C＃winforms使用集合绘制到图片框的问题？

Question

The form contains a picturebox (picturebox1) and a timer control (timer1)...

Basically at start up I create a List collection containing 5 instances of my pixel class. When the timer triggers I call a refresh of the picturebox which calls the paint event. In the paint event I iterate through the list collection and call each pixel's draw method.

The problem I am having is that only one pixel shows up...that is.. unless I set a break at the point where I am adding the pixel, continue, and break again and repeat until all pthe pixels are created. Then for some reason all of the pixels show up...

Can someone tell me why I can only see a single pixel?

public partial class Form1 : Form
{
    List<Pixel> pixels = new List<Pixel>();

    public Form1()
    {            
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        for (int ii = 0; ii < 5; ii++)
            pixels.Add(new Pixel(pictureBox1));  // <- breakpoint here...?
    }

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        foreach (Pixel p in pixels)
            p.Draw(e, pictureBox1);
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        pictureBox1.Refresh();
    }
}

public class Pixel
{
    Random r = new Random(DateTime.Now.Millisecond);

    public Point Position { get; set; }

    public Pixel(PictureBox src) 
    {
        Position = new Point(r.Next(0, src.Width), r.Next(0, src.Height));
    }

    public void Draw(PaintEventArgs e, PictureBox src)
    {
        e.Graphics.DrawRectangle(new Pen(Color.Black), Position.X, Position.Y, 1, 1);
    }
}

My original code does a lot more.. but I stripped it all out and got the same results.

Answer 1

Since Random r = new Random(DateTime.Now.Millisecond) was called each time with the same seed value the pixels showed up in exactly the same spot. I moved the Random declaration to the main class and passed it into the pixel class. Now it works as I expect.

public partial class Form1 : Form
{
    Random r = new Random(DateTime.Now.Millisecond);
    List<Pixel> pixels = new List<Pixel>();

    public Form1()
    {            
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        for (int ii = 0; ii < 5; ii++)
        {
            pixels.Add(new Pixel(pictureBox1, r));
        }
    }

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        foreach (Pixel p in pixels)
            p.Draw(e, pictureBox1);
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        pictureBox1.Refresh();
    }
}

public class Pixel
{
    public Point Position { get; set; }

    public Pixel(PictureBox src, Random r) 
    {
        Position = new Point(r.Next(0, src.Width), r.Next(0, src.Height));
    }

    public void Draw(PaintEventArgs e, PictureBox src)
    {
        e.Graphics.DrawRectangle(new Pen(Color.Black), Position.X, Position.Y, 1, 1);
    }
}

Answer 2

This is because your randomizer on all 5 instances of your pixel class is using the exact same seed (it is all happening on the same millisecond value). Not to condone the use of Thread.Sleep, the quickest "fix" to at least expose the problem would be to change your form_load event to look like the following (you could also refactor how you're randomizing):

    private void Form1_Load(object sender, EventArgs e)
    {
        for (int ii = 0; ii < 5; ii++)
        {
            Thread.Sleep(1);
            pixels.Add(new Pixel(pictureBox1));
        }
    }