在Windows.Forms中绘图

Question

I've got a simple Windows.Forms Form.

I want to fill the pictureBox1 with color Color.Aqua and draw a rectangle.

However nothing is drown untill I move the Form.

Why is this?

How can I force everything to be drawn without moving the Form?

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

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private readonly Graphics _graphics;
        private List<PointF> _points;
        private Bitmap _bitmap;

        public Form1()
        {
            InitializeComponent();
            _bitmap = new Bitmap(1000, 600);
            _graphics = Graphics.FromImage(_bitmap);
            pictureBox1.Image = _bitmap;

            var timer = new Timer
            {
                Interval = 1
            };
            timer.Tick += OnTick;
            timer.Start();
            Invalidate();
        }

        private void OnTick(object sender, EventArgs e)
        {
            _graphics.Clear(Color.Aqua);
           _graphics.DrawRectangle(Pens.Black, 10, 10, 10, 10);
            Invalidate();
        }
    }
}

Answer 1

You must subscribe to Paint event of your picturebox and put your drawing code there,something like this:

public Form1()
{
    InitializeComponent();
    pictureBox1.Paint += PictureBox1_Paint;
}

private void PictureBox1_Paint(object sender, PaintEventArgs e)
{
    e.Graphics.Clear(Color.Aqua);
    e.Graphics.DrawRectangle(Pens.Black, 10, 10, 10, 10);
}

This event is raised everytime the control needs to be redrawn, so you don't need that Timer trick or calling Invalidate

Answer 2

I see four problems:

1. The Invalidate() method is called for the form. This should invalidate the PictureBox, but you can do better by just invalidating the PictureBox directly.
2. You are drawing the bitmap, but not updating the Image property of the PictureBox.
3. One millisecond intervals will kill you. 50 to 100 is much more reasonable, and anything less than 17 is probably faster than the refresh rate of your monitor.
4. The whole the thing with the separate graphics is extra and not needed. The pictureBox has it's own graphics context, and you do better using that.

Put it all together, and you get this:

public partial class Form1 : Form
{
    private Timer _timer;
    private List<PointF> _points;

    public Form1()
    {
        InitializeComponent();

        _timer = new Timer(100);
        timer.Tick += OnTick;
        timer.Start();
    }

    private void OnTick(object sender, EventArgs e)
    {
        pictureBox1.Invalidate();
    }

    private void PictureBox1_Paint(object sender, PaintEventArgs e)
    {
        e.Graphics.Clear(Color.Aqua);
        e.Graphics.DrawRectangle(Pens.Black, 10, 10, 10, 10);
    }
}

Answer 3

You don't need the Invalidate() call at all, since you're drawing to a buffer ( Bitmap ). Just set the bitmap to pictureBox1.Image property:

    private void OnTick(object sender, EventArgs e)
    {
        _graphics.Clear(Color.Aqua);
        _graphics.DrawRectangle(Pens.Black, 10, 10, 10, 10);
        pictureBox1.Image = _bitmap;
    }