C# – Draw on a form by a separate thread

cmultithreadingwinforms

I'm trying to build a multithreaded game where I have a separate thread for painting on the form which is not the main thread. this brings us to thread-safe technics which I've read many articls about, but I'm not really sure I got it correctly.

my problem is that I have a structure where every data object is painting it self on the form so I didn't figure out how to implement it.

this is a snippet of my working mono-thread code:

public partial class Form1 : Form
{
    GameEngine Engine;
    public Form1()
    {
        InitializeComponent();
        Engine = new GameEngine();
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        Engine.Draw(e.Graphics);
    }

}

class GameEngine
{

    Maze Map;
    List<Player> Players;

    public void Draw(Graphics graphics)
    {
            Map.Draw(graphics);
            foreach (var p in Players)
            {
                p.Draw(graphics);
            }
    }

}

so please can anyone give me a hint or a link to good article helping me to learn how to separate the drawing on an another thread?.

[Edit]

I managed to implement what I intended to do
and this is how I coded it

    protected override void OnPaint(PaintEventArgs e)
    {
        formGraphics = e.Graphics;
        DisplayThread = new Thread(new ThreadStart(Draw));
        DisplayThread.Start();
    }

    private void Draw()
    {
        if (this.InvokeRequired)
        {
            this.Invoke(new DrawDelegate(this.Draw));
        }
        else
        {
            Engine.Draw(formGraphics);
        }
    }

but I got an ArgumentException : Parameter is not valid

would you please point to the error in that code

Best Answer

I think you will need to draw to a Bitmap, then in the OnPaint Method, draw that bitmap to the window. I will demonstrate in a moment.

As Hans pointed out, in the OnPaint method you are setting 

formGraphics = e.Graphics;

but at the end of the method e.Graphics is disposed, so you can't use it anymore, if your code got to 

Engine.Draw(formGraphics);

you would get an exception.

So basically you need to have a global 

Bitmap buffer = new Bitmap(this.Width, this.Height)

in your asynced thread you would invoke your drawing to that Bitmap you can use 

Graphics g=Graphics.FromBitmap(buffer);//

To get a graphics object, but remember you have to 

g.Dispose()

it or wrap it in a 

using (Graphics g=Graphics.FromBitmap(buffer))
{
//do something here

}

I am going to play with it for a few moments and see if I can get you a working sample

EDIT Here's your working sample. I started a new form and tossed a button on it. I changed the mainform backgroundimagelayout to none.

I think you need to be using .net 4.0 or better, if not using this, let me know I can change it to match your version... I think.

//you need this line to use the tasks
using System.Threading.Tasks;

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    public void Draw()
    {
        Bitmap buffer;
        buffer = new Bitmap(this.Width, this.Height);
        //start an async task
        Task.Factory.StartNew( () =>
        {
                using (Graphics g =Graphics.FromImage(buffer))
                {
                    g.DrawRectangle(Pens.Red, 0, 0, 200, 400);
                    //do your drawing routines here
                }
            //invoke an action against the main thread to draw the buffer to the background image of the main form.
            this.Invoke( new Action(() =>
            {
                    this.BackgroundImage = buffer;
            }));
        });

    }

    private void button1_Click(object sender, EventArgs e)
    {
        //clicking this button starts the async draw method
        Draw();
    }

}

}

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