Is there a better way than this to draw (sometimes) simultaneously on the form?

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I want to draw on a form from different threads with the same graphics object (sometimes) at the exact same time, for example to draw a fire pixels near the mouse when it moves on my form, OnPaint event gets called rarely and can't animate something like that, is there a better way than the one in the code below?

What I have tried:

public FormThreadsDrawing()
        {
            InitializeComponent();

            System.Windows.Forms.Timer t = new System.Windows.Forms.Timer();
            t.Interval = 10;
            t.Tick += T_Tick;
            t.Start();
        }

        List<Thread> lt = new List<Thread>();

        private void T_Tick(object sender, EventArgs e)
        {
            while (lt.Count > 0)
            {
                lt[0].Start();
                lt[0].Join();
                lt.RemoveAt(0);
            }
        }

        Graphics g;

        private void FormThreadsDrawing_Shown(object sender, EventArgs e)
        {
            g = CreateGraphics();

            lt.Add(new Thread(() =>
            {
                Pen p = new Pen(Color.Red);
                g.DrawLine(p, new Point(100, 60), new Point(200, 50));
                p.Dispose();
            }
            ));

            lt.Add(new Thread(() =>
            {
                using (Pen p = new Pen(Color.Blue))
                {
                    g.DrawLine(p, new Point(200, 600), new Point(200, 50));
                }
            }
            ));
        }

Posted 27-May-21 11:45am

john1990_1

Updated 28-May-21 15:10pm

v6

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[no name] 28-May-21 1:03am

https://docs.microsoft.com/en-us/dotnet/api/system.windows.forms.control.mousemove?view=net-5.0

john1990_1 28-May-21 1:07am

Thx but I know about the mouse move and how to make the fire pixels initially, but I want a way to draw them all each in a different thread and an instance of a method at the same time and animate it down and to the sides randomly (each fire pixel) by itself, what I mean is the drawing with a "Graphics g;" at the same time, is there a better way to do what is in my code that I have supplied to this question?

Richard MacCutchan 28-May-21 3:48am

As I explained yesterday, you cannot do it this way.

john1990_1 28-May-21 12:42pm

Why? maybe I made it.
I edited the code in the question.

Richard MacCutchan 28-May-21 12:49pm

Good luck.

Dave Kreskowiak 28-May-21 13:00pm

You've already been told why and repeatedly ignored what you've been told.

That's why.

john1990_1 28-May-21 13:04pm

I apologize, I learned C# by trial and error and I'm not a professional, and my code works in drawing on the form random fire pixels at the same time and make them move on the form in a recursive method, my code was tested and works, would you please explain like I'm 5 years old why my code is bad?

Dave Kreskowiak 28-May-21 13:15pm

Because the Graphics class is not "thread safe." That means, while it may work most of the time, you WILL run into issues where things go wrong and you will never be able to figure out why.

Oh, and your code leaks resources like crazy and will eventually crash Windows itself.

john1990_1 28-May-21 13:23pm

You mean RAM? where does it not dispose itself and how to make it dispose itself?

Another question: Why doesn't Visual Studio free up resources when their scope ends when not in use (like after leaving a curly brackets) with the garbage collector in a maximum of 2 mins (as the garbage collector fires)?

Dave Kreskowiak 28-May-21 13:40pm

No, I means resource, like handles.

Visual Studio doesn't do anything like that for you. It doesn't run your code. It just lets you edit it and compiles it.

GDI objects all use handles to exist, and there's a limited pool of handles. The handle pool exists system-wide. If you're constantly creating new ones and not freeing up the handles you've used, you will exhaust the handle pool.

The Pens and Graphics objects you're creating and not Disposing are burning through the handle pool. Your code can waste handles faster than the garbage collector can get around to Disposing the objects you're creating.

john1990_1 28-May-21 13:58pm

What can I do about that?

Dave Kreskowiak 28-May-21 14:26pm

I already told you the last time we went round about this, but you, the newb, told me the code would be inefficient.

So here we are.

john1990_1 28-May-21 14:28pm

You told me to make *one* timer and run all actions in it, that's what I did, now I have to create instances of:
new pen()
How to dispose of them? and why doesn't VS dispose of them by itself? why is GC inefficient?

Dave Kreskowiak 28-May-21 17:48pm

"Inefficient" because it doesn't protect you from yourself?

The GC only knows about the allocation of managed objects. It does not know about the underlying unmanaged resources an object may use.

Dave Kreskowiak 28-May-21 17:51pm

...and I told you a lot more than what you just posted.

Remember the game loop? Input, Calculate, Draw, repeat ad nauseum.

john1990_1 28-May-21 20:45pm

Sorry, I don't understand what you mean by "game loop".

Dave Kreskowiak 28-May-21 21:10pm

Of course not. Because you didn't Google for "C# game loop".

john1990_1 28-May-21 14:54pm

I edited the code, is it good now?

Dave Kreskowiak 28-May-21 17:49pm

No. You're still using threads for drawing, which won't work and your code will still leak resources and cause crashes.

john1990_1 28-May-21 19:18pm

The drawing happens only in:

Timer_Tick()

Adding to a list of threads and then running the threads from Timer_Tick() and joining each thread with the main thread till it finishes, would you please tell me what I should do? I need to draw pixel fires like this:

int times=0;
DrawPixelFire(int x, y)
{
g.DrawPixel(x,y,Color.Orange);

Sleep(100);

times++;
if(times==100) return;
DrawPixelFire(x - 5 + Random.Next(0,11),
y - 5 + Random.Next(0,11));
}

And the code above must run without the program being frozen and nonresponsive so this must be in a background thread.

And I must use drawing at the same time, would you guys try and help me instead of just rebuking me? I'm a hobbyist and don't work for money by the way.

