Why isn't my application drawing anything?

Question

I'm very new(read 3 weeks exp) to C#(programming in general),started with html/css and javascript and now on my way with C#.

I'm trying to make my own simple 'Paint' application in windows form. But i've encountered an issue and just cant wrap my head around it, doesnt matter how much i read or follow other mans code, i'm stuck. The following code works fine but when resizing the application window the drawing dissappears.

As a solution ive read that declaring the Graphics method within the panel1_Paint event this should be resolved And here is my issue. See last code sample, ive come up with this(yes like i said, im new to this)and its not drawing anything.

ive simply tried to recreate the first example under the panel1_Paint event but i guess something went wrong during the mouseMove event and i cant figure out what it is.

Could someone explain to me what i am missing here, that would be very appreciated. thanks in advance.

[Old code]

namespace Painter
{
public partial class Form1 : Form
{
    Graphics graphics;
    Pen pen = new Pen(Color.Black, 1);
    Point startingPoint = new Point(0, 0);
    Point endPoint = new Point(0, 0);
    bool mousePaint = false;



    public Form1()
    {
        InitializeComponent();
        this.DoubleBuffered = true;

    }

    private void panel1_MouseDown(object sender, MouseEventArgs e)
    {
        startingPoint = e.Location;
        if (e.Button == MouseButtons.Left)
        {
            mousePaint = true;
        }
    }

    private void panel1_MouseMove(object sender, MouseEventArgs e)
    {
        if(mousePaint == true)
        {
            endPoint = e.Location;
            graphics = panel1.CreateGraphics();
            graphics.DrawLine(pen, startingPoint, endPoint);
        }
        startingPoint = endPoint;
    }

    private void panel1_MouseUp(object sender, MouseEventArgs e)
    {
        mousePaint = false;
    }


  }
}

[New Code]

namespace Painter
{
public partial class Form1 : Form
{
    Pen pen = new Pen(Color.Black, 1);
    Point startingPoint = new Point(0, 0);
    Point endPoint = new Point(0, 0);
    bool mousePaint = false;



    public Form1()
    {
        InitializeComponent();
        this.DoubleBuffered = true;

    }

    private void panel1_Paint(object sender, PaintEventArgs e)
    {
        Graphics graphics = panel1.CreateGraphics();

        if (mousePaint == true)
        {
            graphics.DrawLine(pen, startingPoint, endPoint);
        }
    }


    private void panel1_MouseDown(object sender, MouseEventArgs e)
    {
        startingPoint = e.Location;
        if (e.Button == MouseButtons.Left)
        {
            mousePaint = true;
        }
    }

    private void panel1_MouseMove(object sender, MouseEventArgs e)
    {
        if(mousePaint == true)
        {
            endPoint = e.Location;
        }
        startingPoint = endPoint;
    }

    private void panel1_MouseUp(object sender, MouseEventArgs e)
    {
        mousePaint = false;
    }


   }
 }

Answer 1

The following code works fine but when resizing the application window the drawing dissappears.

This happens because resizing the application window invalidates portion of your panel which causes the portion to be redrawn.

Reason why your second approach is not working (the one labelled as [NEW CODE]) is because the Paint event is called only when relevant component is redrawn. You could partially solve this by forcing redraw of the panel in your MouseDown/MouseMove event handlers but you would still lose your previously painted stuff.

Possible solution is to create instance of Bitmap and paint there. Then just set this Bitmap as BackgroundImage of the panel. You can find more information on that here. Of course you would need to think about stuff like resizing and what should happen to the bitmap if application window gets shrunk or enlarged.

Here is some code that I quickly put together to get you started:

namespace WinForms_PaintTest
{
    public partial class Form1 : Form
    {
        private Pen pen;
        private Bitmap bitmap;

        public Form1()
        {
            InitializeComponent();
            this.pen = new Pen(Color.Black, 1);
            this.bitmap = new Bitmap(this.panel1.Width, this.panel1.Height);
            this.panel1.BackgroundImage = this.bitmap;
        }

        private void panel1_MouseMove(Object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                using (Graphics g = Graphics.FromImage(this.bitmap))
                {
                    g.DrawRectangle(this.pen, e.Location.X, e.Location.Y, 1, 1);
                }
                this.panel1.Refresh();
            }
        }

        private void Form1_FormClosed(Object sender, FormClosedEventArgs e)
        {
            this.pen.Dispose();
            this.bitmap.Dispose();
        }
    }
}

Also regarding this:

this.DoubleBuffered = true;

I believe your intention was to prevent the flickering when relevant control is being redrawn? If that is case you need to set this property against the panel and not against the form itself. It is little bit tricky though because DoubleBuffered property of the panel is protected so you will need to either inherit from the panel or resort to reflection. You can find more information here .

Answer 2

private void panel1_Paint(object sender, PaintEventArgs e)
{
    Graphics graphics = panel1.CreateGraphics();

This is nonsense! Always and only use the e.Graphics object from the Paint param!!

Also: To trigger the Paint event do a panel1.Invalidate(); whenever your drawing data have changed!

Also: Make sure you understand just what your mousePaint flag is supposed to control: the mouse painting (i.e. adding new shapes to draw) or the regular painting (i. all shape previously drawn)!? Note that all drawing, current and previous needs to be done from the Paint event, whenever necessary i.e. over and over again!

To be able to do so: Collect all the shpes' data in a List<T>..

To Doublebuffer a Panel you need to subclass it. Your code turns on DoubleBuffering for the Form, which fine but won't help the Panel..

Instead simply use a PictureBox, which is control meant for drawing on!

A DoubleBuffered Panel subclass is as simple as this:

class DrawPanel : Panel 
{
   public DrawPanel()
    {
        DoubleBuffered = true;
    }
}

---

Update: Instead you can also use a Label (with Autosize=false); it also has the DoubleBuffered property turned on out of the box and supports drawing better than Panels do.