namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public float fpoint;//圆的直径
        private void Form1_Load(object sender, EventArgs e)
        {
            textBox1.Text = hScrollBar1.Value.ToString();
            fpoint = 10;//设一个初始值

        }
        private void hScrollBar1_Scroll(object sender, ScrollEventArgs e)
        {
            
            textBox1.Text = hScrollBar1.Value.ToString();//显示圆直径
            fpoint = hScrollBar1.Value;//想随时改变圆的直径
            //应该加入画图函数，该怎么加
            
            
        }

        private void panel1_Paint(object sender, PaintEventArgs e)//在Panel上画一个圆
        {
            DrawEllipseFloat(e, fpoint);

        }
        private void DrawEllipseFloat(PaintEventArgs e,float ft)//画圆函数
        {
           
            Pen blackPen = new Pen(Color.Black, 1);
            float x = 0.0F;
            float y = 0.0F;
            float width = ft;
            float height = ft;
            e.Graphics.DrawEllipse(blackPen, x, y, width, height);

        }
    }
}
以上是未完成的代码，请高手指点一下，主要就是想在鼠标拖动滚动条时，圆大小不断的变化。
在线等。。。。。

private void DrawEllipseFloat( float ft)//画圆函数
        {
            Pen blackPen = new Pen(Color.Black, 1);
            float x = 0.0F;
            float y = 0.0F;
            float width = ft;
            float height = ft;
            Graphics Gp =Graphics.FromHwnd ( panel1.Handle);
            Gp.Clear(panel1.BackColor);
            Gp.DrawEllipse(blackPen, x, y, width, height);

        }

        private void hScrollBar1_ValueChanged(object sender, EventArgs e)
        {

            DrawEllipseFloat(hScrollBar1.Value);
        }

先谢谢啊
不过还有问题
1.Gp.Clear(panel1.BackColor);这一句要不删除的话，拖动滚动条时panel1就变得全黑色了，什么也看不见
2.去掉上一句之后，虽然可以运行了，但是变化前的圆边仍然在，多次滚动以后就分不清楚了，怎样拖动才能使它只显示一个圆。
3怎样是圆心不变，只变半径？
请指点一下 谢谢

public Form1()
        {
            InitializeComponent();
            trackBar1.Minimum = 0;
            trackBar1.Maximum = pictureBox1.Width;
        }

        private void trackBar1_Scroll(object sender, EventArgs e)
        {
            int banjing;
            banjing = trackBar1.Value;
            Pen pen = new Pen(Color.Red);
            Graphics paper;
            paper = pictureBox1.CreateGraphics();
            paper.Clear(Color.White);
            paper.DrawEllipse(pen, 0, 0, banjing, banjing);
        }

Gp.Clear(panel1.BackColor);

'意思是先把原来的场景的背景色设为跟原来的一样，也就是清除原来画的东西

  圆心不变，只变半径？   
'这个计算下..嘻嘻。。。

那问题还没解决呀
程序该怎么修改呢？

嘻嘻。。乱撞撞出来了。。

       private void DrawEllipseFloat( float ft)//画圆函数
        {
            Pen blackPen = new Pen(Color.Black, 1);           
            float width = ft;
            float height = ft;
            float x = (panel1.Width  / 2) - (ft / 2);
            float y = (panel1.Height / 2) - (ft / 2);
            Graphics Gp =Graphics.FromHwnd ( panel1.Handle);
            Gp.Clear(panel1.BackColor);      
            Gp.DrawEllipse(blackPen, x, y, width, height);

        }

谢谢啊 结题啦