[no name] 28-May-21 14:13pm

You can have multiple threads create "draw requests" that get added to a concurrent queue that a "single" UI thread processes. The "creating" threads can be on a timer to simulate a particular frame rate (FPS).

john1990_1 28-May-21 14:32pm

Please see other comments.

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OriginalGriff · Answer 1 · 2021-05-28T02:58:00

You cannot draw on or otherwise access controls from any thread other than the UI thread: the one that is created when your app starts.

The graphics object and it's methods are not thread safe, and you should never try to use them in background threads.

I don't know why you think this will be a good idea, but you cannot do it.

Dave Kreskowiak · Answer 2 · 2021-05-28T15:10:00

Elephanting hell... You just won't listen.

Here. This creates a control called "LinesCanvas" you can drop on a Form. It demonstrates everything I've been telling you. There's 2 methods, you can call, Start and Stop.

C#

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;

namespace CsLines
{
    public class LinesCanvas : Panel
    {
        Random RNG;
        int lineCount = 100;
        int stepLimit = 10;

        Timer _timer;
        List<Line> _lines;
        Bitmap _canvas;
        Pen _linePen;

        public LinesCanvas()
        {
            this.DoubleBuffered = true;
            SetupInternals();
        }

        public void Start()
        {
            SetupLines(lineCount);
            _timer.Enabled = true;
        }

        public void Stop()
        {
            _timer.Enabled = false;
        }

        protected void SetupInternals()
        {
            RNG = new Random();

            _linePen = new Pen(Color.LightBlue);

            _timer = new Timer();
            _timer.Enabled = false;
            _timer.Interval = 10;
            _timer.Tick += _timer_Tick;
        }

        protected override void OnResize(EventArgs eventargs)
        {
            base.OnResize(eventargs);

            UpdateCanvasSize();
        }

        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);

            if (_canvas != null)
                e.Graphics.DrawImageUnscaled(_canvas, 0, 0);
        }

        private void _timer_Tick(object sender, EventArgs e)
        {
            UpdateLinePositions();
            DrawLinesOnCanvas();
            this.Invalidate();
        }

        protected void UpdateLinePositions()
        {
            foreach (Line line in _lines)
            {
                line.UpdatePosition(ClientSize);
            }
        }

        protected void DrawLinesOnCanvas()
        {
            if (_canvas != null)
            {
                using (Graphics g = Graphics.FromImage(_canvas))
                {
                    g.Clear(Color.Black);

                    foreach (Line line in _lines)
                    {
                        g.DrawLine(_linePen, line.EndPoint1, line.EndPoint2);
                    }
                }
            }
        }

        protected void UpdateCanvasSize()
        {
            if (_canvas != null)
                _canvas.Dispose();

            _canvas = new Bitmap(ClientSize.Width, ClientSize.Height, System.Drawing.Imaging.PixelFormat.Format32bppRgb);
        }

        protected void SetupLines(int count)
        {
            _lines = new List<Line>(lineCount);

            for (int i = 0; i < count; ++i)
            {
                Line newLine = new Line(
                    RNG.Next(ClientSize.Width),
                    RNG.Next(ClientSize.Height),
                    RNG.Next(ClientSize.Width),
                    RNG.Next(ClientSize.Height),
                    RNG.Next(stepLimit) - 5,
                    RNG.Next(stepLimit) - 5,
                    RNG.Next(stepLimit) - 5,
                    RNG.Next(stepLimit) - 5
                );

                _lines.Add(newLine);
            }
        }

        protected override void Dispose(bool disposing)
        {
            _linePen.Dispose();

            base.Dispose(disposing);
        }
    }
}

Another class this needs, called "Line":

C#

using System.Drawing;

namespace CsLines
{
    public class Line
    {
        public Point EndPoint1;
        public Point EndPoint2;

        public Point Velocity1;
        public Point Velocity2;

        public Line(int x1, int y1, int x2, int y2, int vx1, int vy1, int vx2, int vy2)
        {
            EndPoint1 = new Point(x1, y1);
            EndPoint2 = new Point(x2, y2);

            Velocity1 = new Point(vx1, vy1);
            Velocity2 = new Point(vx2, vy2);
        }

        public void UpdatePosition(Size clientSize)
        {
            EndPoint1.X += Velocity1.X;
            EndPoint1.Y += Velocity1.Y;

            EndPoint2.X += Velocity2.X;
            EndPoint2.Y += Velocity2.Y;

            if (EndPoint1.X < 0)
            {
                EndPoint1.X = 0;
                Velocity1.X = -Velocity1.X;
            }

            if (EndPoint1.X > clientSize.Width)
            {
                EndPoint1.X = clientSize.Width;
                Velocity1.X = -Velocity1.X;
            }



            if (EndPoint1.Y < 0)
            {
                EndPoint1.Y = 0;
                Velocity1.Y = -Velocity1.Y;
            }

            if (EndPoint1.Y > clientSize.Height)
            {
                EndPoint1.Y = clientSize.Height;
                Velocity1.Y = -Velocity1.Y;
            }




            if (EndPoint2.X < 0)
            {
                EndPoint2.X = 0;
                Velocity2.X = -Velocity2.X;
            }

            if (EndPoint2.X > clientSize.Width)
            {
                EndPoint2.X = clientSize.Width;
                Velocity2.X = -Velocity2.X;
            }



            if (EndPoint2.Y < 0)
            {
                EndPoint2.Y = 0;
                Velocity2.Y = -Velocity2.Y;
            }

            if (EndPoint2.Y > clientSize.Height)
            {
                EndPoint2.Y = clientSize.Height;
                Velocity2.Y = -Velocity2.Y;
            }

        }
    }
}

Oh, and not a single goddamn thread anywhere.

Is there a better way than this to draw (sometimes) simultaneously on the form?

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