C#画直线的代码!（和windows画图工具一样的效果*）

C#画直线的代码!（和windows画图工具一样的效果*）
我的代码不对,不是射线状,就是没有画的过程.怎么样实现在屏幕上一点,拖出一个可变的直线,像橡皮条,松开后确定的那种!
private void Drawing_MouseDown(object sender,MouseEventArgs e)
{
if ((isDrawing = isDrawing) == true)//判断是否开始绘图
{
startPoint = new Point(e.X,e.Y);
}
}
private void Drawing_MouseMove(object sender,MouseEventArgs e)
{
Graphics g = this.Drawing.CreateGraphics();
if (isDrawing)
{
switch (drawTool)
{
case drawTools.Line:
using (Pen p = new Pen(foreColor,1))
{
graphics.DrawLine(p,startPoint,endPoint);
}
break;
}
}
private void Drawing_MouseUp(object sender,MouseEventArgs e)
{
isDrawing = false;//标识变量
graphics = this.Drawing.CreateGraphics();
switch (drawTool)
{
case drawTools.Line:
using (Pen p = new Pen(foreColor,1))
{
graphics.DrawLine(p,startPoint,new Point(e.X,e.Y));
}
break;
}
}
}
我没有单写类,需不需要单写个类啊?DDA算法是解决我这个问题的么?

我这里是画直线和画曲线的两种都有,楼主看看,错不了的,代码如下：namespace Case02_1{    public partial class Form1 : Form    {        int startX;     //获取鼠标起始点的X坐标        int startY;    //获取鼠标起始点的Y坐标        Graphics g;    //定义Graphics对象实例        public Form1()        {            InitializeComponent();        }        private void Form1_Load(object sender, EventArgs e)        {            this.StartPosition = FormStartPosition.CenterScreen;            this.BackColor = Color.Snow;        }        private void Form1_MouseUp(object sender, MouseEventArgs e)        {            g = this.CreateGraphics();                    Pen p = new Pen(Color.Red , 4);            if (radioButton1.Checked == true)            {                g.DrawLine(p, startX, startY, e.X, e.Y);            }         }        private void Form1_MouseDown(object sender, MouseEventArgs e)        {            startX = e.X;            startY = e.Y;        }        private void Form1_MouseMove(object sender, MouseEventArgs e)        {            g = this.CreateGraphics();            Pen p = new Pen(Color.Blue , 2);            if (radioButton2.Checked == true)            {                g.DrawRectangle(p, e.X, e.Y, 1, 1);            }        }        private void button1_Click(object sender, EventArgs e)        {            g = this.CreateGraphics();            g.Clear(Color.Snow );        }        private void button2_Click(object sender, EventArgs e)        {            this.Close();            Application.Exit();        }    }}