在面板中绘制圆圈

下面是一个Windows窗体的简单演示。

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
// Definition for our ellipse object.
class Ellipse
{
    public int PenWidth;
    public Color Color;
    public Rectangle Rectangle;
    // Paint ourselves with the specified Graphics object
    public void Draw(Graphics graphics)
    {
        using (Pen pen = new Pen(Color, PenWidth))
            graphics.DrawEllipse(pen, Rectangle);
    }
}
class Form1 : Form
{
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1());
    }
    public Form1()
    {
        // Remove "flickering" from the repainting.
        SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.Opaque | ControlStyles.OptimizedDoubleBuffer | ControlStyles.ResizeRedraw | ControlStyles.UserPaint, true);
    }
    // Store all created ellipses so they can be rendered during the Paint event.
    List<Ellipse> ellipses = new List<Ellipse>();
    // Definition of an Ellipse under construction
    class EllipseConstruction
    {
        public Point Origin;
        public Ellipse Ellipse;
    }
    // Storage for ellipse under construction.
    EllipseConstruction ellipseConstruction;
    // Random number generator Ellipse creation.
    private Random Rand = new Random();
    // These are the possible ellipse colors
    static readonly Color[] EllipseColors =
    {
        Color.Black,
        Color.White,
        Color.Red,
        Color.Green,
        Color.Blue,
        Color.Yellow,
        Color.Magenta,
        Color.Cyan,
    };
    protected override void OnMouseDown(MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            // Capture mouse until button up.
            Capture = true;
            // Create a new Ellipse object and record the [X, Y] origin of this click
            ellipseConstruction = new EllipseConstruction
            {
                Origin = e.Location,
                Ellipse = new Ellipse { Color = EllipseColors[Rand.Next(EllipseColors.Length)], PenWidth = Rand.Next(1, 6) },
            };
        }
        base.OnMouseDown(e);
    }
    protected override void OnMouseMove(MouseEventArgs e)
    {
        // If the mouse is captured, the user is creating a new Ellipse so we update its rectangle with the mouse coordinates
        if (Capture)
            UpdateEllipseUnderConstruction(e.Location);
        base.OnMouseMove(e);
    }
    protected override void OnMouseUp(MouseEventArgs e)
    {
        if (Capture && e.Button == MouseButtons.Left)
        {
            // If the mouse is captured and it's the Left button being released, the user is
            //   done creating a new Ellipse.
            // Stop capturing the mouse.
            Capture = false;
            // Final update of the Ellipse under construction
            UpdateEllipseUnderConstruction(e.Location);
            // Add the new Ellipse to our list unless its width or height are zero which would result in a non-visible ellipse
            if (ellipseConstruction.Ellipse.Rectangle.Width > 0 && ellipseConstruction.Ellipse.Rectangle.Height > 0)
                ellipses.Add(ellipseConstruction.Ellipse);
            // Since we are done constructing a new Ellipse, we don't need the construction object
            ellipseConstruction = null;
        }
        base.OnMouseUp(e);
    }
    protected override void OnKeyDown(KeyEventArgs e)
    {
        // Allow Ellipse creation to be cancelled with the Escape key
        if (Capture && e.KeyData == Keys.Escape)
        {
            Capture = false; // End mouse capture
            ellipseConstruction = null; // Remove construction ellipse
            Invalidate(); // Notify operating system that we need to be repainted.
        }
        base.OnKeyDown(e);
    }
    private void UpdateEllipseUnderConstruction(Point point)
    {
        // Calculate new ellipse rectangle based on ellipseConstruction.Origin and point.
        Point origin = ellipseConstruction.Origin;
        int xRadius = Math.Abs(origin.X - point.X);
        int yRadius = Math.Abs(origin.Y - point.Y);
        // Make X and Y radii the same for a true circle unless the Shift key is held down
        if ((ModifierKeys & Keys.Shift) == 0)
            xRadius = yRadius = Math.Max(xRadius, yRadius);
        ellipseConstruction.Ellipse.Rectangle = new Rectangle(origin.X - xRadius, origin.Y - yRadius, xRadius * 2, yRadius * 2);
        Invalidate(); // Notify operating system that we need to be repainted.
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        // Paint the background since we specified ControlStyles.AllPaintingInWmPaint and ControlStyles.Opaque.
        e.Graphics.Clear(Color.SlateGray);
        // Paint the ellipses we have stored.
        foreach (Ellipse ellipse in ellipses)
            ellipse.Draw(e.Graphics);
        // If the user is creating a new ellipse paint it.
        if (ellipseConstruction != null)
            ellipseConstruction.Ellipse.Draw(e.Graphics);
        base.OnPaint(e);
    }
}

1. 在MouseDown事件中，捕捉圆圈的中心位置(或边界框的顶角，根据偏好)，以及正在绘制圆圈的事实。
2. 在MouseMove事件中，如果正在绘制圆圈，则绘制圆圈，使用当前鼠标位置作为圆圈边缘上的点(或根据偏好选择边框的对角)
3. 在MouseUp事件中，捕获圆的半径和它不再被绘制的事实。将新创建的圆圈存储在圆圈集合中，并将其呈现在屏幕上。

在一些较旧的技术中，您必须在步骤2的MouseMove事件中擦除并重新绘制圆圈。如果您正在使用WPF或类似的高级工具，您可以在步骤1中创建一个圆形对象并将其添加到画布上，然后在步骤2中操作其属性。WPF引擎将负责从旧位置擦除圆圈并在新位置绘制圆圈。

您可以使用图形在所有winforms控件上绘制。例如:

private void button1_Click(object sender, EventArgs e)
{
  System.Drawing.Graphics g = System.Drawing.Graphics.FromHwnd(panel1.Handle);
  g.DrawEllipse(Pens.Green, panel1.ClientRectangle);
}

但是在这种情况下，如果重新绘制形式，您的绘画将消失。你需要在OnPaint功能中做。如果想强制重绘表单，只需调用表单的Invalidate方法:

this.Invalidate();

使用图形。DrawEllipse方法:

  // Create pen.
    Pen blackPen = new Pen(Color.Black, 3);
    // Create rectangle for circle.
    Rectangle rect = new Rectangle(0, 0, 100, 100);
    // Draw circle.
    e.Graphics.DrawEllipse(blackPen, rect);

从你想要绘制的表面获取一个图形对象

您可以使用面板的Paint事件处理程序中的Graphics类在面板上绘制。

在CodeProject上看看这篇文章，网上有很多。