is somehow killing the one before it and that the one before it and on... and only the last fires, how to make all bgws in the foreach loop in my code fire into dowork() and RunWorkerCompleted().

Dave Kreskowiak 22-May-21 14:18pm

You're not understanding how threads work or even what I said. I did NOT say 498 threads would be waiting until the first 2 were complete.

I said ALL of them will be spending most of their time waiting for a core to become available to run them. If your machine only has 2 cores, it can only run 2 threads at a time, and they will only run for a very short amount of time before the scheduler puts them back in "wait" and picks two other threads to let run for a bit.

I already told you how "professional" code does this, but you apparently don't believe me so, Google for "C# simple game engine" and now you're on your own.

john1990_1 22-May-21 14:26pm

You don't realize how hard it would be to implement it with timers, like first I create and run a timer, and then I check which time the timer is ticking, and if it's the first time then draw, and then make "NumberOfTimesTimerTicked++" and a switch(NumberOfTimesTimerTicked) statement and make:

case 0: draw() timer1.Interval=300; timer1.Start();

case 1: hideDrawing(); timer1.Stop(); timer1.Dispose();

And also this way I may also access "g" for drawing more than once simultaneously and get an error.

Dave Kreskowiak 22-May-21 18:45pm

ONE TIMER to do it all. To check to see if something scheduled needs to run, you just check the current timer against however many milliseconds need to elapse to kick off some process.

john1990_1 22-May-21 18:50pm

Do you mean only one timer to keep ticking every 100ms and checking for signals in variables from my program to draw and hide some stuff to animate the squares?

Would you please provide me with an answer to the question with an example of how this can be done with a single timer because it seems very complicated to me and in terms of programming: very inefficient to type...

Dave Kreskowiak 22-May-21 19:03pm

Yes, or whatever interval you want to use.

Tell you what. You clean out my garage and I'll write your code.

john1990_1 23-May-21 19:45pm

So would you please tell me what to change in my code so all the DoWork and RunWorkerCompleted fire and not only of the last bgw? how to make multiple bgw in the same foreach loop without them all being overwritten by the last bgw?

Dave Kreskowiak 23-May-21 22:15pm

You're already doing it, and NO, you're BGW's are not "getting overwritten". They all exist in the collection. To see that, all you have to do is set a breakpoint on the curly brace after your loop, then hover the mouse over the collection variable to see them.

You can even put a Debug.WriteLine(Thread.CurrentThread.ManagedThreadId) in the DoWork method to see this.

john1990_1 23-May-21 22:24pm

Exception thrown: 'System.InvalidOperationException' in System.Drawing.dll

I'm getting one of the above for each additional square each time more than one Bgw_DoWork is fired in a foreach loop at the same time, in addition to only one coordinates (x,y) being printed in Debug.WriteLine as shown in my edited code.

Dave Kreskowiak 23-May-21 22:26pm

Remember what I said about Graphics objects being usable only by a single thread? Yeah, that's why you're code is crashing.

john1990_1 23-May-21 22:36pm

Thx, solution accepted.

john1990_1 23-May-21 22:44pm

Can I create a new g (Graphics) for each thread and use them at the same time?

Dave Kreskowiak 23-May-21 22:52pm

I already told you the answer to that. No.

You can only have a single Graphics object per target you want to draw on.

john1990_1 23-May-21 22:59pm

I really need that, should I emigrate from WinForms?

Dave Kreskowiak 23-May-21 23:01pm

Those limits are imposed by GDI+, not WinForms.

john1990_1 23-May-21 23:16pm

Why doesn't the program automatically dispose or leave for garbage collector by stopping the reference of:
{
Pen pen= new Pen();
}
after its scope ends? is it because it's a GDI? so GDI objects don't get disposed alone?

Dave Kreskowiak 23-May-21 23:27pm

They get Disposed when they go out of scope, but Disposal isn't deterministic. It can happen at any time after it goes out of scope. That may be a minute or two later!

john1990_1 22-May-21 6:18am

I edited the code in this question to fix the Pen issue.