C#的System.Drawing命名空间提供了对 GDI+ 基本图形功能的访问
重点在于获取Graphics对象，例如：
Graphicsg =panel1.CreateGraphics
事实上CreateGraphics继承自Control， 即基本每一种控件都有这个方法
Control.CreateGraphics
在pannel、form上画图都一样，这里以pictureBox为例。DrawRectangle函数为例画矩形，其他形状不在这里考虑，自己尝试很简单
画圆是画椭圆，只需g.DrawEllipse后两个int参数width，height要设置相等，同时前两个int参数并不是圆心而是左上角的坐标，没有自带的circle函数只能自己封装
回到正题：
网上给的都是MouseDown MouseMoveMouseUp Paint事件相关的代码，非常的简单。
using System.Drawing;bool bDrawStart = false;Point pointStart = Point.Empty;Point pointContinue = Point.Empty;private void pictureBox1_MouseDown(object sender, MouseEventArgs e){if (bDrawStart){bDrawStart = false;}else{bDrawStart = true;pointStart = e.Location;}}private void pictureBox1_MouseMove(object sender, MouseEventArgs e){if (bDrawStart){pointContinue = e.Location;Refresh();}}private void pictureBox1_MouseUp(object sender, MouseEventArgs e){if (bDrawStart){dicPoints.Add(pointStart, pointContinue);}bDrawStart = false;}private void pictureBox1_Paint(object sender, PaintEventArgs e){if (bDrawStart){//实时的画矩形Graphics g = e.Graphics;g.DrawRectangle(pen, pointStart.X, pointStart.Y, pointContinue.X -pointStart.X, pointContinue.Y - pointStart.Y);}pen.Dispose();}
用完就发现很明显的问题了，一次只能画一个图形
如何才能一次画多个呢？不少都说的重写Paint事件，override之类的函数，多麻烦。
试验修改Paint事件代码即可，定义一个字典表记录画过的矩形（根据对角两个点确定一个矩形，对应字典表的key， value，不考虑矩形相交重叠之类的情况），如下：
Dictionary<Point, Point> dicPoints = new Dictionary<Point, Point>();private void pictureBox1_Paint(object sender, PaintEventArgs e){System.Drawing.Pen pen = new System.Drawing.Pen(Color.LimeGreen);pen.Width = 2;if (bDrawStart){//实时的画矩形Graphics g = e.Graphics;g.DrawRectangle(pen, pointStart.X, pointStart.Y, pointContinue.X - pointStart.X, pointContinue.Y -pointStart.Y);}//实时的画之前已经画好的矩形foreach (var item in dicPoints){Point p1 = item.Key;Point p2 = item.Value;e.Graphics.DrawRectangle(pen, p1.X, p1.Y, p2.X - p1.X, p2.Y - p1.Y);}pen.Dispose();}
