画直线小程序需要改进，来看看吧

Question

谁来帮我看看我的C#小程序
 悬赏分：20 - 离问题结束还有 14 天 6 小时
编了一个画直线的小程序，我的思想是这样的，点了画直线按钮后，再按下鼠标就开始画线，不要松开鼠标键拖动的过程中会产生一条直线。松开鼠标后直线确定。 
using System; 
using System.Collections.Generic; 
using System.ComponentModel; 
using System.Data; 
using System.Drawing; 
using System.Text; 
using System.Windows.Forms; 
namespace DrawLine 
{ 
public class Form1 : Form 
{ 
bool stmove=false;//判断MouseMove操作是否开始 
int x1,y1,x2,y2;//声明线的起点和终点坐标 
bool start=false; 
Graphics g; 
Button btn; 
Pen pen; 
public Form1() 
{ 
this.SuspendLayout(); 
pen = new Pen(Color.Red, 2); 
btn = new Button(); 
btn.Text = "画直线"; 
this.Controls.Add(btn); 
this.ResumeLayout(); 
btn.Click += new EventHandler(btn_Click); 
this.MouseDown += new MouseEventHandler(Form1_MouseDown); 
this.MouseUp += new MouseEventHandler(Form1_MouseUp); 
this.MouseMove += new MouseEventHandler(Form1_MouseMove); 
} 
private void btn_Click(object sender,EventArgs e) 
{ 
start = true; 
g = this.CreateGraphics(); 
} 
private void Form1_MouseDown(object sender,MouseEventArgs e) 
{ 
if(start) 
{ 
stmove = true; 
x1=e.X; 
y1=e.Y; 
} 
} 
private void Form1_MouseMove(object sender, MouseEventArgs e) 
{ 
if (stmove) 
{ 
g.Clear(this.BackColor); 
x2 = e.X; 
y2 = e.Y; 
g.DrawLine(pen, x1, y1, x2, y2); 
} 
} 
private void Form1_MouseUp(object sender,MouseEventArgs e) 
{ 
if(start) 
{ 
y2 = e.Y; 
x2 = e.X; 
g.DrawLine(pen,x1,y1,x2,y2); 
stmove = false; 
} 
} 
static void Main() 
{ 
Application.EnableVisualStyles(); 
Application.SetCompatibleTextRenderingDefault(false); 
Application.Run(new Form1()); 
} 
} 
} 
我发现一个问题就是画完第一调直线后再画第2条，第一条就没了，怎么解决这个问题。

Answer 1

//参考如下代码
//...namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }        Bitmap faceBitmap; // 实际绘制线条的位图
        Graphics faceGraphics; // 位图的图像
        Point downPoint = Point.Empty; // 鼠标按下的坐标，如果为Empty则没有按下
        private void Form1_Load(object sender, EventArgs e)
        {
            faceBitmap = new Bitmap(ClientSize.Width, ClientSize.Height);
            faceGraphics = Graphics.FromImage(faceBitmap);
            faceGraphics.FillRectangle(Brushes.White, faceGraphics.ClipBounds);
            DoubleBuffered = true; // 避免闪烁
        }        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.DrawImage(faceBitmap, new Point(0, 0));
        }        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button != MouseButtons.Left) return;
            downPoint = e.Location;
        }        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (downPoint == Point.Empty) return;
            Graphics vGraphics = Graphics.FromHwnd(Handle);
            vGraphics.DrawImage(faceBitmap, new Point(0, 0));
            vGraphics.DrawLine(Pens.Black, downPoint, e.Location);
        }        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            if (downPoint == Point.Empty) return;
            faceGraphics.DrawLine(Pens.Black, downPoint, e.Location);
            Graphics vGraphics = Graphics.FromHwnd(Handle);
            vGraphics.DrawImage(faceBitmap, new Point(0, 0));
            downPoint = Point.Empty;
        }
    }
}

Answer 2

我的想法是在FROM_PAINT 中画线创建一个Line CLASS把所有画过的线放ARRAYLIST中在Paint中foreach(Line in arrayList)
{
   Line.Draw(g);
}

Answer 3

public partial class Form5 : Form
    {
        public Form5()
        {
            InitializeComponent();
        }        private ArrayList drawingList = new ArrayList();
        private Line currentLine = null;
        private void Form5_MouseDown(object sender, MouseEventArgs e)
        {
            currentLine = new Line(new Point(e.X, e.Y));
        }        private void Form5_MouseUp(object sender, MouseEventArgs e)
        {
            if (currentLine != null)
            {
                currentLine.MouseMove(new Point(e.X,e.Y));
                drawingList.Add(currentLine);
                currentLine = null;
                this.Invalidate();
            }
        }        private void Form5_MouseMove(object sender, MouseEventArgs e)
        {
            if (currentLine != null && e.Button == MouseButtons.Left)
            {
                currentLine.MouseMove(new Point(e.X, e.Y));
                this.Invalidate();            }
        }        private void Form5_Paint(object sender, PaintEventArgs e)
        {
            foreach (Line line in drawingList)
            {
                line.Draw(e.Graphics);
            }
            if (currentLine != null)
            {
                currentLine.Draw(e.Graphics);
            }
        }
    }
    public class Line
    {
        private Point startPoint = new Point(0,0);
        private Point endPoint= new Point(0,0);
        public void MouseMove(Point p)
        {
            endPoint = p;
        }
        public Line(Point p)
        {
            startPoint = p;
        }
        public void Draw(Graphics g)
        {
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
            g.DrawLine(Pens.Black, startPoint, endPoint);
        }
    }